Genetic Inheritance Patterns and Examples

Incomplete Dominance

A form of inheritance where the heterozygote shows an intermediate phenotype (e.g. red x white = pink flowers).

F2 Ratio for Incomplete Dominance

1:2:1 (homozygous dominant : heterozygous : homozygous recessive).

Familial Hypercholesterolemia (FH)

Genetic condition showing incomplete dominance affecting LDL receptor production.

Co-dominance

Inheritance where both alleles are fully and independently expressed (e.g. AB blood type).

Blood Group Alleles

IA and IB are co-dominant; both are dominant over i.

Lethal Alleles

Alleles that cause death when present in certain genotypes, altering expected Mendelian ratios.

Epistasis

Interaction between genes where one gene masks or modifies the effect of another gene.

9:7 Ratio

Reciprocal recessive epistasis; both dominant alleles needed for phenotype.

9:3:4 Ratio

Recessive epistasis; one gene's recessive alleles mask the other (e.g. Labrador fur color).

12:3:1 Ratio

Dominant epistasis; one dominant allele masks all alleles of another gene.

9:6:1 Ratio

Duplicate genes with additive effects; both contribute cumulatively to phenotype.

15:1 Ratio

Duplicate dominant genes; either dominant allele produces same phenotype.

13:3 Ratio

Dominant and recessive interaction; dominant of one gene and recessive of another yield same phenotype.

Polygenic Inheritance

Multiple genes contribute additively to a single trait, causing continuous variation (e.g. height, skin color).

Nilsson-Ehle Wheat Experiment

Demonstrated polygenic inheritance using kernel color with additive alleles.

Edward East Tobacco Experiment

Showed polygenic inheritance in flower length with continuous variation in F2.

Multiple Alleles

More than two alleles exist for a single gene locus (e.g. blood groups IA, IB, i).

Penetrance

Proportion of individuals with a genotype who show the expected phenotype (e.g. polydactyly).

Expressivity

Degree to which a genotype is expressed in an individual (e.g. severity of retinoblastoma).

Environmental Effects - Temperature

Temperature-sensitive alleles affect phenotype (e.g. Himalayan rabbits, Siamese cats).

Environmental Effects - Nutrition

Diet can influence gene expression (e.g. lactose intolerance).

Pleiotropy

One gene influences multiple traits (e.g. PKU affects mental capacity, eye and skin color).

White Cats and Deafness

Example of pleiotropy; gene for white fur linked to deafness, especially in blue-eyed cats.