We know that the parent with long antennae must be homozygous recessive (ss) in genotype, but at first sight the other parent might be either homozygous SS or heterozygous Ss. However, once they have mated, and we see that there are some offspring with long antennae, then we know that the other parent must have been heterozygous itself (see diagram). The phenotype ratio we would expect from a cross between ss and Ss is 50% long and 50% short.
