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GENETICS
study of the mechanism of inheritance
and variation of traits or characteristics
as transmitted from one generation of plants or
animals to another
Contents
Terms
DNA
Homo- & Heterozygous
DNA Structure
DNA Profile
Genetic Screening
DNA Replication
Protein Synthesis
RNA Vs DNA
Dominant
Recessive
Genotype & Phenotype
Phenotype & Environment
Solving Genetic Questions
Punnett Square
Incomplete Dominance
Sex determination
Variations
 Sexual reproduction
 Gene mutation
 Chromosome mutation
 Genetic engineering 2
Species
Is a group of animals or plants that can
interbreed and produce viable, fertile
offspring
- members of a species share the same
characteristics and differ only in minor
details.
3
Heredity
Is the study of the natural law or property of
organisms whereby their offspring have
various physical and mental traits of their
parents or ancestors i.e. certain traits are
transmitted from one generation to the next.
Genetic information is carried on the DNA
molecule as a gene.
4
Gene
Is the unit of heredity found on a
chromosome, and is an instruction (code) to
the cell to make a particular substance,
which helps regulate a trait of an organism.
e.g. the gene for tongue-rolling in humans.
There are two possible genes you can have.
One gives you the ability to roll your tongue.
The other does not give you this ability.
These different forms of the same gene are
5
called alleles.
Alleles
Are alternative forms of a gene or a pair of
genes found at the same locus / position on
homologous chromosomes controlling the
same trait.
6
Gene expression
Possessing a gene does not mean it will be
used.
e.g. in humans the gene for growth hormone is
expressed at different times in your life.
Other factors come into play which will
determine which genes the organism will
use.
When a gene is used it leads to gene
expression which is
- the process of changing the information in a
gene into a protein and the effect that
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protein has on the organism.
DNA (Deoxyribonucleic Acid)
substance found in the cell nucleii in strands
with proteins attached called chromosomes
Chromosome composed of many genes (100s).
A set of chromosomes contains all the genes
needed by an organism to live.
Cells usually have two sets of chromosomes
i.e. they are diploid (2n) and have two genes
for every trait.
These alleles interact to produce the traits in
the organism.
8
Process for gene transmission
In sexual reproduction the offspring get
genetic information from each parent.
Parents produce gametes (sperm and eggs)
which contain one copy of each
chromosome (=> one gene for each trait).
Gametes are haploid (n).
When fertilisation occurs the resultant cell
(zygote) has two copies of each gene.
This process prevents doubling the amount of
DNA at each new generation.
9
Homozygous & Heterozygous
The copies of the gene can be the same or
different.
homozygous: possessing a pair of similar
genes for a trait e.g. TT or tt.
heterozygous: possessing a pair of dissimilar
genes for a trait e.g. Tt i.e. the dominant and
recessive genes.
10
The life cycle
of a human
11
DNA structure
DNA is a long, coiled molecule called a
double helix – like a twisted ladder.
Composed of two strands of sugars and
phosphates – the uprights of the ladder.
Strands are linked by bases – form the rungs.
Four different bases – Adenine (A), Guanine
(G), Cytosine (C) and Thymine (T).
Base pairing rule A always with T, and
C always with G.
Bases can be found in any order along DNA
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strand – but …
Exons
the order of bases is unique for each DNA
molecule.
The order codes for the proteins made by the
cell.
Each message for a particular protein is called
a gene.
The parts of the DNA that code for proteins
are called exons.
13
Interons
Not all of the DNA carries messages.
The majority of it does not code for proteins
and just separates the genes.
These non-coding pieces are called junk genes
or interons and are highly variable.
Found within or between two genes.
These variable parts of DNA are used when
taking a DNA profile (fingerprint).
14
DNA profile – procedure
(1/3)
Take a sample of material containing cells e.g.
blood or semen.
Extract the DNA from the cell by breaking up
the cell membrane.
The DNA is then treated with special enzymes.
These recognise specific sequences of bases,
usually in the junk genes, and cut the DNA
at those sites.
This produces fragments of DNA of various
different lengths.
