In flies, red eyes (r) are dominant to brown eyes (r). a heterozygous fly is crossed with a homozygous recessive fly

The cross of a heterozygous fly with a homozygous fly can be shown as follows:

         r            r

R      Rr           Rr

r        rr            rr

The results show that there is a 50% probability that the flies produced will have red eyes. Their genotypes will be heterozygous.

The results of the punnet square show that there will be a 50% probability that the offsprings will have brown eyes. Their genotype will be homozygous recessive.

A homozygous trait can be described as a trait which has same alleles for the gene which encodes the trait. Heterozygous means that both the alleles for the trait will be different.