Catalyst – April 23, 2010 D = dimples, d = no dimplesF = freckles, f = no freckles

1. Show the dihybrid cross for the following:• Female - Heterozygous for both traits• Male – Heterozygous for both traits

2. Give the phenotypic ratio for the cross in question 1

Incomplete Dominance

• The phenotype of the offspring is a mixture of the two phenotypes of the parents.

• Neither of the alleles is dominant to the other.• This results in a new phenotype

Example: Skin Color

Codominance

• The phenotype of the offspring is a combination of the phenotypes of both parents.

• Both alleles are expressed separately and contribute to the offspring’s appearance

Example - Chickens

A black-feathered (BB) chicken crossed with a white-feathered (WW) chicken results in a black and white feathered chicken (BW).

Show the Cross

• If a black chicken (BB) and a white chicken (WW) mate, what is that probability that the baby chicken will be black? White? Mixture?

• What happens if a black and white chicken (BW) mates with another black and white chicken? What is the phenotypic ratio?

Incomplete Dominance?

Codominance?

Codominance?

Incomplete Dominance?

Codominance?

Incomplete Dominance?

Practice Problems

Codominance: B = Black, W = WhiteIncomplete Dominance: R = Red, Y = Yellow

• Male genotype: BWRR• Female genotype: BBYY• Cross these, give the phenotypic ratio.