T EST C ROSSES AND I NCOMPLETE
D OMINANCE
Agenda: Take up Question #7 and #8 from monohybrid cross problems
Ratios for dibybrid crosses
Mendel’s 2 nd Law
Introduce Test Crosses and Incomplete Dominance

M ENDEL ’ S 2 ND L AW
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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.

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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

T EST C ROSSES
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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.

I NCOMPLETE D OMINANCE
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What do you think would happen, if these two snapdragon flowers were crossed?
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Take a minute to make a prediction of the expected phenotype of the F1 generation of the white and red snap dragon.

I NCOMPLETE D OMINANCE
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In the F1 generation all the flowers are pink!?
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How does that happen?

I NCOMPLETE DOMINANCE
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When neither gene is completely dominant over the other gene, an intermediate phenotype is expressed. In this case it is a pink flower colour

I NCOMPLETE D OMINANCE
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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
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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?
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Discuss with a partner what you think the offspring would look like. Use rainbow or foil method to find the genotypes!

W HAT ARE YOUR PREDICTIONS ?
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Discuss, with a partner what is the phenotype ratio? What is the genotype ratio?
What pattern do you notice between the genotype and phenotype?

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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.

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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.

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Some examples of human traits exhibiting incomplete dominance: hair, skin and eye colours!

P RACTICE P ROBLEM
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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
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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.

C ODOMINANCE
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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:

S O H OW D OES I NCOMPLETE D OMINANCE
D IFFER FROM C O -D OMINANCE ?
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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

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While in incomplete dominance, there is a blending of traits. Resulting in an intermediate phenotype, relative to the parents.
Which flower exhibits co-dominance?
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Incomplete Dominance and Co-Dominance

P RACTICE P ROBLEM
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Sickle Cell Anemia displays codominant inheritance.
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Sickle-cells do not transport oxygen as effectively and cannot pass through small blood vessels, leading to clots and tissue damage

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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
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S= sicked-cell allele
N= normal-cell allele
Let Hb S Hb S represent the homozygous sickled cell individual
Let Hb N Hb N 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
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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

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The sickle cell trait is an example of the principle of heterozygous advantage.
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Being heterozygous for the sickle cell trait has benefits over being homozygous normal or homozygous sickle-cell.