1. DIHYBRID CROSSES
A dihybrid cross is a cross of two individuals that differ in two traits due to two different genes on different chromosomes.
Mendel designed dihybrid crosses to see whether the pattern of inheritance for one trait had any impact on the pattern of inheritance for another trait.
Mendel crossed true breeding plants that produced yellow, round seeds with true breeding plants that produced green, wrinkled seeds.
Mendel designed experiment following two traits using true breeding plants

2. 9:3:3:1 phenotypic ratio is always obtained
No matter which two traits were followed in a dihybrid cross, the same phenotypic ratio 9:3:3:1 was always obtained in F2
Which traits are the dominant ones? How do we know?
What are the traits reappear in F2? What are the new traits in F2?

3. Using a Punnett square to model Mendel’s results
F1 x F1: YyRr x YyRr
Each parent in F1 could produce four possible gametes (YR, Yr, yR and yr.)
Both parents must be heterozygous for both traits in order to get 16 possible genotype combinations to explain for the observed 9:3:3:1 ratios.

4. Punnett square can help explain mendel results

5. The Law of Independent Assortment
Mendel’s second law states that the movement of each pair of homologous chromosomes is INDEPENDENT of the movement of other pairs of homologous chromosomes.
i.e. Allele Y and G don’t have to always go together. Neither do y and g.
This law is applicable only for genes on different chromosomes.
The ratio observed can only be explained for it the inheritance of one trait has no influence on the inheritance of another trait. (they occur independently of each other). This forms the law of independent assortment of mendel
During gamete formation, the two alleles for one gene segregate independently of the alleles for other genes

6. Learning Check
1. List all the possible different gametes that could be produced from parents with the following genotypes:
TtGg b. AABbcc
2. A tall true-breeding plant with purple flowers is crossed with a short true-breeding plant with white flowers. Assuming tall and purple are dominant phenotypes, find both the F1 and F2 generation’s genotypic and phenotypic summaries.
Answer:1 a. TG, Tg, tG, tg b. ABc, Abc
2. P: TTGG x ttgg
F1: TtGg

7. Homework Do DIHYBRID CROSS PROBLEMS 1-6.
We will check them next class.

8. BEYOND MENDELIAN GENETICS

9. Incomplete Dominance
Flower Colour of Snapdragons
A condition in which neither allele for a gene completely conceals the presence of the other; it results in an intermediate expression of the trait.
3 phenotypes are observed in F2

10. Incomplete dominance
However, the alleles of flower colour do segregate according to Mendel’s Law of Independent Assortment.
In human, traits showing incomplete dominance include:

11. How do we represent incomplete dominance?
Use superscripts rather than upper-case and lower-case letters since neither allele is dominant over the other.
Eg. Snapdragon flower colour: both alleles affect the colour of flower - C
2 alleles are therefore CR (red) and CW (white)

12. Codominance
A condition in which both alleles for a trait are equally expressed in a heterozygous offspring; both alleles are dominant.
E.g.
Shorthorn roan cattle can be red (HRHR = all red hairs), white (HWHW = all white hairs), or roan (HRHW = red & white hairs together)
Human AB blood type (two types of proteins ("A" & "B") appear together on the surface of blood cells.)
Roan cow is the product of mating of shorthorn cattle: a red bull and a white cow.

13. Codominance
Why small r?

14. What’s the difference between incomplete dominance and codominance?
a. Incomplete dominance:
- Both alleles are partially expressed
- Offspring shows a third phenotype that is something between the parents’ phenotypes
b. Codominance:
- Both alleles are fully expressed
- Offspring shows a third phenotype that is completely different from both the parents’ phenotypes

15. Multiple Alleles
So far we’ve studied traits controlled by one gene with two alleles.
An individual has only 2 alleles for a gene but many different alleles for a gene can exist within a population.
A gene with more than 2 alleles is said to have multiple alleles.
The ABO Blood Type system in humans is an example of a trait with multiple alleles.

16. Introduction to Blood Typing
An antigen is a substance that activates an immune response.
Antibodies, found in the blood plasma, are the body’s immune response to antigens.
People will produce antibodies in response to the presence of antigens that their own cell’s lack.
There are antigens present on the cell membrane of red blood cells. The type of antigen(s) present determines the type of blood a person can be given during a transfusion.
If they are given a blood type having antigens that they lack, their antibodies will attack the new cells causing agglutination, or clumping, of the blood.

17. Blood Typing Rules Blood Type Antigens Antibodies Can Donate
Can Receive To
From A A
Anti - B
A, AB
O, A B B
Anti - A
B, AB
O, B
O, A, B, AB AB
A & B
Neither AB
Universal
Recipient
Anti - A
&
O, A, B, AB O
Neither
Anti - B
Universal O
Donor

18. Human Blood Types: An example of codominance and complete dominance