Phenotypic Ratios - Genetics

Monohybrid cross (complete dominance)

3:1 phenotypic ratio from Aa × Aa

Dihybrid cross (independent assortment)

9:3:3:1 phenotypic ratio from AaBb × AaBb

Incomplete dominance

Heterozygote shows intermediate (blended) phenotype

1:2:1 ratio

Codominance

Heterozygote expresses both parental phenotypes simultaneously

1:2:1 ratio

Monohybrid testcross

Aa × aa cross used to determine genotype

1:1 ratio

Dihybrid testcross

AaBb × aabb cross with unlinked genes

1:1:1:1 ratio

Recessive epistasis

Homozygous recessive genotype masks another gene

9:3:4 ratio

Dominant epistasis

A single dominant allele masks another gene

12:3:1 ratio

Duplicate recessive epistasis

Both genes must have a dominant allele for phenotype

9:7 ratio

Duplicate dominant epistasis

Either dominant allele produces phenotype

15:1 ratio

Lethal allele

One genotype is nonviable, removing a class

2:1 ratio among survivors

Polygenic inheritance

Trait controlled by many genes

continuous variation, no fixed ratio