15
DNA profile – procedure
(2/3)
It is almost impossible for two members of a
species to produce the exact same DNA
fragments (unless they are identical twins).
The fragments are placed at one end of a gel.
An electric charge is passed through the gel
and the fragments move down the gel.
The smallest pieces move fastest.
The DNA is then transferred onto a nylon
membrane for ease of use.
16
DNA profile – procedure
(3/3)
Radioactive DNA probes are put onto the
membrane.
These attach to the fragments.
The membrane is then put in contact with Xray film and the distance travelled by the
fragments can be seen.
The result is a series of bands similar to bar
codes.
17
DNA profile – use
Can be used for a number of purposes:
 Identify criminals – from blood, semen or
other tissue left at the scene of a crime.
 Identify fathers in paternity cases – the old
method of using blood types only proved
that a man was not the father.
18
The
production
of a DNA
profile
19
Genetic screening
(1/3)
This is the use of DNA profiling to identify
harmful genes possessed by an individual.
A couple’s DNA can be searched to see if
they are carriers of a particular gene e.g.
cystic fibrosis.
If a couple know they are carriers, and there is
a high probability of having a child with a
genetic disorder, then they can decide
whether or not to have a family.
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Genetic screening
(2/3)
There are no cures for genetic disorders.
Treatments are available to reduce their
effects e.g.
with haemochromatosis a person accumulates
a dangerous level of iron in the body.
The damage due to this can be prevented by
removing blood on a regular basis from the
sufferer.
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Genetic screening
(3/3)
Ethical problems
 should people be tested for genetic disorders?
 Should they be told the results if there is nothing
the medical profession can do for them?
Consequences for insurance business
 Could you be refused to be insured if you have a
genetic disorder?
 Would people refuse to employ you?
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 Would you be refused a house loan?
DNA Replication
(1/3)
DNA is vital for a cell to survive.
When a cell divides it is essential that an exact
copy of the DNA is passed on to the new
cells.
This ensures that the new cell can produce the
same substances and perform the same
functions as the original cell.
DNA has the ability to make an exact copy of
itself.
23
DNA Replication
(2/3)
Enzymes control the process of DNA
replication.
They pull apart or unzip the two strands of
DNA.
They then match the exposed bases with their
partners using the base pairing rule i.e. A
with T and C with G.
The matchhing bases are taken from the pool
of free bases, with attached sugar and
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phosphate, surrounding the DNA.
DNA Replication
(3/3)
This process results in two identical strands of
DNA produced from the original strand.
One side of each new DNA molecule comes
from the original and one is new.
The order of the bases on the new molecules
is identical to the original.
As a result the genes on each chromosome are
the same.
25
The
process of
DNA
replication
26
Functions of DNA
Replication is one function of DNA .
Converting its coded message into proteins is
another.
The proteins then control the activities of the
cell.
Proteins are made from amino acids.
There are about 20 different amino acids that
make up most proteins.
The number and order of amino acids
determines the type of protein that is made. 27
Protein synthesis – DNA codes
DNA has a code that determines the order of
amino acids in a protein.
The code is made up of groups of three bases.
Each group codes for a specific amino acid
which will be placed in that specific
position.
There are more codes than amino acids =>
some amino acids have more than one code
e.g. GCA and GGG code for the same
amino acid.
28
Protein synthesis – how it works
1.
2.
3.
4.
The piece of DNA which codes for a
protein is rewritten – transcribed into a
new molecule called messenger RNA
(mRNA) – see slide 31.
The RNA leaves the nucleus and travels to
the ribosome.
Here the message is translated and amino
acids are assembled in the correct
sequence in a long chain to make the
protein.
The chain will fold into the threedimensional shape of the protein – which 29
will then carry out its function – slide 32.
RNA Vs DNA
RNA
 Contains U (uracil)
 Contains ribose
 Single strand
 Nucleus &
cytoplasm
DNA
 Contains T
 Contains
deoxyribose
 Double strand
 Nucleus only
RNA is complimentary to DNA, e.g. if the
order of bases in the DNA is
GGCCAATT then in the RNA it is
CCGGUUAA.
