Extensions of Mendelian Analysis

• Modifications of Mendelian Ratios

• Single Gene modifications

• Two gene pair modifications: modified 9:3:3:1 ratios

• Modification of Expression

Single Gene modifications

• Incomplete dominance (partial dominance) • Multiple alleles • Lethal alleles

Incomplete dominance (partial dominance)

• Expression of a heterozygous phenotype that is distinct from and usually intermediate to either homozygous phenotype

Incomplete dominance (partial dominance)

Tay Sachs and Sickle Cell Anemia

Heterozygous are intermediate at the biochemical level.

Multiple Alleles &Co-Dominance

Bombay phenotype

• H allele affected

• Recessive epistasis

• Show O phenotype

• Genotype variable

Lethal alleles

• Yellow dominant to Agouti coat color• No purebred yellow• Deletion mutation• Recessive for lethality

Two gene pair modifications: modified 9:3:3:1 ratios

• Epistasis • Suppression • Complementary gene action

Recessive Epistasis

Dominant Epistasis

Suppression

Complementary gene action

Complementary gene action

Modification of Expression

• Pleiotropic effects

• Penetrance and Expressivity

Pleiotropic effects

Penetrance

No expression

Expressivity