Chapter 4 - Gene Interactions

Wildtype

Most common allele, can be dominant or recessive

Blue eyes

Example of neutral mutation

Albinism

Example of deleterious mutation

CCR5 mutation (near immunity to HIV)

Example of advantageous mutation

Wildtype is haplosufficient when...

The mutant allele is recessive to the wildtype, one wt allele is sufficient for wt phenotype

Wildtype is haploinsufficient when...

Mutant allele is dominant to wildtype, one wt allele is insufficient for wt phenotype

Null allele

Does not produce a protein, can be homozygous or heterozygous with another allele

Loss of function allele

Produces less protein with a diminished function

Dominant negative mutations

Often due to impaired interactions with other proteins

Gain of function allele

Produces excess protein (hypermorphic mutation)
Has a novel function (neomorphic mutation)

Incomplete/partial dominance

Heterozygous individuals have a median trait

Co-dominance

NOT an intermediate heterozygote, but different from either homozygote

Allelic series

Dominance relationships may vary in a hierarchal way
E.g. coat color in bunnies

Achondroplasia

Heterozygous results in dwarfism, homozygous dies in utero or shortly after birth due to breathing problems

De novo mutations

New mutation not previously in the lineage
Age of father increases the chance

Mutation-selection equilibrium

Balance between removal by natural selection and introduction by new mutation

Variable phenotype

One gene with multiple phenotypes

Cause of variable phenotypes

Sex limited traits, sex influenced traits, incomplete penetrance, variable expressivity

Sex limited traits

Both sexes carry the allele but only one expresses it

Sex-influenced traits

Both sexes carry a gene, but phenotype varies between sexes
E.g. hetero- & homozygous males express trait, but only homozygous females do

Incomplete penetrance

Traits that are nonpenetrant in some individuals but penetrant in others
Some individuals may have the dominant trait but not express it

Complete penetrance

Every individual with the allele expresses the trait

Variable expressivity

One genotype has multiple phenotypes with a range or combination of characteristics rather than being present or absent

Variable expressivity & incomplete penetrance

Can be caused by interactions with other genes that may differ among individuals &/or environmental effects

Pleiotropy

One gene affects many phenotypes
E.g. Mutations that cause sickle-cell anemia

Epistasis

The effect of one gene/locus is dependent on the allele(s) of one or more modifier genes

9:3:3:1 dihybrid ratio

No epistatic interaction

9:7 dihybrid ratio

Complimentary epistatic interaction

13:3 dihybrid ratio

Dominant suppression

Linked genes

Genes are so close together that alleles cannot assort independently

Syntenic genes

On the same chromosome

Alleles of syntenic genes

Can produce recombinant chromosomes due to crossing over

Genetic linkage mapping

Plots position of genes and relative distances from each other

Incompletely linked genes

Recombination does occur but is less common