The different forms of a gene. Y and y are different alleles of the gene that determines seed color.
Alleles occupy the same locus, or position, on chromosomes.
A locus on any chromosome but a sex chromosome. Two different alleles at a locus are responsible for different phenotypes, and both alleles affect the phenotype of the heterozygote. For example, consider the situation where there are three alleles A,B, and O that determine human blood type. Both the A and B alleles contribute to the phenotype of the heterozygote. Thus the alleles A and B are said to be co-dominant.