(a)  Non-homologous chromosome The law of independent assortment holds true as long as two different genes are on separate chromoses. When the genes are on separate chromosome, the two alleles of one gene (A and a) will segregate into gametes independently of the two alleles of the other gene (B and b).

Equal numbers of four different gametes will form AB, aB, Ab, ab. But if the two genes are on the same chromosomes, then they will be linked and will segregate together during meiosis, producing only two kinds of gametes.

Genes A and B on different chromosomes         Genes A and B on some chromose

Homologous chromosomes are similar but not identical. Each carries the same gene in same order but the alleles for each trait may not be the same. Extranuclear genetic elements are also called as plasmids and shows the pattern of material inheritance.