1. What if we consider two genes at one time?
Mendel’s law of independent assortment.
What happens with one gene has no effect on the other.

3. Outcome 9/16 Smooth, Yellow 3/16 Wrinkled, Yellow 3/16 Smooth, Green
1/16 Wrinkled, Green
Combining these, you’ll find that
9/16 + 3/16 = 12/16 = 3/4 are smooth, 1/4 wrinkled
9/16 + 3/16 = 12/16 = 3/4 are yellow, 1/4 green

4. Law of independent assortment
Each gene assorts independently, not affected by other genes.
This occurs if each gene is on a different chromosome.
Story is different if 2 genes are on the same chromosome (linkage) but we won’t discuss that in detail.

6. Dihybrid cross AaBb x AaBb Consider outcome of other crosses.
phenotypic ratios 9:3:3:1
9/16 dominant phenotype for both traits (A-B-)
3/16 are A-bb
3/16 are aaB-
1/16 are aabb (recessive for both traits)
Consider outcome of other crosses.

7. 2-gene crosses: multiplying probabilities instead of punnett square
Problem: AaBb x Aabb
Consider outcome of each gene separately:
Aa x Aa = ¾ A- and ¼ aa
Bb x bb = ½ Bb and ½ bb
Fraction of offspring with each phenotypic combination can be derived by multiplying
A-Bb = (3/4) (1/2) = 3/8
A-bb = (3/4) (1/2) = 3/8
aaBb = (1/4) (1/2) = 1/8
aabb = ((1/4) (1/2) = 1/8

8. Extensions to Mendelian genetics
Multiple alleles
Incomplete dominance
Polygenic inheritance
Pleiotropy
Linkage
Sex-linkage

9. Multiple alleles
Across the whole population, there may be many more than just two alleles of a given gene.
(Each diploid individual has at most two alleles, if they are heterozygous.)
For example, there are 4 alleles for fur color in rabbits. Particular color of a rabbit depends on which allele it possesses. (See fig but don’t sweat the details unless you want to.)

10. Incomplete dominance
Up till now, the heterozygote always looked just like one homozygote, but that’s not always the case.
Sometimes the heterozygote is intermediate between the two homozygotes.
However, this is not blending inheritance. The alleles remain distinct and will segregate in the next generation.

12. Human ABO Blood Groups: an example of both multiple alleles and codominance (a kind of incomplete dominance)
There are four phenotypes: A, B, AB, and O
These are controlled by three alleles: IA, IB, and IO
(Io is often written as “i” because it is recessive)
Genotypes responsible for the phenotypes:
IAIA and IAIO both have Type-A blood
IBIB and IBIO both have Type-B blood
IAIB has Type-AB blood (codominance)
IOIO has Type-O blood (homozygous recessive)

14. ABO Transfusion Compatibility
Both the IA and IB alleles put a protein on the rbc’s that is recognized by the immune system.
The immune system of a Type-A person will attack any rbc’s with B protein (cannot accept B or AB blood).
The immune system of a Type-B person will attack any rbc’s with A protein (cannot accept A or AB blood).
A type-AB person will accept both A and B proteins.
The IO allele does not put any protein on the rbc’s, therefore, type O blood is accepted by anyone.
The immune system of a Type-O person will attack any rbc’s with either A or B protein (can only accept type O).

15. ABO blood group compatibility
Can donate to:
Can receive from: A
A or AB
A or O B
B or AB
B or O AB
(universal recipient)
AB only
A, B, AB, or O O
(universal donor)
O only

16. Polygenic inheritance
Sometimes many separate genes control the same characteristic.
Each gene may have two or more alleles.
The effects of the genes are often additive: each adds a little to the character in question.
This explains continuous variation in characters like height, weight, etc.

18. Continuous variation
Continuous characters like height and weight are also affected by the environment (e.g. nutrition during development).
Thus, there are no apparent categories in the offspring.
Another reason that Mendel’s work was ignored at first was that everyone thought that continuous characters were more important than 2-state characters, but couldn’t see how Mendelian genetics explained them. Polygenic inheritance was not worked out until the 1920’s.

19. Pleiotropy One gene may affect unrelated characters.
Gene for coat color in Siamese cats also causes crossed eyes.
Gene that causes sickle-cell anemia (when homozygous) also confers resistance to malaria (when heterozygous).

20. Sex determination
A variety of mechanisms exist in various species. Many (not all) involve sex chromosomes.
In humans, one pair of chromosomes determines sex.
Individuals with two X chromosomes are female.
Individuals with an X and a Y are male.
The Y chromosome is very small and carries few genes other than those that code for maleness.
The X chromosome is large, and carries many genes that have nothing to do with gender.

21. Sex-linked inheritance
Applies to genes located on the X chromosome.
Examples include color-blindness, male pattern baldness, and hemophilia.
These traits are recessive, but in males they are always expressed, since the male has only one X.
Sons cannot inherit these from their fathers, because they get a Y, not an X, from dad.

22. Sex-linked genetics problems
What happens if a normal man marries a normal woman whose father was color-blind.
C = normal vision, c=color-blindness
Dad is CY. Mom is Cc.
Half of their daughters are CC; half are Cc. All have normal vision.
Half of their sons are CY (normal); half are cY (color-blind)