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BLUEPRINT OF LIFE
KEY WORDS AND TERMS USED IN THIS TOPIC
As you study this topic you should write the definitions for the following syllabus terms.
Term
evolution
natural selection
palaeontology
biogeography
comparative embryology
divergent evolution
convergent evolution
pedigrees
monohybrid crosses
Punnett squares
homozygous
heterozygous
allele
gene
hybridisation
dominant and recessive alleles
phenotype
genotype
meiosis
crossing over
haploid
diploid
DNA
gamete
sex linkage
co-dominance
gene expression
DNA replication
mutation
mutagen
variation
Punctuated Equilibrium
artificial insemination
artificial pollination
cloning
transgenic species
genetic diversity
polypeptide synthesis
Definition
BLUEPRINT OF LIFE
SUMMARY OF THIS TOPIC
The theories of evolution and natural selection,
which have been supported by fossil evidence,
biogeography, comparative embryology and
anatomy and biochemistry, suggest that inherited
characteristics must be able to vary in order to
produce new species over time.
The work of Gregor Mendel helped to improve our
knowledge of the inheritance of these
characteristics, although it was not formally
recognised at the time. Further work by scientists
such as Sutton, Boveri and Morgan helped to
explain Mendel’s work by recognising that genes,
the factors responsible for inheritance, are located
on chromosomes. The role of gamete formation
and sexual reproduction in producing offspring
variability was also recognised, as was the
influence of the environment on the expression of
phenotypes.
The determination of the structure of the DNA
molecule by Watson, Crick, Franklin and Wilkins
and the recognition of its role in polypeptide
synthesis by Beadle and Tatum gave further
insight into the mechanisms involved in
inheritance. Other research has revealed that the
structure of DNA can be changed, producing
mutated alleles that can be inherited.
Current reproductive technologies, such as
artificial insemination, artificial pollination and
cloning, and genetic engineering techniques such
as recombinant DNA technology, make use of our
current knowledge of genetics to the point where
man is in fact capable of altering the path of
evolution.
BLUEPRINT OF LIFE
MAJOR OBJECTIVES OF THIS TOPIC
As indicated in the HSC Biology syllabus, the
major outcomes of this topic include the ability to:
 describe how environmental changes and
competition for resources can lead to a
change in species
 explain how the theory of evolution is
supported by palaeontology,
biogeography, comparative embryology
and biochemistry
 use Darwin and Wallace’s theory of
evolution by natural selection to explain
divergent and convergent evolution
 outline the historical development of
evolutionary theories and describe how
these have been affected by technological
advances
 describe Mendel’s experiments and predict
the outcomes of simple monohybrid
crosses
 construct pedigrees and family trees to
trace the inheritance of various
characteristics
 understand the terms ‘homozygous’,
‘heterozygous’, ‘dominant’ and ‘recessive’
alleles , ‘genotype’ and ‘phenotype’
 give a specific example of hybridisation
within a species
 account for the fact that Mendel’s work
was not initially recognised
 describe the chemical nature of genes and
chromosomes and the roles of Sutton and
Boveri in linking genes with chromosomes
 construct a model showing the different
stages of meiosis
 describe the structure of the DNA
molecule
 explain the role of gamete formation,
crossing over and sexual reproduction in
creating variability among offspring
 understand how co-dominance and sexlinkage do not produce typical Mendelian
ratios among offspring
 outline Morgan’s contribution to our
understanding of sex-linkage
 investigate ways in which the environment
can affect phenotype
 describe DNA replication and the role of
DNA in polypeptide synthesis







outline Beadle and Tatum’s ‘one gene- one
polypeptide’ hypothesis
explain how mutations can occur and
describe how they can result in altered
DNA sequences and therefore altered
genes
describe how the concept of genetic
variation supports Darwin’s model of
natural selection
compare the theory of punctuated
equilibrium with Darwin’s theory of
evolution
outline the importance of the work of
Watson, Crick. Franklin and Wilkins in
determining the structure of DNA
describe current reproductive technologies
such as artificial insemination, artificial
pollination, cloning and recombinant DNA
technology and discuss their impact on the
genetic diversity of species
discuss any ethical issues arising from
current reproductive technologies
1. Evidence of evolution suggests
that the mechanisms of
inheritance, accompanied by
selection, allow changes over
many generations
ancestral giraffe, for example, was thought to have
been able to compete more successfully for leaves
high on trees because of its slightly longer neck.
This animal survived to pass its ‘long neck’ gene
on to its offspring, which could in turn compete
more successfully with shorter-necked animals for
food, and eventually a new long-necked species of
giraffes arose.
Environmental changes, competition
and evolution

The mechanism for evolution is known as natural
selection. It was proposed in 1858 by Darwin and
Wallace. This theory states that within a
population, physical and chemical environmental
changes and competition within a species can act
as selecting agents for favourable genetic
characteristics. Organisms possessing these
favourable characteristics survive, passing them on
to their offspring. Eventually, these characteristics
predominate, resulting in a change in the
population. For natural selection to occur, there
must first be genetic variation within a species.
An example of physical environmental changes
acting as a selecting agent can be seen in the
changes in the peppered moth in industrial
England in the late nineteenth century. Dark
coloured moths tended to survive to maturity when
the environment was polluted because they were
camouflaged. The gene for dark colour was passed
on to the next generations, while the gene for light
colour almost disappeared because light coloured
moths were spotted and eaten by birds before they
could reproduce. With the reduction of pollution in
recent years, the gene for light-coloured moths is
now predominating while the gene for dark
colouring is no longer favourable for survival.
An example of a population change brought about
by a chemical change in the environment is the
action of the insecticide DDT on mosquitoes. This
insecticide initially killed most of the mosquitoes it
was sprayed upon, but among the mosquitoes that
survived were some that did so because of the
possession of a ‘DDT resistant’ gene. This
favourable gene was passed on to the offspring of
the survivors, eventually resulting in a population
of mosquitoes with a genetic resistance to this
insecticide. DDT is now only effective in high
concentrations.
Competition within a species may also select for
particular favourable characteristics which could in
turn result in population changes over time. The
As a requirement of this topic, you are
expected to perform a first hand
investigation to model natural selection
Cut-out models of dark and light
species of the Peppered moth could be placed first
on dark coloured cloth to represent polluted
surroundings and then on light coloured cloth to
represent unpolluted surroundings. A student
could represent a predatory bird by randomly
selecting the first moths they can see clearly
against each background. Numbers of each moth
type ‘preyed’ upon could be recorded in each case.
Palaeontological (fossil) evidence for
evolution
The theory of evolution states that the many
different species alive today have developed from
a simple common ancestor or ancestors. The fossil
record is useful in providing some of the evidence
for this gradual change from simple to more
complex life forms. A fossil is defined as any trace
of past life and thus not only the actual remains are
regarded as fossils, but also things such as
footprints, burrows, moulds, casts and relics of
animal droppings.
One of the main ways fossils provide evidence for
evolution is the fact that the fossils of simpler
organisms have been found in the oldest rocks
while those of more complex organisms are only
found in more recent rock strata, indicating the
development of life on earth as being a gradual
unfolding from simpler to more advanced
organisms. One of the major trends revealed by
fossils has been the evolution of life from aquatic
to terrestrial forms in both the plant and animal
kingdom. The development of vertebrates, for
instance, appears to originate with fish and then
progresses to amphibians, reptiles, birds and
finally mammals. Similarly, the oldest plants
would seem to be those of algae, with subsequent
land plants progressing through the partly water
dependent mosses, ferns and seed ferns to the more
terrestrially adapted conifers and flowering plants.
The evolution of certain animal species alive today
is particularly well recorded by fossil evidence;
examples include the present day horse and
elephant. Fossils of ancestors of the modern horse
reveal its evolution from the tiny ‘dawn horse’,
Eohippus, over 50 million years ago, through
successively larger animals to its present form.
Accompanying these changes in size have also
been changes in the teeth and feet of the horse; this
is thought to be a result of the change in the habitat
of these animals over the years from forest to
grassland environments. The evolutionary history
of the modern elephant is also well represented by
fossils of ancestral forms dating back to more than
45 million years ago.
Fig.2-3 A Crossopterygian fish
A transitional form between reptiles and mammals
may well have been the therapsid, a reptile which
possessed a mammal-like jawbone and skeletal
structure. In the plant kingdom, fossils of ancient
‘seed-ferns’, Fig. 2-4 below, reveal them to have
been intermediate in form between ferns and seedbearing conifers, thus supporting the theory that
conifers evolved from ferns.
Fig. 2-4 A seed fern
Fig. 2-1 The evolution of the foot of the horse
reflected a change in habitat from softer to harder
ground and an increase in the size of the animal.
Fossils of ‘transitional’ organisms also help to
support the proposed sequence of evolution. The
discovery of fossils of Archaeopteryx, shown in
Fig. 2-2 in which bird-like wings and tail feathers
and reptilian teeth and skeletal structure are both
apparent, suggests that birds may have evolved
from reptiles.
Fig.2-2 Archaeopteryx
In a similar manner, fossils of the Crossopterygian
fish, an ancient lungfish capable of obtaining
oxygen from the air and with lobed (jointed) fins,
tend to suggest that this animal (shown in Fig. 2-3)
had characteristics of both fish and reptiles.
THINK!!! Can you re-write the
plants below in the correct order in which they
appeared in the fossil record?
flowering plants, mosses, conifers, algae, ferns
Biogeographical evidence for
evolution
Biogeography is the distribution of plants and
animals in various regions of the world. The
biogeography of unique species of animals such as
the marsupials in Australia helps to provide
evidence that organisms have become adapted to
their own particular habitat in the process of
natural selection. Although some marsupials are
also present in South America, Australian
marsupials such as the kangaroo, platypus and
echidna have evolved as an adaptation to their own
particular ecological niche since the continents of
Gondwanaland moved apart 136 million years ago.
In his travels, Darwin also noted evolution
occurring independently due to geographic
isolation, especially in areas such as the Galapagos
islands. The biogeography of flightless birds
(ratites) also provides evidence for evolution.
Originally, an ancestral bird existed in
Gondwanaland, but as the continents drifted apart,
a variety of forms developed. Examples include
the Cassowary and Emu in Australia, the ostrich in
Africa, the rhea in South America and the kiwi in
New Zealand. A comparison of the DNA among
these species suggests a common ancestor, but
their different forms and their biogeography again
imply that they evolved to become adapted to their
own specific niches.
The fact that members of the
Proteaceae family (which includes waratahs
and proteas) are found in both Australia and
South Africa helps to support the idea that the
land masses of the southern hemisphere were
once connected to form a super land mass
called Gondwanaland.

