If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. Though the term allele was originally used to describe variation among genes, it now also refers to variation among non-coding DNA sequences.

An organism in which the two copies of the gene are identical — that is, have the same allele — is called homozygous for that gene. An organism which has two different alleles of the gene is called heterozygous.

Explained in greater detail, each gene resides at a specific locus (location on a chromosome) in two copies, one copy of the gene inherited from each parent. The copies, however, are not necessarily the same. When the copies of a gene differ from each other, they are known as alleles.

Function of Chromatids Chromatids allow cells to store two copies of their information in preparation for cell division. This is vital to ensure that daughter cells are healthy and fully functional, carrying a full complement of the parent cells’ DNA.

Chromatids help the cells to duplicate and in turn, aid in cell division. A chromosome is present throughout the life cycle of the cell. A chromatid is formed only when the cell undergoes through either mitosis or meiosis stages. Sister chromatids, on the other hand, are identical copies of one another.

A pair of sister chromatids is called a dyad. Once sister chromatids have separated (during the anaphase of mitosis or the anaphase II of meiosis during sexual reproduction), they are again called chromosomes, each having the same genetic mass as one of the individual chromatids that made up its parent.