Variations on Mendelian Genetics

Incomplete dominance

• Phenotype expression is in between dominant and recessive.
• Example: red and white flowered snapdragons crossed to produce pink.
• One allele being expressed isn't enough to produce a red flower.
• Alleles are still there and will be expressed in future crosses.
• Alleles are represented with letter super scripts: C^rC^r (red), C^wC^w (white), C^rC^w (pink).

Codominance

• Both alleles are fully expressed.
• Example: human blood types. Both A and B blood cell surface proteins expressed.
• Most often seen in biochemical traits.
• Alleles represented with letter super scripts: I^AI^B.

Multiple alleles

• More than two alleles are possible for a trait.
• Only two can be present in an individual!
• Different combinations possible though.
• Example: A (I^A), B (I^B) and O blood (i) alleles.

Epistasis

• More than one gene affects single phenotypic expression.
• Example: fur and skin color in mammals.

Pleiotropy

• A single gene affects many phenotypic expressions.