As a requirement of this topic, you need to
prepare a case study to show how an
environmental change can lead to a change
in species.
The evolution of the horse over the last 60 million
years is a good example of this. The changes in
this animal over time were accompanied by
changes in its environment from swampy forests to
dry grasslands. As the surroundings changed, those
with better adaptations survived: smaller animals
with 3-4 toes and teeth without large grinding
surfaces thrived in the forests of the Eocene period
while larger animals resembling today’s horses
predominated in the grasslands of the more recent
Pliocene period. The possession of molar teeth that
were adapted for grinding grass was a definite
advantage in grassland environments. In addition,
a reduced number of toes meant that the increased
weight of these larger horses could be centred
more effectively. This also enabled the animals to
run faster over the hard ground when being chased
by predators. Table 2-1 outlines the evolution of
the horse, as indicated by fossil evidence.
THINK!!! Modern day horses also
have eyes which are positioned higher up their
their heads. What adaptive advantage would this
have?
Era and
type of
environment
Name of
genus
Structural
features/
adaptations
Eocene
period (60
million years
ago) swampy
forests
Hyracotherium
( Eohippus)
3 toes on hind
feet, 4 toes on
front feet. Only a
small grinding
area on teeth
surfaces.
Oligocene
period (40
million years
ago) -Forests
and some
grassland
Mesohippus
Larger than
Hyracotherium,
middle digit of
each foot wider
and weight
bearing; other
two digits
reduced in size,
molars capable
of grinding
Miocene
period (26
million years
ago) - mainly
grassland
Merychippus
Eyes higher in
the head, larger
than
Mesohippus,
second and third
toes reduced
even more in
size, molars with
large grinding
surface.
Pliocene
period (7
million years
ago) - dry
grasslasnd
Pliohippus
Second and third
toes absent, large
grinding surface
on molars,
almost as big as
modern horse.
Pleistocene
period (1.5
million years
ago to
present) - dry
grassland
Equus
Single toe
surrounded by
protective hoof,
large grinding
molars, larger
than all its
ancestors.
Table 2-1 The evolution of the horse
Comparative embryology and
anatomy as evidence for evolution
The theory of evolution states that organisms have
arisen over time from common ancestors .If this is
the case then we would expect to find similarities
among groups of descendant organisms today.
This is indeed the case when certain structural
features of the vertebrates are examined. Firstly,
when vertebrate embryos are compared, it is found
that there is a strong similarity between fish,
amphibians, reptiles, birds and mammals,
especially in the very early stages of development.
All vertebrate embryos possess gill pouches but it
is only in fish that these later develop into true
gills.
Secondly, a comparison of the anatomy of the
forelimbs of vertebrates reveals a similarity that
suggests they have evolved from a common origin.
Characteristics such as this are known as
‘homologous structures’. An observation of the
bone structure of the bird wing, the bat wing, the
whale fin and the forelimbs of reptiles and
mammals indicates that they are all composed of
the same fundamental units. Each consists of five
finger-like bones connected to a modified radius,
ulna and shoulder blade, as shown below. This
feature is often referred to as the ‘pentadactyl
limb’.

As a requirement of this topic, you are
expected to observe, analyse and
compare the structure of a range of
vertebrate forelimbs.
Fig.2-5, below, shows some modifications of the
pentadactyl limb in five vertebrate species
a) Fin of Crossopterygian
fish
(a primitive lungfish)
b) Whale fin
c) Bird wing
d) Bat wing
e) Human
forearm
Fig. 2-5 Modifications of the pentadactyl limb
in five vertebrate species
Biochemical similarities as evidence
for evolution
The fact that the basic chemistry of the cells of all
living things is similar provides further evidence of
common ancestry; all cells contain proteins,
carbohydrates, fats, water, DNA and RNA. In
addition, all vertebrates possess haemoglobin and
can manufacture hormones.
A more revealing method of comparing the
evolutionary relationship between two species is to
observe similarities in the number and type of
proteins in each. The longer groups of organisms
have been separated from each other over time, the
greater the protein difference will be; it is not
surprising, then, to find that a comparison between
humans and turtles reveals 15 different amino
acids in the cytochrome C protein whereas there is
only a difference of one amino acid between
humans and monkeys, as shown in table 2-2.
Related to this is the fact that the more closely
related one organism is to another, the more
similar their DNA will be.
Scientists can compare the proteins of
different organisms by mixing the blood of each.
The amount of antibody production that occurs is
an indication of the genetic difference between the
two species.
Human
Duck
Turtle
Yeast
Horse
Monkey
Cow
Dog
Kangaroo
Human
0
11
15
45
12
1
10
11
10
Duck
Turtle
Yeast
Horse
Monkey
Cow
Dog
Kangaroo
0
7
46
10
10
8
8
10
0
49
11
14
9
9
11
0
46
45
45
45
46
0
11
3
6
7
0
9
10
11
0
3
6
0
7
0
Table 2-2 The number of different amino acids in the cytochrome C protein in different organisms
Type of Galapagos
Island Finch
woodpecker finch
THINK!! Based on the table
above, which organism is the least closely
related to humans?
How natural selection and isolation
accounts for divergent and
convergent evolution
Darwin and Wallace stated that within any
population of a species, there exists genetic
variation. Divergent evolution arises when
members of a species develop different adaptations
in different environments. Examples of divergent
evolution include the finches of the Galapagos
islands, as shown in Fig, 2-6. Darwin noticed that
the 14 different species of this bird had developed
different diets and beaks on each island. In order
for completely new species to have developed
from the original finches on the mainland, genetic
isolation must have occurred. Divergent evolution
can also be seen in the kangaroo family, with the
tree kangaroo evolving in rainforests and the rat
kangaroo evolving in deserts, for example.
Convergent evolution occurs when different
species develop similar adaptations in similar
environmental niches. Examples include the shark,
penguin, dolphin and turtle, which all possess
streamlined bodies, flippers or fins.

