Download Unit III: GENETICS

Unit III: GENETICS
Classical (Mendelian)
Genetics
Classical Genetics
 Genetics:
This is the branch of
biology that studies the ways in which
hereditary information is passed on
from parents to offspring. It includes
principles in variation and inheritance.
 Heredity:
this is the passing on of
trait from parents to offspring.
The Blend Theory of Inheritance
 This
is the early belief (before
Mendel) that factors from the parents
were blended in their offspring.
 This theory was not able to explain
the appearance or disappearance of
distinct traits (dominant or recessive)
from one generation to another.
Gregor Mendel and
the Scientific Process
 Gregor
Mendel used the scientific
process to lay the foundation for the
science of genetics.
 Mendel used garden peas.
 They were a good choice because
they were easy to grow, matured
quickly and showed distinct traits.
Mendel studied seven pairs of
contrasting traits as follows:
Trait
Dominant
Recessive
Seed Shape
Round
Wrinkled
Seed Color
yellow
Green
Seed Coat
Color
Pod Color
Grayish brown
White
Green
Yellow
Pod Shape
Inflated
Wrinkled
Stem Length
Long
Short
Flower Position
Lateral
Terminal
Mendel’s Pea Plants
 Pea
plants normally self pollinated
because they contain the stigma and
anther enclosed in the petals.
 Mendel could easily cross fertilize by
removing the stamens before they
cross fertilize and dust the pollen from
another anther onto the stigma.
Cross Pollination
 Mendel
cross pollinated pure plants
(purebreds) with contrasting traits.
 These pure plants were called the
parent or P generation.
 In the offspring, Mendel observed that
one trait would completely disappear.
 For example, in a cross between tall
and short plants, only tall plants would
be present.
 The
offspring are considered to be the
first filial or F1 generation.
 The offspring of purebreds are known as
are known as hybrids.
 Mendel allowed the hybrids of the F1
generation to self pollinate.
 This resulted in the F2 generation. In this
case, ¾ of the offspring were tall and ¼
were short.
 This showed that the factor that contained
the shortness had not disappeared.
 The
“factors” described by Mendel were
later called genes.
 The
traits expressed in the F1 generation
are considered to be dominant because
they mask another trait.
 The
hidden traits that appeared in the F2
generation were considered to be
recessive.
Unit Theory of Inheritance
 Mendel
reasoned that for every trait there
must be two governing factors (unit
characters).
 One of these factors came from the
mother and the other factor came form the
father during zygote formation.
 These factors are separated during
gamete formation.
 This lead to the following of Mendel’s
Laws:
Law of Dominance
 Law
of Dominance: When an
organism is hybrid for a pair of
contrasting traits, only the dominant
trait can be seen in the hybrid. A
dominant trait is indicated by an
upper case letter (ie. R) whereas a
recessive trait is indicated by a lower
case letter (ie. r)
Law of Segregation
 Law
of Segregation: The idea that
“factors” (genes) occur in pairs (alleles)
and are separated from each other in
gamete formation and recombined in
fertilization.
 For
more on Mendel and his experiments
go to:

http://anthro.palomar.edu/mendel/mendel_1.htm
Law of Independent
Assortment
Law of Independent Assortment: genes for
different trait are separated and distributed to
gametes independently from each other.

This law is not considered true today
because many genes are found on homologous
chromosomes and can not be separated.
Genes found on the same chromosome are said
to be linked.
 http://www.sumanasinc.com/webcontent/
anisamples/majorsbiology/independentass
ortment.html

Terms
Trait:
a characteristic determined
by a gene ie. eye color.
Gene: a distinct unit of hereditary
material found in chromosomes.
Allele: Two or more alternate
forms of a gene for a trait.
Homozygous: A genotype in which both
genes of a pair are identical.
 Heterozygous:a genotype in which the gene
pairs are different.
 Co-dominant alleles: Alleles that are
simultaneously expressed in the
heterozygous condition (AB blood type). Can
be symbolized by upper case letters.
 Incomplete dominant alleles: Alleles that
are simultaneously expressed in an altered
form – snap dragons hybrids are pink not
white or red.


Genotype: The genetic make-up of an
organism.

Phenotype: The physical traits an organism
develops as a result of its genotype.

