So a certain place on chromosome where gene is, it's called locus. And gene contains two allels. So in my book is said that homologous chromosomes contain set of allels and this is what confuses me. So how come is that possible if a single chromosome contains genes and each gene contain two allels. In case of homologous chromosomes it should be 4 allels that is two pairs of allels or two genes because on each chromosome there is a similar gene right ?