During this topic you are required to use a
named example to analyse how advances in
technology have changed scientific thinking
about evolutionary relationships
Diet and beak type
insects; strong bill that
can bore into wood
warbler finch
small insects; curved
beak
ground finch
seeds; small beak
cactus finch
nectar; straight beak
and tongue
tree finch
buds, berries or
insects; beak strongly
curved
Fig. 2-6 Divergent evolution in the Galapagos
Islands finches
DNA hybridisation, a technique in which ‘hybrid’
DNA molecules are produced with one strand from
each of two different organisms, is a modern
technology that has changed scientific thinking
about some evolutionary relationships. The degree
of matching between base pairs from each strand
indicates the similarity in amino acid sequences
between the two species. Recent DNA
hybridisation studies have revealed, for instance,
that the giant panda is more closely related to bears
than raccoons, and that humans bear the closest
genetic resemblance to chimpanzees, not gorillas.
The historical development of
theories of evolution
Originally, the idea of ‘Special Creation’, endorsed
by the church, was generally accepted by the
masses. This theory stated that all living things
were the products of a divine creation and that
most were created for the service of mankind.
Among those to first question this was Georges
Buffon (1707-1788), who suggested that species
might undergo some changes in the course of time
by a process of ‘degeneration’. James Hutton
(1726-1797) proposed that the earth had been
formed gradually over many years, and the
geologist Charles Lyell (1797-1875) produced
evidence that the earth had a long history. This
helped to provide enough time for the evolutionary
changes proposed by Darwin and Wallace to have
taken place. In 1858 Both Darwin and Wallace
suggested that the many species on earth have
evolved from a common ancestor by the process of
natural selection. This theory, although generating
much controversy, was more widely accepted than
Lamarck’s (1744-1829) idea that evolution
occurred through the inheritance of acquired
characteristics.
2. Gregor Mendel’s experiments
helped advance our knowledge
of the inheritance of
characteristics
Mendel’s experiments
The work of the Austrian monk, Gregor Mendel,
has helped us to understand how characteristics in
living things are inherited, and, in certain cases, to
predict the genetic and physical outcomes of
sexual reproduction.
Mendel did not know of the existence of genes, but
his experiments (presented in 1865 but mostly
ignored) led him to conclude that the factors
responsible for the inheritance of characteristics
occurred as discrete units, and that they were
inherited in pairs with one factor coming from
each parent. This idea is known as Mendel’s first
law, or the principle of segregation, and we now
know that the factors he was referring to are genes.
Each alternate member of a gene pair is known as
an ‘allele’, and these alleles separate from each
other during gamete formation.
Mendel chose to investigate the inheritance of
particular ‘traits’ (characteristics) in pea plants.
These plants were easy to grow and reproduced
quickly, with distinctive traits that could be easily
tested such as flower colour and seed form.
Moreover, they were not easily cross pollinated, so
allowing Mendel to carry out his experiments in a
controlled manner.
Mendel pollinated the plants himself, and initially
tested the inheritance of one particular trait at a
time; these traits included seed form, flower
colour, seed colour, stem length, pod form and pod
colour. Each trait existed as two alternate forms so
that with flower colour, for instance, purple
flowers or white flowers were produced by the
plants. With each of these experiments he found
that when pure breeding forms were crossed, one
of the alternate traits completely disappeared in the
F1 or first generation but reappeared in the
offspring whenever two F1 plants were crossed.
Hence, when flower colour was the trait
investigated, Mendel’s results were as follows:
Parents:
Purple
flowers x
white
flowers
F1(first)
generation:
All purple
flowered
↓
Two F1
plants
crossed
F2(2nd)
generation:
↓
3 purple
flowered:
1 white
flowered
↓
↓
Fig. 2-7 The outcome of crossing pure
breeding purple with pure breeding white pea
flowers
In this particular example, Mendel called the
purple colour the ‘dominant’ trait because it was
the only factor expressed in the F1 generation, and
the white colour the ‘recessive’ trait because it
disappeared or receded for a generation. He
concluded that the factor (gene) for white
colouring must have been present all the time and
that when it combined with the factor (gene) for
purple it was not expressed.
We can predict the outcome of the monohybrid
cross (i.e. involving the inheritance of one pair of
alternate characteristics only) above using a
‘Punnett square’, as shown below.
Gametes
( i.e.
alleles
present)
p
p
P
Pp
Pp
gametes of
parent 2
P
produced when two heterozygous black guinea
pigs are mated.
Answer
A Punnett square can be drawn up, as shown, and
the resulting genotypes can be filled in.
gametes of
parent 1
Pp
Pp
genotypes of
offspring
The genotype of an individual is its genetic
makeup, hence all the offspring above are of
genotype Pp. The phenotype of an individual is its
outward appearance. In this case, the phenotype of
all offspring would be purple coloured flowers,
because the purple allele (P) is dominant to the
white allele (p).
Gametes B
b
BB Bb
B
Bb bb
b
From the Punnett square we can see that the
genotypes of the offspring are BB, Bb, Bb and bb.
Their corresponding phenotypes are black, black,
black and white, or 3 black: 1 white.
ii) Two purple flowering pea plants were crossed.
All the offspring plants produced purple
flowers. However, when these plants were
crossed, there were some white flowering
plants in the F2 generation.
Explain why it is impossible that each of the
original plants was homozygous for purple
flowers. Use diagrams where necessary.
Answer
Pure breeding plants possess two
identical alleles and are referred to as being
‘homozygous’. Examples are PP,pp, WW,ww
etc.Hybrid individuals, that is, individuals carrying
a dominant and a recessive gene, are referred to as
‘heterozygous’. Examples would be Pp, Ww etc.
One of the original plants must have been
heterozygous because the two F1 plants that were
crossed needed to be heterozygous to produce a
white flowering offspring.
i.e. a) Original parents crossed:
PP x Pp
↓
F1: PP, Pp, PP, Pp
b) Heterozygous members of F1
THINK!!! Circle the
homozygous genotypes from the ones listed
below:
pp, Pp, Bb, BB, ww, Ww, aa, AA. Which of
these are homozygous and dominant?

crossed:
F2:

