1. Revisions to Mendel
Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes.
So special rules must be used for these cases.
Incomplete dominance
Codominance
Multiple Alleles
Lethal Alleles
Sex Linked Inheritance
Polygenic Traits

2. Incomplete Dominance Neither allele is dominant.
The heterozygous phenotype is somewhere in between the two homozygous phenotypes.
Example: Four o’clock flowers

3. Practice : Your Turn!
In Australia there is a creature known as wild dogs or dingoes. It comes in three colors, black, grey, and white. This trait is controlled by a single gene with incomplete dominance. A homozygous (BB) individual is black, a homozygous (bb) individual is white, and a heterozygous (Bb) individual is grey.
What would be the genotypes and phenotypes of the offspring if a black dingo were crossed with a white dingo?

4. Check your work B b Bb Genotypes: 100% or 4/4 Bb
Phenotypes: 100% Grey dingo pups.

5. Codominance Both alleles are dominant.
Both alleles appear in the phenotype separately.
Each allele produces both types of protein, so they both appear.
Example: A black chicken mating with a white chicken will create an erminette chicken (also known as checkerboard).

6. Practice : Your Turn!
A white cow (WW) mates with a brown cow (RR) to create a roan calf (RW). This trait is controlled by a single gene with codominance.
Suppose this roan calf grows up to mate with another roan cow.
What would be the genotypes and phenotypes of the offspring?

7. Check your work R W RR RW WW Genotypes: ¼ RR or 25% RR
2/4 RW or 50% RW
¼ WW or 25% WW
Phenotypes: ¼ or 25% Brown calves
½ or 50% Roan calves
¼ or 25% White calves

8. Multiple Alleles
In a population, there might be more than 2 alleles for one single gene.
Any individual cannot have more than 2 alleles, even if many more exist in their gene pool.
One from the mother
One from the father
Example: Laborador coat color, mouse coat color, human blood types

9. Blood Typing
Human blood works of a system of Codominance with the AB blood type and Multiple Alleles due to three alleles, A, B and O .
Charles Drew is famous for his blood preservation methods.
We’ll talk more about blood later!

10. Blood Typing

11. Practice : Your Turn!
A man is concerned about if he is the father of a child. He has blood type B and the mother has blood type O. The child’s blood type is A.
Is he the father of this child?

12. Check your work IB i IBi IB i IBi ii
Since the mother is O, she is recessive and has the genotype of ii. Since the father is B, he could be homozygous (IBIB) or heterzygous (IBi) so you should complete two separate punnett squares for each possibility.
Punnett #1: All children will have B blood type
Punnett #2: ½ children can have B blood type, ½ can have O blood type.
Result? If the child’s blood type is A, then he is not the father. 

13. Lethal Alleles
This defies Mendel’s laws of 3:1 because one of the Genotypes results in a lethal (deadly) Phenotype.
Also called Pseudodominance
Example: Sickle Cell Anemia – the offspring that gets C1C1 will die immediately from the blood disorder.
Then how do lethal alleles still persist in the population if those with lethal alleles die before birth!?

14. Sex-Linked Inheritance
Sex linked inheritance varies the Mendel number of 3:1 by having males a 50/50 percent chance of inheriting the characteristic on the X chromosome.
Females have XX and Males are XY.
The Y carries little genetic information, mainly those that contribute to male characteristics.
The X carries a lot more genetic information.
Genes for certain traits are on the X chromosome, and boys only receive one X chromosome then they are more likely to inherit disorders passed to them from their mother.
Women are affected when they get the disorder from their mother and paternal grandmother.

15. Practice: Your Turn!
Hemophilia is a sex-linked trait where XH gives normal blood clotting and is dominant to the hemophilia allele Xh.
Identify the genotypes of…
1) a woman with normal blood clotting whose father had hemophilia
2) a normal man whose father had hemophilia.
What is the probability that a mating between these two individuals will produce a child, regardless of sex, that has hemophilia?

16. Check your work
1) the woman has normal clotting so she has one XH but she got a Xh from her father, so she is XHXh
2) the man is XHY since he got the Y from his father and he is normal so must be XHY XH Xh
XHXH
XHXh Y
XHY
XhY
Genotypes: ¼ XHXH Phenotypes: ½ unaffected girls
¼ XHXh ¼ unaffected boy
¼ XHY ¼ affected boy
¼ XhY
Notice how girls are “protected” from disorders and carry them.

17. Polygenic Traits
Instead of a trait being controlled by a single pair of homologous chromosomes, some traits are controlled by several genes.
Poly = many
Genic = genes
You get more a bell shaped curve of diversity
Examples: Skin color, hair color, etc…
Example: AaBBCcddEe