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: 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
Molecular aspects of Lethal allele
Two gene pair modifications:
modified 9:3:3:1 ratios
• Epistasis
• Suppression
• Complementary gene action
Epistasis
Recessive Epistasis
Dominant Epistasis
Suppression
Complementary gene action
Complementary gene action
Modification of Expression
• Pleiotropic effects
• Penetrance and Expressivity
Pleiotropic effects
Penetrance and Expressivity
Penetrance and Expressivity
No expression
Expressivity
Eyeless mutation
Position and Temperature Effects
Lower temperature at extremities allows
Pigment production, higher temperature
Suppresses production.
Heterozygous for white:
Inactive wild type allele causes
Patches.