As a requirement of this topic you need to
be able to solve problems involving
monohybrid crosses using Punnett squares
or other appropriate techniques.
Sample problems
i) The gene for black hair colour in guinea pigs
(B) is dominant to the gene for white hair (b).
Determine the genotypes and phenotypes
Pp x Pp
↓
PP, Pp, Pp, pp
As a requirement of this topic you need to
be able to construct pedigrees or family
trees, trace the inheritance of selected
characteristics and discuss their current
use.
In cases where genetic breeding experiments are
not possible or convenient, pedigrees of known
relatives can be drawn up to study the inheritance
of a particular trait over the generations. Symbols
often used are shown below.
= normal female
= affected female
= normal male
= affected male
marriage line
Ι
offspring
line
ΙΙ
Fig. 2-8 A typical pedigree
Sample problem
Cocker spaniels have either black or red coats. Use
the pedigree below to answer the following
questions. The trait for red coat is shaded.
1
3
2
4
5
6
7
8
a) Is the characteristic for red coat colour
dominant or recessive? Explain.
b) Determine the phenotypes and genotypes for
individuals 1, 3 and 6.
c) What possible genotypes could cocker spaniel
8 be? Explain.
Answer
a) Recessive, because if individuals 5 and 6 were
recessive they would only produce recessive
offspring, and it can be seen that their
offspring display both the dominant and
recessive phenotypes.
b) 1 = Bb; 3 = Bb; 6 = Bb
c) Cocker spaniel 8 could be either Bb or BB
because its parents are both Bb; BB and Bb are
thus both possible when 5 and 6 are crossed.
Why Mendel’s work was not initially
recognised
Mendel’s two laws, the principle of segregation
and the law of independent assortment (which
states that during meiosis either allele of a gene
pair can combine with either allele of another gene
pair), are still used today to explain the behaviour
of genes in reproduction. Despite this, his 1865
paper had little impact until the early twentieth
century. One reason for this may have been that he
was not a high profile member of the scientific
community in Austria at the time; also, scientific
developments as a whole were not encouraged
during this period. In addition, Mendel’s findings,
because they were unprecedented, were not
generally understood by other members of the
scientific community.
Hybridisation within a species
Hybridisation involves crossing two genetically
different individuals to produce offspring with
more favourable characteristics than the parent
plants. These superior characteristics among the
offspring are often known as ‘hybrid vigour’, and
are a result of their heterozygous nature. Crop
plants that have been improved by hybridisation
include corn, potatoes and tomatoes. Improved
qualities include disease resistance and better
nutritional quality.
To produce hybrid corn plants, pairs of ‘inbred’
strains are crossed: each is pure breeding for a
different allele of the same trait. The offspring of
these crosses are then crossed, producing a plant
that is heterozygous for one or more
characteristics.
In 1970, 15% of the US corn crop
was destroyed by leaf blight. A similar situation
occurred in the Irish potato famine of 1846 where
the bulk of the potato crop was wiped out by a
fungal attack. This was due to the genetic
uniformity of the plants, which had been
hybridised to produce greater yields, but not
disease resistance. It is therefore now commonly
recognised that samples of original ‘wild stock’
plants should be retained and crossed with modern
hybrids to maintain genetic diversity within these
particular species.

During this topic you are required to
construct a model that demonstrates
meiosis and the processes of crossing over,
segregation of chromosomes and the
production of haploid gametes.
Modelling clay could be used to reconstruct each
stage of meiosis, using a diagram similar to that
shown in Fig. 2-9. Meiosis is the term given to
describe the cell division that occurs in the sex
organs of animals and plants to produce haploid
(half the normal chromosome number) gametes
from diploid (the full chromosome number) body
cells. Notice that in Metaphase 1, homologous
chromosomes have paired up with each other and
exchange genetic material by ‘crossing over’. In
anaphase 1 members of each homologous pair
separate from each other and in anaphase 2 their
chromatids separate. The resulting gametes, seen
in Telophase 2, have half the original chromosome
number.
3. Chromosomal structure
provides the key to inheritance
Sutton, Boveri and the importance of
chromosomes
Theodore Boveri, a microbiologist who studied
meiosis in sea urchins, and William Sutton, a
scientist who studied reproduction in grasshoppers,
proposed the “Chromosome theory of inheritance’
in1902.This theory stated that a) chromosomes
occur in pairs in the body cells of organisms; b)
each member of a chromosome pair separates into
separate gametes during meiosis; c) new pairs of
chromosomes form when gametes unite in
fertilization and d) hundreds of genes are located
on each chromosome.
They arrived at the last part of this theory by
noticing that the behaviour of chromosomes during
meiosis was similar to the way Mendel proposed
his ‘factors’(genes) to behave. As a result, they
concluded that genes must be located on
chromosomes and that since there are more
inheritable characteristics in an organism than
there are chromosomes, each chromosome must
have many genes located on it.
Sutton referred to similar
chromosomes within a pair as ‘homologous’
chromosomes
Fig. 2-9 The stages of meiosis
DNA and the chemical nature of
chromosomes and genes
Chromosomes occur in the body cells and gametes
of organisms. They consist of a DNA molecule
coiled around a protein core. The protein
component of chromosomes makes up about 60%
of their mass, while the DNA component
constitutes about 40%.
The DNA molecule, first described by Watson and
Crick in the early 1950s, forms the shape of a
double helix. This looks somewhat like a ladder
that has been twisted, with the ‘uprights’
consisting of deoxyribose sugars and phosphate
groups. The ‘rungs’, which are attached to the
sugar units, are made up of nitrogen bases. There
are four of these bases: adenine, thymine, cytosine
and guanine. Thymine will only bond with
adenine and guanine will only bond with cytosine.
Each subunit of a DNA molecule is called a
nucleotide, and consists of a sugar, a phosphate
and a base, as shown below.
phosphate
nitrogen
base
sugar
Fig. 2-10 A nucleotide
The diagram below of a simple model of a DNA
molecule shows the complementary base pairs and
alternating sugar and phosphate groups along the
‘uprights’ of the molecule.
T
A = adenine
C =cytosine
T = thymine
G = guanine
Organisms that reproduce by mitosis alone will
always produce identical offspring. Meiosis, on
the other hand, ensures variability among the
offspring in several ways: i) When the gametes of
both parents unite, the resulting diploid zygote
will carry genes from both parents and will not be
identical to either parent; ii) According to the
chromosome theory and Mendel’s law of
independent assortment, during meiosis each
member of a homologous chromosome pair
separates independently of members of other
homologous pairs. Because these chromosomes
are carrying genes it means there will be a greater
range of possible gene combinations in the
gametes and thus in the zygotes produced after
fertilization; iii) During the first metaphase in
meiosis, homologous chromosomes may exchange
portions of their chromatids. This process of
‘crossing over’ will also inevitably result in the
exchenge of genetic material between these
chromosomes. The result is that there will be a
greater range of gene combinations in the gametes,
thus ultimately ensuring more variety in the
offspring after sexual reproduction takes place.
Fig 2-12, below, shows a pair of homologous
chromosomes undergoing crossing over. It can be
seen that the chromosomes in the gametes contain
genetic material (alleles B , b, T and t) that has
been ‘swapped’ around in the crossing over
process.
A
G
A
Gamete formation, sexual
reproduction and variability of
offspring
C
sugar
T
phosphate
C
G
Fig. 2-11 A section of a DNA molecule
Fig. 2-12 Crossing over during meiosis
Genetic variability greatly improves
the survival chances of a particular species
because unfavourable characteristics are not likely
to be duplicated throughout the whole population.
With greater genetic variation there is also the
possibility of better adaptations arising through
natural selection if, for example, the environment
changes.
crossed with a pure breeding red cow. The result is
a calf with both red and white hairs in its coat.
In other cases, the characteristics of both genes
blend, because neither is completely dominant
over the other. This is known as ‘incomplete
dominance’ and occurs, for instance, in
snapdragons. In these flowers, the presence of a
gene for red colouring and the presence of a gene
for white colouring results in a pink flower, as
shown below.
Sex linkage and co-dominance
Mendel’s predicted monohybrid and dihybrid
ratios are not adhered to when the results of some
crosses are examined. One situation where this is
the case is found in ‘sex linkage’. This is where
genes for certain characteristics are located on the
‘X’ chromosome, one of the two sex
chromosomes present in most organisms. Females
possess two X chromosomes but males possess an
X and a Y chromosome. Because the Y
chromosome carries very little genetic
information, if a male inherits a recessive gene on
his X chromosome, he will exhibit that
characteristic because there is no corresponding
dominant gene present to mask it. Hence
haemophilia and colour blindness, for instance,
which are caused by recessive genes carried on the
X chromosome, are diseases more commonly
found in males. T.H. Morgan first discovered the
phenomenon of sex linkage when studying
inheritance in fruit flies. He concluded that the
gene for white eyes was recessive and carried on
the X chromosome. As a result, when a white eyed
female was crossed with a red eyed male, the
following outcome occurred:
White eyed female x Red eyed male
X wX w
XWY
↓
XWXw,
XWXw
2 red eyed females,
XwY,
X wY
2 white eyed males,
Another situation where Mendel’s predicted
outcomes does not occur involves crosses where
one gene does not completely dominate the other.
In some cases, both genes are expressed in the
individual carrying them. This is known as ‘codominance’ and an example includes coat colour
in cows. Red and white genes are both expressed
in the offspring when a pure breeding red bull is
Red flowers x White flowers
RR
WW
↓
All pink flowers
RW, RW, RW, RW