30
The production of mRNA from DNA
Back to slide 29
31
The production of a protein from DNA
Back to slide 29
32
Dominant
(1/2)
A human has two genes for each trait (slide 11)
– one from each parent.
They may be the same or different (slide 12).
e.g. tongue-rolling in humans
There are two types of allele (gene) – one
allows you roll tour tongue (R), the other does
not (r).
If you are homozygous for the trait (RR or rr)
then you can (RR) or cannot (rr) roll your
tongue. There is no in between.
33
Dominant
(2/2)
If you are heterozygous for the trait (Rr) i.e.
you have one of each type of allele then
only one of the two traits is expressed
– the same trait is always expressed
In this case you can roll your tongue.
The allele for tongue rolling is dominant.
Dominant alleles are the ones that are
expressed in the heterozygous condition.
34
Recessive
This refers to alleles that can only be expressed
when they are in the homozygous condition
e.g. rr = tongue non-roller; r is recessive.
35
Genotype and Phenotype
genotype: genetic makeup of an individual or
the genes that they inherit e.g. RR, Rr or rr
– three types.
phenotype: physical appearance of an
individual as a result of the interaction of
the genotype with the environment e.g. can
or cannot roll the tongue – two types.
Genotype and phenotype are not always the
same.
36
The possible genotypes and
phenotypes for the tongue rolling
allele
GENOTYPE
PHENOTYPE
RR
Ability to roll tongue
Rr
rr
Inability to roll tongue
37
Phenotype and environment
Having the alleles is no guarantee of their
expression e.g.
you may inherit a genotype that will make you
two meters tall, but if you do not get enough
food then you cannot grow to this size.
The genotype will not be expressed.
The genotype (nature) and the environment
(nurture) are both important.
38
Solving genetic questions
Question: If a man homozygous for tongue
rolling (RR) reproduces with a woman
homozygous for non-tongue rolling (rr),
what type of tongue rolling ability will
their children have?
To answer this we must examine:
1. the possible gametes that can be produced.
2. the possible combinations of these
gametes at fertilisation, e.g.
39
Homozygous roller x non-roller
40
Non-diagramatic representation
Parents
Male roller homozygous
Female nonroller
Parental
genotype
RR
rr
gametes
All R
All r
F1 genotype
All Rr
F1 phenotype
All rollers
41
A cross between two
heterogygous parents
42
Heterozygous rollers
Parents
Male roller homozygous
Female roller homozygous
Parental
genotype
Rr
Rr
gametes
R
r
R
r
F1 genotype
RR
Rr
Rr
rr
Roller
Non43
roller
F1 phenotype
Roller
Roller
Punnett square
When the possible gametes of the parents are
known they are placed along the side of a
grid.
Then the possible combinations at fertilisation
are worked out and placed in the remaining
squares.
Using the previous example:
Parents
Rr x Rr
Gametes R, r x R, r
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Now using the Punnett square
Use of Punnett square
Male
Female
Possible
gametes
R
r
R
RR
Rr
r
Rr
rr
F1 genotypes
RR
F1 phenotypes
Rr
Possible genotypes
of offspring
Rr
rr
Roller Roller Roller Non-roller
Next: A problem to solve
45
Parents
Yellow
Yellow
Parental
genotype
Yy
Yy
gametes
Y
y
Y
y
F1 genotype
YY
Yy
Yy
yy
F1 phenotype
Yellow
White
Go back to Slide 49 Back Back again And again Last time
46
Phenotypic ratio
F1 genotype
YY
Yy
Yy
Yy
F1 phenotype
Yellow
Yellow
Yellow
White
Expected
Phenotypic
ratio
3
1
Actual results
294
89
Actual ratio
3.3
1
47
Go to Slide 49
A problem to solve
(1/2)
In a species of plant, yellow flower colour is
dominant to/over white. Two yellow
flowers were crossed and their seeds
produced 294 yellow flowers and 89 white
flowers. Explain.
Answer:
Yellow – dominant =
Y
Go to Slide 46
White – recessive
=
y
Remember each parent must have two alleles.
Both parents are yellow ⁂ both contain Y 48
A problem to solve
(2/2)
What is the other allele – Y or y?
