Take two parental individuals (mom and dad). One is RED and the other is YELLOW (these colors would be their phenotypes). In this case, both parentals are homozygous (both alleles are identical) for locus A (gene A). When they are crossed, each can only contribute one type of allele (A or a). For this locus, A and a are co-dominant. Thus, their combination will produce an intermediate ORANGE phenotype, compared to that observed in the parentals.
Co-dominant Inheritance
Allele
A
a
P
AA
X
aa
F1
Aa
The result of the cross is a first generation (F1) in which all individuals are heterozygotes, and have a ORANGE phenotype.
F1
Aa
X
Aa
When one crosses F1 individuals, each can contribute either the A or the a allele.
A
a
A
AA
Aa
a
Aa
aa
F2 Phenotypic frequencies - 1:2:1
The result of the cross is a second generation (F2) in which ¼ of the individual are homozygous for the A allele, ½ are heterozygous, and ¼ of the individual are homozygous for the a allele. This generates a genotypic frequency of 1:2:1. The A allele homozygotes will have a RED phenotype, the heterozygotes will have an ORANGE phenotype, and the a homozygotes will have a YELLOW phenotype.
Please note that when inheritance is co-dominant, the genotypic and phenotypic frequencies match.