As a requirement of this topic, you are
expected to solve problems involving codominance and sex linkage.
Sample problems
i)
In humans, colour-blindness is caused by a
sex-linked recessive gene. What are the
genotypes and phenotypes of the offspring in
the following crosses?
a) A normal man x a colour-blind woman
b) A normal woman who has a colour-blind
father x a normal man
Answer
a) The cross involved is
Offspring:
genotypes
XCY x XcXc
↓
XCXc, XCXc, XcY, XcY
Phenotypes: 2 normal females, 2 colour blind
males
b) The cross involved is
XCXc x XCY
↓
C C
Offspring
X X , XCY, XCXc, XcY
genotypes:
Phenotypes:
normal female, normal male,
normal female, colour blind male
ii) In cattle, the two alleles for coat colour are codominant. When genes for white colouring and
genes for red colouring occur together, the
resulting colour is known as ‘roan’. Work out the
genotypes and phenotypes produced in the
offspring when a roan cow is crossed with a white
bull.
Answer
The cross involved isRW x WW
↓
RW, RW, WW, WW
(2 roan, 2 white)
iii) a) How would you explain the fact that when
red flowered and white flowered
snapdragons are crossed their offspring
always produce pink flowers?
b) What will be the phenotypes of the
offspring produced when two pink flowers
are crossed?
in features such as growth rates, habit, flowering
and fruiting. The water buttercup’s leaves
(Ranunculus peltatus) express their genes
differently, depending on whether they are
submerged or above the water, as shown in Fig. 213, below. Himalayan rabbits and Siamese cats
display different markings on their coats,
depending on the surrounding temperatures.
Similarly, the colouring of Mexican fighting fish is
partly determined by genes, and partly by the
presence of another fish. In addition, studies of
identical twins have shown that although they are
genetically identical, if they are raised in different
environments their phenotypes may differ (in, for
example, their height, health etc.).
floating
leaves
Answer
a) This must be a situation in which incomplete
dominance is occurring: neither allele
dominates the other, resulting in the
characteristics of both alleles blending.
b)
RW x RW
↓
RR, RW, RW, WW 1 red flowered plant, 2
pink flowered plants, 1 white flowered plant
THINK!!! Morgan initially crossed a
white eyed male fruit fly (XwY) with a red eyed
female (XWXW) to produce offspring which all had
red eyes (XWXw, XWXw, XWY, XWY). Explain why
a cross between a male and a female of these
offspring could never produce a white eyed
female, and thus would not follow Mendel’s ratios.
Gene expression and the environment
The characteristics of an individual are determined
both by genes and by the influence of the
environment on the expression of these genes. This
can be seen when essential plant requirements such
as nutrients, moisture, sunlight and temperature are
varied; experimental plants will display differences
submerged
leaves
Fig. 2-13 Gene expression in the water buttercup

As a requirement of this topic, you are
expected to perform a first-hand
investigation to demonstrate the effect of
environment on phenotype
Data on this subject usually involves comparisons
involving sets of twins; when, for instance, a
disease is more commonly shared between
identical twins than fraternal twins, the difference
is likely to be caused by genetic rather than
environmental factors. Another method is to
compare characteristics of identical twins raised
together with these characteristics in identical
twins which have been raised apart. The table
below, for instance, suggests that the environment
does have an effect on IQ and height.
Characteristic
studied
Difference in
identical twins
raised together
IQ
Height
3.2%
2%
Difference
in identical
twins
raised apart
6.9%
2.7 %
Table 2-3 The effect of environment on IQ and
height in identical twins
4. The structure of DNA can be
changed and such changes may
be reflected in the phenotype of
the affected organism
DNA replication
functions. Each of the polypeptide molecules are
in turn composed of a chain of amino acids, and
the sequence of these depends on the sequence of
bases in the DNA molecule. The segment of a
DNA molecule that codes for a particular
polypeptide is known as a gene. The stages
involved in polypeptide synthesis are as follows:

DNA replicates during the resting stage
(interphase) between nuclear divisions in mitosis
and meiosis. This process involves the DNA
‘unzipping’ to form two separate strands. As this
happens, nucleotides, consisting of a sugar,
phosphate and complementary nitrogen base
attach themselves to the single parent strands at
the appropriate bases (i.e. adenine with thymine,
guanine with cytosine). The enzyme, DNA
polymerase is required for this process.




THINK!!! In the diagram below
showing DNA replication, can you name the six
bases on the two newly formed strands?
newly
formed
strands


DNA in the nucleus ‘unzips’, exposing
unpaired nitrogen bases.
Messenger RNA copies the code on a single
stranded DNA molecule in the nucleus. This
process is known as transcription. RNA is
single stranded and contains a ribose instead
of a deoxyribose sugar.
The messenger RNA moves from the nucleus
and attaches itself to the small subunit of a
ribosome in the cytoplasm.
Transfer RNA molecules carry a ‘triplet’ of
bases at one end. Each triplet codes for a
particular amino acid, and these are picked up
by the tRNA molecules in the cytoplasm.
The transfer RNA molecules match up with
their complementary base triplets on the
messenger RNA.
Further amino acids, carried by their transfer
RNAs, become attached to the messenger
RNA and are joined to each other by peptide
bonds.
Eventually, a chain of amino acids is formed.
This process is known as translation.
Fig 2-14 shows a diagram of polypeptide synthesis
parent
strands
DNA and polypeptide synthesis
The particular base sequence of each DNA
molecule determines which polypaptides are
formed in the cells of organisms. Polypeptides are
the basic units of proteins and it is the proteins in a
cell thast essentially control all its metabolic

As a requirement of this topic, you need
to develop a simple model for
polypeptide synthesis
Coloured pegs representing base pairs
and wool could be used to represent a DNA
molecule. The pegs are then unclipped and
complementary base pegs clipped on to the single
parent strands to represent transcription on to
mRNA. Note that on your mRNA a new coloured
peg representing uracil must be used to replace
thymine. ‘Triplets’ of appropriately coloured pegs
are threaded together to represent transfer RNA,
and the amino acid corresponding to this triplet is
taped to the tRNA. To represent transcription,
triplets corresponding to the base sequence on your
mRNA can then be lined up along the mRNA to
form a sequence of amino acids.
ii)
aa
mRNA
i)
iii)
DNA
aa
Fig. 2-14 Polypeptide synthesis. In i), the DNA code is being copied by mRNA; in ii), mRNA moves to a
ribosome in the cytoplasm; in iii), tRNA carries amino acids to the ribosome where the base triplets match uphere, the amino acids will link together to form a polypeptide .

As a requirement of this topic, you need
to outline the evidence that led to
Beadle and Tatum’s ‘one gene-one
ptotein’ hypothesis and to explain why
this was altered to the ‘one gene-one
polypeptide’ hypothesis.
In 1941 these two American biochemists
attempted to alter or inactivate specific genes in
the bread mould Neurospora so that they could
observe whether this produced any changes in the
organism’s cells. After exposing the mould to Xrays they found that some of the offspring could
not produce the amino acid arginine. Further
investigation revealed that each of four strains of
mould failed to produce a separate enzyme needed
for arginine synthesis. They concluded that for
each defective enzyme ther was one gene on one
specific area of a chromosome that had been
mutated by irradiation. Their’one gene-one
enzyme/protein’ theory has been re-named the
‘one gene-one polypeptide’ theory because some
proteins are made of more than one polypeptide
chain.
Mutations in DNA and the formation
of new alleles
A mutation is a change in a gene. It usually occurs
during DNA replication. Mutations are inherited
when they occur in a gamete, but not when they
occur in a normal body cell. Favourable mutations
can result in more variation within a population.
Factors that cause mutations are known as
mutagens. They include certain types of radiation
and chemicals. Spontaneous mutations are called
background mutations. Some mutations occur
during cell division. Down’s syndrome, for
instance, results from the inheritance of an extra
chromosome 21 and is the result of chromosome
pairs failing to separate during meiosis. Another
situation, in which chromosomes fail to separate
during mitosis, can result in an organism
inheriting more than one set of chromosomes. This
is known as polyploidy.
Mutations in which there is a change in the base
sequence of the DNA molecule include;
i) Substitution mutations - here, an incorrect base
replaces one of the original ones in the DNA
strand.
ii) Insertion mutations - in this type of mutation,
an extra base is added to the DNA sequence.
iii) Deletion mutations - in this type of mutation a
base is deleted from the DNA code.
iv) Inversion mutations - here, due to a mistake
during DNA replication, a whole base triplet is
back to front.
The DNA below has undergone a
substitution mutation because the first thymine has
been replaced with an adenine.
original DNA C A G T A G G T C
mutated DNA C A G A AG G T C