Look at the offspring some are white ⁂ must
have a genotype yy and (Slide 46)
must have gotten a y allele from each parent
(Slide 46)
⁂ both parents have genotype Yy (Slide 46).
Finish the cross (Slide 46) and
examine the phenotypic ratio. (Go to Slide 48)
What about the number of offspring?
Can you accept this as a 3:1 ratio?
49
Solution Template
Parental Phenotype
Parental Genotype
Gametes
F1 Genotype
F1 Phenotype
Depending on Question asked: Ratio / F2
50
Incomplete dominance or
Co-dominance
In snapdragons
this is seen
when neither
allele for colour
is dominant,
and both are
expressed in
the
heterozygous
condition.
51
Incomplete dominance
In short-horn cattle the heterozygous roan
colour results from a lack af dominance
between red and white coat colours.
e.g. if a red bull mates with a white cow, what
results will be produced?
Allele for White = CW
Allele for Red = CR
Genotype of cow = CW CW
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Genotype of bull = CR CR
Red bull X White cow
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White cow X Roan bull
– do yourself
54
Sex determination
Humans have 23 pairs of chromosomes i.e.
22 pairs of autosomes and a pair of sex
chromosomes.
Sex chromosomes contain the genes that
determine the sex of an individual.
Male and female sex chromosomes are
different.
 Females have two the same size called X
chromosomes.
 Males have one X chromosome and a much
55
smaller Y chromosome.
Types of gametes produced
All females gametes (eggs) contain an X
chromosome.
Male gametes (sperm) will be of two types.
Half the sperm will contain an X chromosome
and half will contain a Y chromosome.
56
How sex is determined
Everyone receives an X chromosome from
their mother.
The other chromosome (X or Y) is received
from the father’s sperm.
The father determines the sex of the child.
XX = female
XY = male
There is a 50:50 chance that the child will be
57
male (or female).
Sex determination in humans
58
Exceptions
Bird, butterfly & moth
Females – XY
Males – XX
59
Variation
No two members of a species are identical.
Variations are due to environmental and
genetic causes.
Genetic variations are the important ones for
evolution.
Can you explain why?
60
Causes of genetic variation
1.
Sexual reproduction
2.
3.
4.
Gene mutation
Chromosome mutation
Genetic engineering
61
1. Sexual reproduction
Offspring get one set of chromosomes from each
parent.
⁂ they can have a different combination of genes
than either parent
and will be different from both of them.
Meiosis allows genes to be reshuffled e.g. in a cell
where 2n = 6, homologous chromosomes are not
identical but do carry genes for the same trait
(red & white flower).
A gamete contains a copy of each chromosome.62
The possible ways three pairs of
chromosomes can be combined
Gametes get one from each parent.
Possible combinations are: r
A
R
a d
D
R
A
D
r
A
D
R
a
D
r
a
D
R
a
d
r
A
d
R
A
d
r
a
d
63
There are eight ways of combining three pairs
of homologous chromosomes i.e. 23 = 8.
Humans have 23 pairs of chromosomes so the
possible number of gametes from one
parent is
223 = 8,388,608 (8.4 million)
The other parent is capable of producing a
similar number of gametes also.
The total possible number of offspring is ⁂
70,368,744,177,664 (= 7 x 1013)
all slightly different from each other i.e. all
64
variations Back to causes of genetic variation
2. Gene mutation
A mutation is a spontaneous change in the
sequence of nitrogenous bases in a gene or
chromosome.
Genes contain the codes that are responsible
for making proteins (e.g. enzymes).
If this code is altered then different protein
may be produced which may not work in
the same manner as the normal protein and
may have serious consequences.
Mutations may give rise to variations.
65
They are permanent changes in the genes or
chromosomes.
If they occur in gamete producing cells the
changes can be passed on to the next
generation.
If they occur in somatic cells (non-gamete
producing cells i.e. body cells) they cannot
be passed on.
Mutations are rare, occur randomly and are
usually harmful.
Sickle-cell anaemia and cystic fibrosis result
66
from gene mutations.
Cystic fibrosis
(1/4)
This is one of the commonest genetic disorder
diseases in children.
