GN 301 Module 6: Complexities in Mendelian Inheritance
Epistasis
Define Epistasis: when two or more loci interact to determine the phenotype of one characteristic
Two deaf parents can have a child with normal hearing: Diagram cross of two parents
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)
| D | D | |
|---|---|---|
| d | Dd | Dd |
| d | Dd | Dd |
| e | e | |
|---|---|---|
| E | Ee | Ee |
| E | Ee | Ee |
\n All children are hearing (Dd Ee).
Two parents with normal hearing can have a deaf child. Show Branch for DdEe x DdEe cross:
Freckles and Albinism
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)
Penetrance and Expressivity
- Define Penetrance
- Pentrance: the percent of individuals which show a phenotype expected for a particular genotype. 100% penetrance is when a genotype always causes an expected phenotype
- Define Expressivity
- 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
Polydactyly: additional fingers or toes. Displays incomplete penetrance in humans. Some P_ individuals have exactly 5 fingers and 5 toes. 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
Pleiotropic Effects (Pleiotropy)
- Define Pleiotropy: 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.
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