As a requirement of this topic, you need
to construct a flow chart that shows
that changes in DNA sequences can
result in changes in cell activity
The table below can be used to construct an
appropriate flow chart. Notice that the mutation
has resulted in a totally new sequence of triplets,
with the third triplet becoming a ‘stop’ codon
(UAA) . This results in premature termination of
polypeptide synthesis.
Normal
protein
synthesis
Code( base
sequence) on
original DNA
↓
Transcribed
code on mRNA
↓
Sequence of
amino acids
formed on
ribosome
↓
Final
polypeptidefunctional or
nonfunctional?
↓
Cellular
activity
proceeds/ does
not proceed
normally?
CAG, TAG,
AGT, TAA,
CGC
↓
GUC, AUC,
UCA, AUU,
GCG
↓
Valine,
isoleucine,
serine,
threonone,
alanine
↓
Faulty
protein
synthesis
following
mutation
CAG, TAG,
ATT, AAC,
GC
↓
GUC, AUC,
UAA, UUG,
CG
↓
Valine,
isoleucine,
stop
Functional
polypeptide
↓
Nonfunctional
poypeptide
↓
Cellular
activity
proceeds
normally
↓
Cellular
activity does
not proceed
normally
Table 2-4 The effect of a mutation on cellular
activity.
Evidence for the mutagenic nature of
radiation
Radiation is of two main types; ionizing (high
energy) radiation, which includes X-rays and
gamma rays, and non-ionising radiation such as
ultraviolet light. Ionising radiation has been shown
to produce free radicals or electrons when
absorbed, resulting in deletions, translocations (the
movement of whole DNA sections to other
locations) and base substitutions in DNA.The
atomic bombs dropped on Hiroshima and
Nagasaki, for instance, produced a ten-fold
increase in cancer deaths in these areas. Mutations
such as deletions and translocations have been
found in the cancer cells of some of the bomb
victims. The Chernobyl incident in 1986 has also
been linked to increased cancer rates in Russia, the
Ukraine and Belarus. In 1927 Hans Muller was
awarded the Nobel Prize for demonstrating that
genes in Drosophola mutated when exposed to Xrays.
Ultraviolet radiation has been shown to create
bonds between adjacent pyrimidine bases (i.e.
thymine, cytosine and uracil). This blocks the
normal replication of DNA.
The disease Xeroderma pigmentosum
is a disease in which the sufferer lacks an enzyme
needed for the repair of UV-induced mutations.
These people must avoid excessive exposure to
sunlight.
Variation and natural selection
Darwin and Wallace recognized that individuals
within a population vary, and that those with
favourable variations survived to reproduce. Our
current idea of natural selection includes the
concept that there is a gradual change in the allele
frequency within a population. Darwin and
Wallace did not understand how variations came
about, but we now know that inheritable variations
are carried on genes. Genetic variation can arise
from haploid gametes uniting randomly during
fertilization, independent assortment of
homologous chromosome pairs during meiosis and
from mutations.
Although most mutations are harmful, some may
result in the development of favourable
characteristics. Examples include the mutation to
form dark coloured peppered moths (this feature
enabled them to become camouflaged from
predators) and the sickle-cell gene in Africans,
which confers resistance to malaria in the
heterozygous form.

As a requirement of this topic, you are
expected to explain a modern example
of ‘natural’ selection.
The development of resistance to insecticides
among many species of insects and other
arthropods has been demonstrated to be a result of
natural selection at work. The mechanism for this
phenomenon initially involves the eradication of
the most susceptible members of the population
when sprayed with the insecticide, leaving behind
the more resistant individuals to produce offspring.
After a number of generations the incidence of
resistance increases, eventually resulting in a
situation where the insecticide is no longer of any
use.
Table 2-5 relates the development of DDT
resistance in insects to the different steps involved
in natural selection as proposed by Darwin and
Wallace in 1848.
Steps involved in
Natural Selection
Within a population
genetic variation exists
as a result of sexual
reproduction,
independent assortment
of chromosomes and
mutations
There is a constant
struggle for survival
among organisms, with
those best adapted to the
environment surviving
(‘survival of the fittest’)
The organisms that
survive will pass on
their favourable genetic
characteristics to their
offspring
Eventually a new
population emerges,
with one or more new
genetically determined
characteristics
predominating
Steps involved in the
development of DDT
resistance in insects
Insect populations often
include individuals
possessing a mutant
gene that produces
resistance to DDT
The DDT resistant
insects survive
The DDT resistant gene
is passed on to the
offspring of these
surviving insects
Eventually a new
population emerges in
which DDT resistance
predominates
Table 2-5 DDT resistance - a modern example of
natural selection
Punctuated equilibrium and
gradualism
Darwin’s concept of evolution was that it was a
very slow process and that changes in populations
occurred gradually over time. However, although
the fossil record shows that for most organisms
little change has occurred over millions of years,
new species seem to appear in the rock layers
within a relatively short space of geological time.
Moreover, for many species, few or no transitional
forms have been found at all.
In 1972, Niles Eldridge and Stephen Jay Gould
used their ‘Punctuated equilibrium’ model to
explain this phenomenon. Their theory was that
evolution through natural selection did occur, but
usually relatively rapidly and within small groups
of a population that had become isolated from the
rest of the species. Because these groups were
small and selection pressures were probably high
(perhaps because a drastic environmental change
had been what had caused them to move away in
the first place), natural selection and evolutionary
change occurred more rapidly. In these small areas
fossils of intermediate forms have in fact been
found for some species. The new species
eventually moved back into the original area, and
this appears as an apparent ‘gap’ in the fossil
record.
Apparent evidence for punctuated
equilibrium has been found for certain species,
including the marine microfossil Globorotalia
crassaformis. In a small region of the South Pacific
fossils have been found of this species, a
transitional species and the species Globorotalia
truncatulinoides. The change from the first to the
third species has been estimated to have taken
500,000 years, which is relatively short in
geological time. In nearby areas the only species
that has been found has been G.truncatulinoides.
This more recent species therefore seems to make
a ‘sudden’ appearance in the fossil record, but
according to Eldridge and Gould this is simply
because it has migrated here from the original
South Pacific region.

As a requirement of this topic, you need
to discuss the relative importance of the
work of Watson, Crick, Franklin and
Wilkins in determining the structure of
DNA, and the extent to which they
collaborated with each other .
The work of Watson and Crick was influenced by
the findings of Rosalind Franklin and Maurice
Wilkins, who worked on the structure of DNA at
King’s College, London. Together, they conducted
X-ray diffraction studies of crystallized DNA and
found that it is a long molecule of regularly
repeating units that are arranged in a helical shape.
James Watson and Francis Crick went on to
conclude that DNA is a double stranded helix,
consisting of alternating sugar and phosphate
groups which are linked by pairs of bases. They
also deduced that the two DNA strands are
complementary, that is, each chain contains a
sequence of bases that is complementary to the
other. Rosalind Franklin died at the age of 37,
missing out on the Nobel Prize her three other
colleagues received in 1962.