It is caused by a disorder in a gene on
chromosome seven, and is a recessive
condition, so both parents may be carriers
without having the disease.
The correct gene codes for a protein found in
the cell membrane that controls the flow of
chloride ions into and out of the cell.
67
Cystic fibrosis
(2/4)
The mutated protein does not allow the
correct flow of chloride ions.
This results in a disturbance of the mucussecreting glands of the lung, pancreas,
mouth, skin and gastrointestinal tract.
Symptoms include failure to gain weight, with
frequent chest infections and loose, pale
stools.
Sodium chloride concentration is increased in
sweat.
68
Cystic fibrosis
(3/4)
It is possible that in the future cystic fibrosis
screening may become a routine antenatal
investigation.
At the moment, genetic counselling is offered
to those whose children might be affected.
Treatment consists of taking pancreatic
enzymes, as well as preventing and treating
respiratory infections.
69
Cystic fibrosis
(4/4)
Heart and lung transplants, as well as genetic
manipulation, may provide the answer for
future sufferers.
It is possible that in the future cystic fibrosis
screening may become a routine antenatal
investigation.
At the moment, genetic counselling is offered
to those whose children might be affected.
70
The inheritance of a genetic
disorder e.g. cystic fibrosis
The mutation is recessive i.e. you need to
inherit it from both parents.
A person who possesses one gene but has no
symptoms for the disorder is called a carrier.
If both parents are carriers there is a one in
four chance that any child they may have
will get the disorder.
71
The inheritance of a genetic
disorder
Back to causes of
genetic variation
72
3. Chromosome mutation
Arise as a result of
 a change in an individual chromosome or
 a change in the number of chromosomes
present e.g. Down’s syndrome.
These children have almond-shaped eyes and
a round face,
are usually mentally disabled
and often have congenital heart defects.
The effected children have an extra copy of
chromosome number 21.
73
The chromosome mutation
leading to Down’s syndrome
74
How it happens
The gamete producing cells divided by
meiosis, but the chromosomes (No.21)
failed to separate (segregate).
As a result two of the gametes had no copy of
chromosome no. 21, and two gametes had
two copies of it.
When this second type of gamete is fertilised
by a normal gamete the result is Down’s
syndrome.
The incidence of Down’s syndrome increases
with increasing age of the parents.
75
Causes of mutations
Agents such as X-rays,
gamma radiation, and
chemicals such as cigarette smoke,
formaldehyde and
mercury
cause mutations.
Most mutagenic agents are carcinogens and
vice versa.
76
Examples
Chernobyl – children born with birth defects
– parent’s gonads irradiated by nuclear
material in the environment.
Incidence of childhood thyroid cancer has
increased by 1000 times.
Animals also affected – produce young with
birth defects.
Japan - children born with birth defects parents eating shellfish contaminated with
mercury.
77
Back to causes of genetic variation
4. Genetic engineering
modern techniques or processes used to
artificially alter the genetic information in the
chromosome of an organism.
Plant breeders and farmers have used selective
breeding for generations to produce new
varieties of plants and variations in animals.
Now it is possible to take genes from one
species and insert them into another species.
Look at how human growth hormone / insulin is
78
produced by bacteria.
How is it done?
The gene for growth hormone is identified and isolated
on a human chromosome.
A copy of this gene is cut (restriction) from human cells.
Gene is then inserted into a bacterial plasmid (small
circular piece of DNA in bacteria) – transformation
(ligation) – introduction of base sequence changes.
Plasmid is then inserted into a bacterial cell.
The transformed (transgenic) bacteria make many copies
of the plasmid.
These bacterial cells then express the human gene and
make the human growth hormone.
79
The production
of transgenic
bacteria that
produce human
growth hormone
80
Examples of genetic engineering
Tomato plants – gene for producing the
enzyme needed to soften the fruit on
ripening has been altered and no longer
functions – fruit remains hard – easier to
harvest – used to make tomato ketchup.
Sheep – have been given the human gene for
factor VIII – needed for blood clotting –
haemophiliacs are missing this gene – it is
hoped that the factor VIII will be able to be
81
extracted from the sheep’s milk
END
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