Download Incomplete and Codominance

TEST CROSSES AND INCOMPLETE
DOMINANCE
Agenda: Take up Question #7 and #8 from
monohybrid cross problems
Ratios for dibybrid crosses
Mendel’s 2nd Law
Introduce Test Crosses and Incomplete Dominance
MENDEL’S 2ND LAW

The Law of Independent Assortment: when two
or more pairs of characteristics are considered at
one time, each pair shows dominance and
segregation, independently of each other.

In a dihybrid cross involving heterozygous individuals
for both traits. The ratio will always be 9:3:3:1, 9
dominant/dominant, 3 dominant/recessive, 3
recessive/dominant and 1 recessive/recessive
TEST CROSSES
Imagine, Mr. Law wanted to know if he had
purebred black lab. He was too cheap to pay for a
genetic test and did not neuter his dog. Black
coat colour is dominant over gold coat colour
 What can he do!?
 A test cross: is a cross-fertilization between a
parent of unknown genotype and a homozygous
recessive parent.

INCOMPLETE DOMINANCE

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What do you think would happen, if these two
snapdragon flowers were crossed?
Take a minute to make a prediction of the
expected phenotype of the F1 generation of the
white and red snap dragon.
INCOMPLETE DOMINANCE

In the F1 generation all the flowers are pink!?

How does that happen?
INCOMPLETE DOMINANCE

When neither gene is completely dominant over
the other gene, an intermediate phenotype is
expressed. In this case it is a pink flower colour
INCOMPLETE DOMINANCE
In an RR homozygous dominant red plant,
enough red pigment protein is produced to give
the flower a red appearance.
 But in the heterozygous pink (Rr) snapdragon
flower, there is only sufficient red pigment
protein produced to make the flower appear pink
 The R allele does not completely dominate
expression over the r allele.
 So we can say, the R allele is only partially
expressed in a heterozygous Rr snapdragon plant


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What if I cross-fertilized my F1 generation
plants? What do you think the resultant offspring
would look like?
Discuss with a partner what you think the
offspring would look like. Use rainbow or foil
method to find the genotypes!
WHAT ARE YOUR PREDICTIONS?
Discuss, with a partner what is the phenotype
ratio? What is the genotype ratio?
 What pattern do you notice between the
genotype and phenotype?

In the F2 generation we observe a 1:2:1
white:pink:red phenotype, which happens to be
the exact same as the genotype ratio.
 This only happens when there is incomplete
dominance between alleles.

This demonstrates the law of segregation still
holds because the alleles are able to separate into
their own gamete.
 Therefore, there is no blending of genetic
material.


Some examples of human traits exhibiting
incomplete dominance: hair, skin and eye colours!
PRACTICE PROBLEM
A homozygous white flowered plant is crossed
with a homozygous plant that produces purple
flowers. Describe the phenotype of the offspring,
if the alleles show incomplete dominance.
 Let WW represent the white flowered plant
 Let W’W’ represent the purple flowered plant

W’ = allele for purple plants
 W= allele for white plants

We need to write them as W and W’ because they
do not display dominance over one another.
CODOMINANCE
Is the condition in which both alleles for a trait
are equally expressed in a heterozygote offspring,
meaning both alleles are dominant.
 An example is the roan (red and white) cow:

SO HOW DOES INCOMPLETE DOMINANCE
DIFFER FROM CO-DOMINANCE?

In co-dominance, both alleles are expressed
completely. Each individual hair in the coat of the
cow is either red or white. Both parental phenotypes
can be distinguished in the heterozygote
While in incomplete dominance, there is a
blending of traits. Resulting in an intermediate
phenotype, relative to the parents.
 Which flower exhibits co-dominance?


Incomplete Dominance and Co-Dominance
PRACTICE PROBLEM


Sickle Cell Anemia displays codominant
inheritance.
Sickle-cells do not transport oxygen as effectively
and cannot pass through small blood vessels,
leading to clots and tissue damage
Suppose an individual homozygous for the gene
coding for sickle cell anemia had children with an
individual who was homozygous for the normal
hemoglobin gene. What would the F1 generation
be?
 Hb= gene coding for hemoglobin
 S= sicked-cell allele
 N= normal-cell allele
 Let HbSHbS represent the homozygous sickled
cell individual
 Let HbNHbN represent the homozygous normalcelled individual



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Individuals who are heterozygous for the sickle
cell gene have cells that are both sickled and
normal
However, they do not experience any symptoms of
anemia
They are also more resistant to malaria because it is
thought, that the parasite cannot infect sickled cells
The sickle cell trait is an example of the principle
of heterozygous advantage.
 Being heterozygous for the sickle cell trait has
benefits over being homozygous normal or
homozygous sickle-cell.