As a requirement of this topic, you are
expected to process information from
secondary sources to describe a
methodology used in cloning
The stages involved in the nuclear transfer cloning
method are shown in Fig. 2-15 below.
d)
a)
5. Current reproductive
technologies and genetic
engineering have the potential to
alter the path of evolution
Artificial insemination, artificial
pollination and cloning
Rather than waiting for natural selection to occur,
desirable crops and livestock are now produced
using ‘artificial selection’. Artificial insemination
involves inserting semen from selected male
livestock into a female animal. This allows farmers
to decide which characteristics will be inherited.
Artificial pollination involves dusting the pollen
from desirable plants over fertile stigmas, and is a
method of controlling the genetic composition of
offspring plants.
Cloning is a process which produces genetically
identical offspring to the parent. The process of
‘tissue culture’ involves cloning plants. Here, the
cells of the parent plant are cultured on a nutrient
medium to form a ‘callus’ of identical cells, and
new plants are grown from this tissue. Animal
cloning , still in the developmental stage, can
involve either ‘twinning’, in which an embryo is
split during its very early stages, or use of the
‘nuclear transfer’ method. Nuclear transfer
involves transferring the nucleus from a normal
body cell to an egg cell that has had its own
nucleus removed. The egg cell is then implanted
into another female.
The production of the cloned sheep,
‘Dolly’, in 1997 represented the first successful
attempt to clone a mammal from an adult cell.
Prior to this, frogs and mice had been cloned with
limited degrees of success.
c)
e)
b)
Fig. 2-15 The stages involved in the nuclear
transfer method of cloning;
a) nucleus is taken from an adult ewe udder
cell; b) UV radiation destroys nuclear material
in egg cell; c) nucleus is inserted into egg cell
using a micropipette; d) egg is implanted into
surrogate mother; e) adult sheep
THINK!!! Tissue culture is a more
recent cloning method used in plants. Other
cloning methods, however, have been used by
plant breeders for centuries. Can you name any of
these methods?
Transgenic species
A transgenic species is a plant, animal or
bacterium that has been genetically modified to
contain a gene from a different species. Areas in
which it has proved successful include
a) Genetically improving the characteristics of
plants and livestock- features such as resistance
to insects or disease have been incorporated into
some of these organisms by the insertion of genes
from other species. A gene from the bacterium
Bacillus thuringiensis, for example, has been
inserted into cotton plants to form a new variety
that is toxic to insect pests such as bollworm, but
not to humans or other animals.
b) Manufacturing pharmaceutical productstransgenic animals such as goats, pigs and rabbits,
certain transgenic plants and bacteria can be used
to manufacture haemoglobin, human protein C
anticoagulant, insulin, vaccines and growth
hormones, among other things. In animals these
products can often be harvested from their milk.
This process is known as ‘pharming’.
restriction enzymes; each particular enzyme will
only cut the DNA near a certain base sequence.
Other enzymes called DNA ligases are used to join
the ‘sticky’ ends of each piece of DNA together
when recombinant DNA is made.

