Genetics Exam 4

penetrance

for individuals with the same genotype, the percentage of who displayed a trait

incomplete penetrance

less than 100% risk

expressivity

the severity of a phenotype

variable expressivity

trait displayed with different severity in different individuals

lethal phenotypes

causes death of an individual

sex-influenced trait

autosomal gene but dimorphic

incomplete dominance

heterozygous individual displays both phenotypes, with blending (e.g., snapdragon flower color)

codominance traits

heterozygous individual displays both phenotypes without blending (e.g., shorthorn cattle coat color)

glycosyltransferase

enzyme that modifies RBC antigens; the human blood group gene I encodes this

gene interaction

two different traits can alter one another

duplicate genes

• alleles at two genes cause no phenotype alone

• when combined (double mutant), they cause a new recessive phenotype

complimentary genes

• two alleles cause the same recessive phenotype

• the alleles affect two different genes

9:7

ratio of a dihybrid cross with complimentary genes

15:1

ratio of a dihybrid cross with duplicate genes

epistasis

when two genes cause different phenotypes AND when a trait caused by one gene masks/hides a trait caused by another gene

hypostatic gene

a gene hidden by a trait that’s caused by an epistatic gene

series pathway

pathway that’s typical for recessive epistasis

repressor pathway

pathway that’s typical for dominant epistasis

recessive epistasis

when a recessive trait masks another trait

dominant epistasis

when a dominant trait masks another trait

9:3:4

ratio of a dihybrid cross between recessive epistatic genes

12:3:1

ratio of a dihybrid cross between dominant epistatic genes

discontinuous traits

the phenotype of individuals in a population falls into discrete categories

continuous traits

the phenotype of individuals in a population varies and can be described by a distribution

polygene hypothesis

continuous traits are observed if these are true…

1. incomplete dominance

2. controlled by a very large # of genes

3. trait is additive

heritability

the fraction of phenotype variance that is contributed by genetic variance

twin studies

hypothesis: MZ twin pairs are more similar than DZ twin pairs