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The Codominance Calculator predicts offspring phenotype ratios for crosses where both alleles are fully expressed in the heterozygote. In codominance, neither allele is dominant or recessive; instead, both allele products are simultaneously visible in the phenotype. Classic examples include AB blood type and roan coat color in cattle.
Enter each parent's genotype as the number of A alleles (2=AA, 1=AB, 0=BB) to see predicted offspring ratios.
In codominance, each genotype produces a distinct phenotype:
AA → Phenotype A only, BB → Phenotype B only, AB → Both A and B expressed simultaneously.
For AB × AB: The cross produces 25% AA : 50% AB : 25% BB (a 1:2:1 ratio), just like incomplete dominance, but the heterozygote shows both phenotypes rather than a blend.
For AB × BB: 50% AB : 50% BB (1:1 ratio).
Inputs
Results
Crossing two heterozygotes produces 25% AA (phenotype A), 50% AB (both expressed), and 25% BB (phenotype B). This 1:2:1 ratio yields 3 distinct phenotype classes.
Inputs
Results
Crossing red (AA) with white (BB) cattle produces 100% roan (AB) offspring, where both red and white hairs are present in the coat.
Codominance occurs when both alleles at a locus are fully and simultaneously expressed in the heterozygote. Unlike incomplete dominance where the phenotype is blended, codominant traits show both phenotypes independently. The AB blood type is the classic example: individuals with genotype IA IB express both A and B antigens on their red blood cells.
Both produce three phenotypes from three genotypes (1:2:1 ratio), but the heterozygote differs. In incomplete dominance, the heterozygote shows a blended intermediate (red × white = pink). In codominance, the heterozygote shows both parental phenotypes simultaneously (red × white = red AND white patches, as in roan cattle).
ABO blood type involves both codominance and simple dominance. Alleles IA and IB are codominant with each other (AB blood type expresses both), but both IA and IB are dominant over the i allele (which produces type O). This is a multi-allele system with three alleles: IA, IB, and i.
Roboculator Team
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