As a requirement of this topic, you are
expected to debate the ethical issues
arising from the development and use
of transgenic species.
Some of these issues are listed in table 2-6, below.
c) Gene therapy- this involves altering the genetic
make-up of an individual in an attempt to cure
genetic diseases. Only the body cells are affected
in this process, so the altered genes are not
inherited. At present this technique is still in the
experimental stage.
d) Studying disease- transgenic animals are used
to study the effects of human diseases and to test
new forms of medication for these diseases. This
process involves inserting human genes into the
animals to produce the effects of the particular
disease studied.
Transgenesis was originally produced in mice in
the 1970s using DNA injection of mice ova. This
is still the most commonly used method and
involves introducing DNA from another organism
into a sperm nucleus which is then used to fertilise
an egg cell. Another method, known as ‘retrovirusmediated gene transfer’, involves using a carrier
such as a bacterial plasmid (a circle of DNA) or a
‘bacteriophage’ virus which is then introduced into
the host cells. The introduced gene can only be
passed on to offspring if it enters the host’s
gametes. Both of the above methods, shown in Fig.
2-16, have very low success rates but once
produced, transgenic embryos can be frozen and
stored for later use. The enzymes used to cut
required sections of DNA are known as
Reasons supporting
the use of transgenic
species
Bacteria containing the
human genes for the
production of insulin
and growth hormone
can be cultured and
these useful hormones
can be used to treat
people with diabetes or
growth deficiencies.
Plants can be produced
that are insect and
pesticide resistant, have
improved yields and
better nutritional
quality.
Reasons against the
use of transgenic
species
Transgenesis can be
seen as interfering with
nature and may result in
a decrease in
biodiversity.
Concerns have been
raised regarding
whether genetically
modified food is safe to
eat, whether it should
be labeled and whether
its nutritional value has
been affected.
Environmental concerns
include debate over
whether G.M.
organisms will affect
ecosystems ; genetically
modified Canola, for
instance, can
successfully hybridise
with related weeds and
so pass on its herbicide
resistance to these
weeds.
Table 2-6 The transgenic species debate
enzymes
cut desired
section of
DNA
Donor
cell
gene is introduced into a
sperm cell which then
fertilises an egg
gene is inserted
into bacterial
DNA
Fig. 2-16 The production of transgenic species
Bacterial DNA
is introduced
into host cells
or is cultured
itself to make
useful products.
The impact of genetic technology on
diversity
Useful websites to refer to in this
topic
Reproductive technologies tend to reduce genetic
diversity because the genes for favourable
characteristics are selected at the expense of genes
for unfavourable characteristics or those
considered unimportant. Genetic diversity among
members of a population is important because it
reduces the species’ risk of extinction. Examples
of crops in which genetic diversity has been
decreased are cotton and wheat. The Convention
on Biodiversity, established in 1992, has set in
place procedures aimed at preserving wild seed
varieties in seed banks to reduce the risk of crop
‘monocultures’ in the future.
Cloning animals such as sheep will further reduce
the gene pool because all the animals will be
genetically identical. In some situations, however,
cloning may in fact help to revive the gene pool:
re-creating extinct animals such as the Thylacine is
an example of this.
i) Evolution and the theory of natural selection:
anthro.palomar.edu/evolve/evolve_2.htm - 36k
ii) Mendel and genetics:
www.biopoint.com/engaging/MENDEL/MENDEL.H
TM
iii) The chromosome theory of inheritance:
www.emc.maricopa.edu/faculty/
farabee/BIOBK/BioBookgeninteract.html
iv) DNA and protein synthesis:
photoscience.la.asu.edu/photosyn/
courses/BIO_343/lecture/DNA-RNA.html
v) Mutations and evolution:
www.talkorigins.org/faqs/mutations.html
vi) Transgenic crops:
www.colostate.edu/programs/
lifesciences/TransgenicCrops/what.html
REVIEW QUESTIONS
4.
Three scientists involved in determining the
structure of DNA werea) Mendel, Wallace and Morgan
b) Sutton, Boveri and Beadle
c) Muller, Lyell and Eldridge
d) Franklin, Watson and Wilkins
5.
An example of convergent evolution
occurring would bea) The different species of finch in the
Galapagos islands which have adapted to
their own particular environmental
niches
b) The Thylacine of Tasmania and the wolf
of the Northern hemisphere which are
both adapted to a predatory lifestyle.
c) The development of DDT resistant
insects in farming areas
d) The rapid evolution of new species
without the appearance of transitional
forms
6.
A bull with a red coat was crossed with a
white-coated cow. All the offspring
possessed ‘roan’ coats, in which separate red
and white hairs were present. This is an
example ofa) sex linkage
b) incomplete dominance
c) co-dominance
d) mutation
7.
The scientists who developed the ‘One gene
one polypeptide’ theory werea) Watson and Crick
b) Darwin and Wallace
c) Sutton and Boveri
d) Beadle and Tatum
8.
The genetic code of a DNA molecule is
determined bya) The number and type of sugar groups in
the molecule
b) The sequence of phosphate groups in the
molecule
c) The number of base pairs per turn of the
double helix
d) The sequence of base pairs in the
molecule
9.
Which of the following techniques would be
the best method of determining the
evolutionary relationship between two
species?
a) MULTIPLE CHOICE
1.
Part of a DNA strand has a sequence in it
that reads AAGCTA. The corresponding
sequence in the messenger RNA will bea) TTCAT
b) CCTAGC
c) UUCGAU
d) AAGCTA
2.
Colour blindness is a sex linked trait. A
colour blind man marries a woman with
normal vision whose father was colour blind.
The chance of their producing a colour blind
daughter is:
a) 1 in 4
b) 1 in 2
c) 2 in 3
d) zero
3.
The pedigree below shows the inheritance of
a certain disease in humans. Affected
individuals are shaded.
I
II
III
Which of the following best describes the
gene for this disease?
a) non sex-linked and recessive
b) sex- linked and dominant
c) non sex -linked and dominant
d) sex-linked and recessive
a) Studying structural similarities between
the two organisms
b) Using DNA hybridisation techniques or
comparing blood proteins
c) Observing the fossil record and
identifying any transitional forms
between the two species
d) Attempting to produce offspring from a
mating of the two species
10)
15. The pedigree below shows the inheritance of
albinism (lack of pigmentation) in a family.
Affected individuals are shaded.
1
A section of a DNA molecule is 30 base
pairs long. This would code for:
A) 6 amino acids
B) 15 amino acids
C) 20 amino acids
D) 10 amino acids
3
b) SHORT ANSWER AND
LONGER RESPONSE QUESTIONS
11.
12.
13.
14.
Give an example of natural selection in a
population being brought about by the
following types of environmental changes.
a) physical
b) chemical
c) competition for resources
Explain the meaning of the following terms:
a) convergent evolution
b) divergent evolution
c) homologous structures
d) transitional forms
In humans the gene for brown eyes (B) is
dominant to the gene for blue eyes (b).
a) What is the genotype of a woman with
blue eyes?
b) The woman’s husband is heterozygous
for eye colour. What is his genotype?
c) Write down the possible gametes
produced by the woman’s husband.
d) Use a Punnett square or other means to
determine the percentage of their
children that will have blue eyes.
Give two reasons why Mendel’s work was
not recognised until the early 20th century,
despite the fact that he published his paper in
1866.
2
4
5
6
7
8
a) Is the gene for albinism dominant or
recessive? Use the pedigree to explain your
answer.
b) Determine the genotypes for individuals
1, 2, 3, 5 and 6
c) What possible genotypes could individual
8 be?
16.
Write in the missing nitrogen bases on the
section of DNA shown below, and label a)
and b) as either a sugar or phosphate
molecule.
A = adenine
C =cytosine
T = thymine
G = guanine
C
a)
A
b)
G
T
17.
Explain how the following enable genetic
variation to occur within a population.
a) Independent assortment during gamete
formation:
b) Crossing over during gamete formation:
c) The random union of gametes in sexual
reproduction:
18.
19.
20.
a) During the first stage of polypeptide
synthesis, the DNA molecule unzips.
The code of bases on this single strand is
then copied in the process of
‘transcription’. Name the molecule that
does this copying.
b) To where is this transcribed code now
carried?
c) What do transfer RNA molecules carry
and how many bases are involved?
d) Briefly describe how transfer RNA and
messenger RNA are involved in the
formation of polypeptides at the
ribosome.
a) What is a mutation?
b) Explain why a mutation can result in
serious consequences for cellular
metabolism.
c) Explain the difference between a
substitution mutation and an insertion
mutation.
a) Describe what is meant by a ‘transgenic’
organism.
b) Briefly outline the process involved in
the production of transgenic organisms.
c) Name three organisms that have been
genetically ‘improved’ using transgenic
techniques. For each of these new
species, include the name of the
organism that has ‘donated’ some of its
DNA to them.
had been recessive, all their offspring
would have the disease. The disease is
not sex-linked because no males with the
dominant gene could possibly be
produced from the parents in generation
I if their genotypes were XbXb and XBY.
4.
d)
5.
b); Convergent evolution occurs when
organisms that are completely unrelated
evolve in a similar way because they
occupy similar environmental niches.
6.
7.
c)
d)
8.
d)
9.
b); Comparing biochemical characteristics
is a more accurate way of determining
evolutionary relationships because we
are effectively comparing the DNA of
each organism.
10.
d); Each ‘triplet’ of bases on a tRNA
molecule is attached to one amino acid.
b) SHORT ANSWER AND
LONGER RESPONSE
QUESTIONS
11.
a) The peppered moth population changed
in appearance because of an alteration in
its physical environment; the polluted
surroundings favoured the survival of
dark coloured moths.
b) The action of DDT on mosquitoes; this
selected for resistant individuals.
c) Giraffes with longer necks were thought
to have competed more successfully for
food than those with shorter necks.
12.
a)
ANSWERS
a) MULTIPLE CHOICE
1.
2.
3.
c); Note that in RNA uracil replaces
thymine, so that whenever adenine
appears in the DNA it will combine with
uracil in the corresponding RNA.
a); The cross involved is
XcY x XCXc
↓
XCXc, XcXc , XCY, XcY
(1 normal female, 1 colour blind female, 1
normal male, 1 colour blind male)
c); The disease must be dominant, because if
the two affected parents in generation II
Convergent evolution occurs when
different species develop similar
adaptations in similar environmental
niches.
b) Divergent evolution occurs when
members of a species develop different
adaptations in different environments.
c) Homologous structures are similar
structures found within groups of related
organisms; e.g. the pentadactyl limb.
d) Transitional forms are organisms
displaying characteristics which are
transitional between consecutive species
in the fossil record.
18.
a) Messenger RNA
b) It is carried to a ribosome in the
cytoplasm
c) Transfer RNA molecules carry amino
acids, each coded for by three bases.
d) Transfer RNA carries amino acids to the
mRNA. Each amino acid matches up
with a codon (base triplet) on the
messenger RNA and attaches to the
mRNA at this position. Eventually a
chain of amino acids forms, each amino
acid being joined to the next by a
peptide bond.
19.
a) A mutation is a change that occurs in a
gene.
b) This is because genes are responsible for
manufacturing proteins, which include
the enzymes controlling metabolism.
c) A substitution mutation involves the
substitution of one base for another,
whereas an insertion mutation involves
the addition of an extra base to the DNA
sequence.
20.
a)
13. a) bb
b) Bb
c) B, b
d) Bb x bb
↓
Bb, Bb, bb, bb;
so 50% have blue eyes
14.
i)
He was not a high profile member of the
scientific community
ii) Many of the scientists who read the
paper did not understand it or realise its
significance.
15. a) Recessive, because individuals 5 and 6
have produced both dominant and
recessive offspring, so the unaffected
condition must be dominant.
b) 1= aa; 2= Aa; 3= aa; 5= Aa; 6= Aa
c) Individual 8 could be AA or Aa
16.
A = adenine
C =cytosine
T = thymine
G = guanine
G
C
a)
b)
sugar
A
T
b)
C
G
phosphate
T
A
17. a) The fact that each member of a
chromosome pair separates independently
of each member of another pair enables a
greater number of allele combinations in
the gametes: 2n, in fact, where n is the
haploid chromosome number.
b) This enables the ‘unlinking’ of linked
genes to occur, thus increasing the number
of allele combinations in the gametes.
c) This ensures that offspring are never
genetically identical to their parents and
makes possible a large range of gene
combinations in the offspring.
c)
ii)
iii)
A transgenic organism is a plant, animal
or bacterium that has been genetically
modified to contain a gene from a
different species.
The required section of DNA is cut from
a donor cell’s DNA with restriction
enzymes. At the same time a bacterial
plasmid is split with restriction enzymes
→ The ‘sticky’ ends of the two pieces of
DNA join together, with the help of
DNA ligase, to form a recombinant
bacterial plasmid. →The recombinant
bacterial plasmid is introduced into host
cells using microinjection techniques or
particle guns, or the DNA is inserted
into bacterial cells which are themselves
cultured to produce useful products.
i) Genetically modified (Bt) cotton:
Contains a gene from a bacterium that
makes it insect resistant.
Genetically modified sheep: Contain a
gene from a plant that produces
resistance to blowflies.
Genetically modified tomatoes: Contain
a fish gene that improves the red
colouring of their skin.