1. Honors Biology Genetics
Exceptions to Mendel

2. Incomplete Dominance
When two or more alleles influence the phenotype, the heterozygous phenotype is a blend of both homozygous phenotypes the trait is considered incompletely dominant.
White FF
Pink Ff
Red ff

3. Incomplete Dominance: Example
The trait for hair texture in humans is incompletely dominant.
People with curly hair have two HH alleles
People with straight hair have two hh alleles
People with Wavy hair are heterozygous Hh hh Hh HH

4. Incomplete Dominance: Problem
A wavy haired man marries a wavy haired woman, they are expecting their first child, what is the chance they will have a child with wavy hair? ____ x ____ Hh Hh

5. Codominance
When both alleles for a gene are expressed in a heterozygous offspring the trait is considered codominant (there is no blending in codominant traits both traits are expressed.)

6. Codominance: Example Problem
The trait of fur color in horses and cows follows a codominant inheritance pattern.
The allele for brown fur and white fur are codominant meaning a heterozygous horse will have both brown and white furs on their body (ROAN).

7. Codominance: Example Problem
A brown horse (R) and a white horse (r) are mated, what are the phenotypes and genotypes of their offspring? ____ x ____ RR rr

8. Multiple Alleles
Definition: A trait that is controlled by three or more alleles that can fit into one gene.
C= full coat color, cch chinchilla, ch Himalayan, c albino

9. Multiple Alleles: Example
Example Problem:
ABO blood type
Phenotype
Genotype(s)
A type
IAIA or IAi
B type
IBIB or IBi
AB type
IAIB
O type ii
Phenotype
Genotype(s)
A type
B type
AB type
O type

10. Multiple Alleles
If a person who is heterozygous for B type blood marries a person who is homozygous for A type blood what is the chance they will have a child with O type blood?

11. Sex Linkage
Refers to genes that exist only on the X chromosome. Males have only one allele for these traits females have two alleles.

12. Sex-Linkage: Example
Ichthyosis forms scales on the skin because of the build up of dead skin cells
This genetic disorder is X-linked and recessive.
When a trait is X-linked it means it is only found on the X chromosome.
In these problems you will need to include both the gender AND the allele
A male genotype is written XY and a female genotype is XX.
Ichthyosis is represented by a Xi the i represents the recessive allele for this trait.

13. Sex-Linkage: Example Problem
If a healthy woman and a man with ichthyosis have a child, what is the chance the child will also have the disease?
XI XI x XiY

14. Polygenic
A trait that is controlled by many genes combined together

15. Polygenic: Example Problem
Three genes help to determine skin color: gene A, gene B and gene C.
if a person has all dominant alleles: AABBCC they have a very dark skin tone
If they have all recessive alleles: aabbcc they will be very pale
If they have half of the dominant alleles AaBbCc they are medium tone.
How many different skin tones are possible?
2x2x2 = 8