Codominance Genetic Crosses

Genetic crosses with codominant alleles are set out in the same way as other monohybrid crosses, but the heterozygote has its own phenotype.Worked example 1: two roan shorthorn cattle are crossed. Red is C^R and white is C^W.Parental phenotypes: roan × roan
Parental genotypes: C^RC^W × C^RC^W
Gametes: C^R or C^W from each parent

	CR	CW
CR	CRCR	CRCW
CW	CRCW	CWCW

Offspring: 1 red : 2 roan : 1 white. The expected phenotypic ratio is 1 : 2 : 1, the same as the genotypic ratio, because each genotype has a different phenotype.Worked example 2: a red bull (C^RC^R) is crossed with a roan cow (C^RC^W). The gametes are C^R from the bull and C^R or C^W from the cow, giving 1 C^RC^R (red) : 1 C^RC^W (roan). So the probability of a roan calf is 0.5 (50%).Worked example 3: a red bull crossed with a white cow gives only C^RC^W offspring, so all calves are roan.Remember that ratios are expected probabilities. In small families the observed numbers often differ from the expected ratio by chance, which can be tested with the chi-squared test.