WHEN HEREDITY FOLLOWS DIFFERENT RULES INCOMPLETE DOMINANCE Two

WHEN HEREDITY FOLLOWS DIFFERENT RULES

INCOMPLETE DOMINANCE • • Two alleles appear to blend An intermediate form of traits from parents The heterozygote phenotype is between the two homozygotes Snap dragons – red X white = pink

NOTATION • When working with incomplete dominance use two of the same capital letters…. . add a prime to one of them – Example • RR-Red • R’R’-White • RR’-Pink

RATIOS • The ratios are written: – Homozygous: Heterozygous: Homozyg ous – The • genotype ratio is always the same as the phenotype ratio. How does this differ from a pattern that shows complete dominance?

Homozygous Red (R) x Homozygous White (R’) R R R` R` Geno Ratio = Pheno Ratio:

Heterozygous (Pink) x Heterozygous (Pink) R R` R R ` Geno Ratio = 1: 2: 1 Pheno Ratio= 1: 2: 1

Codominance: condition in which both alleles of a gene are expressed in heterozygous offspring.

Example 1: In cows, black coat color and white coat color are codominant. Heterozygous offspring will be spotted. 1. Black cow and White cow B BB x WW B Genotypic Ratio: W Phenotypic Ratio: W

Example 1: In cows black coat color and white coat color are codominant. Heterozygous offspring will be spotted. 2. Black cow and Spotted cow B B W B BB x BW Genotypic Ratio: 50% BB 50% BW Phenoty pic Ratio: 50% Black cow 50% Spotted cow

Example 1: In cows black coat color and white coat color are codominant. Heterozygous offspring will be spotted. 3. Both Spotted cows BW x BW Genotypic Ratio: B B W 25 % BB 50% BW 25% WW Phenoty W pic Ratio: 25% Black cow 50% Spotted cow 25% White cow