Chapter 4 Genetics

polygenic trait

trait is controlled by multiple genes

pleiotropy

when a single gene has multiple effects on the phenotype (ex. Cystic Fibrosis)

Variations of Dominance

describes multiple ways a dominant allele can effect the phenotype

epistasis

interaction between alleles in which one allele masks the effects of the other

penetrance

percentage of individuals with a given allele who exhibit the phenotype

expressivity

measures the degree to which a given allele is expressed at a phenotypic level

What are the 3 main ways pleiotropy occurs?

1. expression of a single gene can have multiple functions

2. gene is expressed in different cell types in multicellular organisms

3. gene is expressed at different stage of development

main example of pleiotropy

cystic fibrosis

complementation test

performed to determine if two mutants have mutations in the same gene

conduction of a complementation test

performed by mating two recessive mutants with the same phenotype together

What does it mean if the offspring of a complementation test are WT?

the two mutations are in different genes

What does it mean if the offspring of a complementation test are mutant?

the two mutations are in the same gene

expected ratio of a non-epistatic cross

9:3:3:1

dominant epistasis

occurs when a dominant allele of one gene masks the effects of either allele of the 2nd gene

dominant epistasis ratio

12:3:1

recessive epistasis

occurs when a recessive allele of a gene masks the effects of either allele of the 2nd gene

recessive epistasis ratio

9:3:4

rare example of recessive epistasis in humans

Bombay phenotype

complementary gene action

occurs when the two genes interact because they're in the same pathway

complementary gene action ratio

9:7

Complete Dominance

when the dominant allele is always expressed when at least one copy is present (heterozygous and homozygous dominant are phenotypically the same)

Incomplete Dominance

when the number of dominant alleles affects the phenotype (homozygous dominant appears different than heterozygous)

Co-Dominance

when there are two dominant alleles, and both are expressed

Co-Dominance example

ABO blood types

SRY gene

gene that determines maleness

SRY gene is ____ because there is only one Y chromosome.

hemizygous

pseudoautosomal regions 1 and 2

helps pair X and Y chromosomes together within the cell

Nondisjunction

occurs when the chromosomes fail to separate properly (can occur at any divisional step in meiosis)

sex-limited inheritance

when expression of a phenotype is absolutely limited to one sex (ex. color or size difference)

sex-influenced inheritance

when the sex of an individual influences the expression of a phenotype (ex. male pattern baldness)