GN 301 Module 6: Complexities in Mendelian Inheritance

Genetics in Human Affairs

epistasis

when two or more loci interact to determine the phenotype of one characteristic

Hearing ability is controlled by many genes.

* D locus → production of the auditory nerve. dd = deaf
* E locus → cochlea function. ee = deaf
* Two deaf parents can have a child with normal hearing  (DD ee x dd EE → Dd Ee)

Can two deaf parents produce a hearing child?

Two deaf parents can have a child with normal hearing  (DD ee x dd EE → Dd Ee)

Can two hearing parents have a deaf child?

Yes, if they each have a recessive d or e that binds to the other recessive d or e.

Freckles and Albinism 

freckles are dominant, albinism is recessive

Mom is albino and does not have freckles. Dad has normal skin pigment and does not have freckles. They have three children: an albino daughter, a son with normal skin pigment and no freckles and a son with normal skin pigment and freckles. Determine the genotypes for the parents. 

**Mom’s Genotype**: aaFf (although we don’t know if she has freckles, one of her sons has freckles so she must have at least one F. Because she also has a son without freckles, she must also pass down a recessive f)

**Dad’s Genotype**: Aa ff (definitely no freckles, so ff, but he must have a recessive a in order to make his daughter albino, so he has Aa)

freckles genotype

Ff or FF

albinism genotype

aa

Can a person with albinism carry an allele for freckles?

Yes, the freckles are just masked by the albinism, but the individual could have FF or Ff alleles

Penetrance

the percent of individuals who show a phenotype expected for a particular genotype.

100% penetrance is when

a genotype always causes an expected phenotype

Expressivity

the variation in severity of a particular trait.

Eg. All individuals are genotype H_ which produces hair

Complete: 100%
Incomplete: (7/9) x 100 = 77.78%
Variable Expressivity: (6/8) x 100 = 75%

Variation in Expressivity: Beagles

All the beagles have the dominant Sp allele. All of them have spotting, so 100%

Polydactyly

additional fingers or toes. Displays incomplete penetrance in humans. Some P_ individuals have exactly 5 fingers and 5 toes.

What is the penetrance of polydactyly?

Penetrance is about 80%

List Factors that can influence phenotype:

* Why isn’t a trait 100% penetrant?
* Other genes coil have a role in the trait
* Environmental factors

Define Pleiotropy:

when one gene has an effect on several seemingly unrelated aspects of an individual’s phenotype. One gene affects the phenotype of several unrelated traits.

Describe how Nail-Patella Syndrome is an example of pleiotropy

There are abnormalities in the finger and toenails and the knees. This is pleiotropic, since one gene affects multiple traits.

Describe how Marfan Syndrome is an example of pleiotropy

Individuals with Marfan Syndrome are pleiotropic; they have long legs, arms, and fingers, along with flexible collagen.