Codominance Genetic Crosses

Genetic crosses involving codominant alleles are set out in the same way as other monohybrid crosses, but each genotype gives a different phenotype, so the phenotypic ratio matches the genotypic ratio.Take coat colour in shorthorn cattle: C^RC^R red, C^WC^W white, C^RC^W roan.Cross 1: red × white
Parents: C^RC^R × C^WC^W
Gametes: all C^R and all C^W
Offspring: all C^RC^W, so all roan.Cross 2: roan × roan
Parents: C^RC^W × C^RC^W
Gametes: C^R or C^W from each parent

	CR	CW
CR	CRCR	CRCW
CW	CRCW	CWCW

Genotypic ratio 1 C^RC^R : 2 C^RC^W : 1 C^WC^W
Phenotypic ratio 1 red : 2 roan : 1 whiteCross 3: roan × white
C^RC^W × C^WC^W gives 1 roan : 1 white.Worked example
Two roan cattle produce 80 calves over several years. How many would you expect to be roan?expected roan = 2/4 × 80 = 40Remember that ratios are probabilities. Real numbers of offspring rarely match them exactly, because fertilisation is random.