Punnett Square: a chart used by geneticists to
show the possible combinations of alleles in an
offspring.
P E Question
 Which
are alternate forms of genes?
 (A) alleles
 (B) chromatids
 (C) hybrids
 (D) sub units
P E Question
Which refers to the physical appearance of
an organism?
 (A) genetic drift
 (B) genetic heritage
 (C) genotype
 (D) phenotype

P E Question
How did Mendel obtain the F1 generation
of all hybrid tall pea plants?
 (A) hybrid tall × hybrid tall
 (B) hybrid tall × pure tall
 (C) pure short × pure short
 (D) pure tall × pure short

P E Question
See . 531… The Product Rule
 What is the probability of parents having
the following three children: two boys in a
row followed by a daughter?
 (A) 1/16
 (B) 1/8
 (C) 1/4
 (D) 1/2

Crosses
Monohybrid Crosses
 This is a chart to show the results of a cross
between parents for a single trait. (2 x2)
 Dihybrid Crosses
 This is a chart to show the results of a cross
between parents for two traits. (4 x4)

Homozygous Male with
Homozygous Female
 A yellow
round seed male is crosses
with a green wrinkled seed female.
 Yellow- Round seed Male (YYRR)
 Green – Wrinkled seed female (yyrr)
Test Cross
 A test
cross is used to determine an
unknown genotype that may either be
homozygous dominant or heterozygous for
a trait.
 A individual of the unknown genotype is
mated with an individual showing the
contrasting recessive trait.
Example of a Test Cross:

A breeder wishes to know if a pea plant is TT or
Tt. He will cross the plant with a homozygous
recessive plant (tt).

If the results show all tall plants than the
unknown plant must be TT. If 50% are short,
then the unknown plant must be Tt. Why?
Attempt the crosses in the space below.
Co-dominance
The condition in which both alleles of a gene are
expressed. (ie. AB blood type)
 Example: A cross between homozygous red
shorthorn cattle and homozygous white
shorthorn cattle result in heterozygous offspring
with a roan coat. Roan is a mixture of red and
white hairs
CR CR x CW CW  CR CW

Incomplete Dominance
 Both
alleles contribute to the
phenotype of a heterozygous
individual to produce a trait which is
not exactly like either parent.
 Example: Red flowers (RR) combine
with white flowers (RR) to produce
pink flowers (RR).
P E Question
In horses, roan coats (red and white
hairs) result from codominance. If two
roan coat horses are crossed, what would
be the expected phenotype ratios?
 (A) all roan
 (B) ½rroan, ¼ red, ¼ white
 (C) ½ roan, ½ red
 (D) ½rroan, ½ white

P E Question
Which type of inheritance causes variation
in height and skin colour in humans?
 (A) co-dominance
 (B) incomplete dominance
 (C) multiple alleles
 (D) polygenic inheritance

Polygenic Inheritance (Multiple
Gene Inheritance)
The traits studied by Mendel are controlled by
the alleles of a single gene. Many traits of both
plants and animals do not appear in just two
contrasting forms. For example, humans are not
just tall or short. Traits that vary between two
extremes are controlled by the alleles of two or
more different genes.
 When two or more independent genes affect a
characteristic, it is called polygenic or multiplegene inheritance. Examples of traits controlled
by polygenic inheritance include: eye color, skin
color, height, facial features.

P E Question
Eye colour for fruit flies is determined by
genes X, Y and Z. Which is illustrated if X
produces black eyes, Y produces brown
eyes and Z produces magenta (purple)
eyes?
 (A) co-dominance
 (B) incomplete dominance
 (C) multiple alleles
 (D) polygenic inheritance

Multiple Alleles

In this case, there are more than two
alleles for a particular trait. In humans,
there a three alleles that control blood
type: A, B and O. A and B are equally
dominant. O is recessive.
Genotype Blood Type
A
A
I I
A
I i
A
IB IB or IBi
B
A
B
I I
AB
ii
O
or
Example of Multiple alleles
 A woman
with type A blood marries a
man who has type B blood. They
have five children, with type AB blood.
What are the most probable parental
genotypes?
P E Question

Which are possible blood types for the
parents of a child with AB blood?
Walter Sutton and Theodore Boveri
1902 - studied phases of meiosis and
realized that the behaviour of chromosomes
during meiosis was similar to the behaviour of
Mendel’s “factors.”
 Noticed that chromosomes occur in pairs and
that these separate during anaphase 1 of
meiosis.
 Also noticed that chromosomes align
independently along the cell’s equator.
 Result is that each gamete receives one
chromosome of each pair and that this one
chromosome does not influence which other
chromosomes will be found in that gamete.

Chromosome Theory of Inheritance
These observations formed the basis for the
chromosome theory of inheritance. It states :
 1. Mendel’s factors , or genes are carried on
chromosomes.
 2. It is the segregation and independent
assortment of chromosomes during meiosis that
accounts for the patterns of inheritance.
 * The chromosome theory can also account for
patterns of inheritance that do not follow
Mendel’s laws.

Thomas Morgan ( 1910)
 Investigated
eye colour in fruit flies and
produced a white- eyed male by crossing
two red-eyed parents.
 This was similar to other monohybrid
crosses.
 However , when he crossed a red eyed
female offspring of the white eyed male
with a normal red eyed male the results
were :


All females - red eyes
Males - half had red eyes and half had white eyes.
Morgan …




Morgan deduced that the gene for eye colour was
located on the X chromosome. This was the first
time that a gene had been linked to a specific
chromosome.
He also found that some genes do not follow the law
of independent assortment because they tend to be
inherited together.
For example : genes on the same chromosome
cannot be separated. They are called linked genes.
However , later in his studies he found that
sometimes linked genes do separate.
How can this happen ?
Gene-chromosome theory :
 1. Genes exist at specific sites arranged in
linear fashion along chromosomes.
 2. When pairs of homologous chromosomes
separate during gamete formation, they form
two gametes.
 3. Each gamete will contain a separate allele
for each trait.
 4. During fertilization , chromosomes from
one gamete will combine with another
gamete.

Therefore : genes on chromosomes which
are very close together will almost always be
inherited together.
 And : genes located far apart are more likely
to be separated during a crossing over event.
 The likelihood of crossing over increases
with the distance between the two genes.
 Morgan’s work indicates that Mendel’s law of
independent assortment can be restated as:

P E Question
What is one of Morgan’s major
contributions to the field of genetics?
 (A) concept of codons
 (B) double helix
 (C) Law of Dominance
 (D) sex-linked traits

P E Question
What does the Law of Independent
Assortment state?
 (A) Gene pairs always sort in the same
order.
 (B) Gene pairs sort randomly and
independently from each other.
 (C) One allele is always dominant.
 (D) Sister chromatids separate during
gamete formation.

Independent Assortment
Restated:
 If
crossing over does not take place ,
genes that are located on different
chromosomes will assort
independently while genes that are
located on the same chromosome will
be inherited together.
Sex - Linked Inheritance
 Some
traits are passed from one
generation to the next depending on the
sex of the parent carrying the trait.
 This is because the genes for these traits
is carried on the sex chromosomes
 The transmission of genes that are located
on the sex chromosomes , X or Y , is
called sex-linked inheritance.
Y-marks the spot?
 A gene
that is only on the X
chromosome is called X-linked
 A gene that is only on the Y
chromosome is called Y-linked.
 This may be due to the smaller size of
the Y chromosome.
Why are sex-linked defects more
common in males than females ?
Sex linked inheritance involves pairs of genes
on the X chromosome.
 Note: In terms of gene expression , autosomal (
non-sex chromosomes) inheritance typically
involves pairs of genes , with gender being
irrelevant to gene expression.
 Most sex-linked traits are X-linked.Very few Ylinked traits are known.
 Sex-linked inheritance involves pairs of genes
on the X chromosome in the female, and a
single X in the male. In this case , gender is
important in gene expression and must be
considered a part of the phenotype.

P E Question
If a colour blind man and a woman who is
a carrier of colour blindness have a son,
what are the chances that their son will be
colour blind?
 (A) 25%
 (B) 50%
 (C) 75%
 (D) 100%







Three women gave birth to children in the same
hospital at the same time. The babies are mixed
up in the nursery. The hospital checked the blood
type of the parents and children and the following
data was obtained:
Who could have produced Baby Bob?
(A) either the Olsens or Jones
(B) either the Smiths or Jones
(C) only the Olsens
(D) only the Smiths