Final answer:
Different versions of genes are called alleles. They are alternative forms of a gene that cause genetic variation among individuals through the combination of two alleles, one from each parent, leading to either homozygous or heterozygous expressions of a trait.
Step-by-step explanation:
Genes come in different versions known as alleles. These alleles are alternative forms of a gene that contribute to the genetic variation among individuals. In humans, differences in individual DNA base pairs within alleles are largely responsible for this variation. For instance, the gene that determines blood type in humans can have three allele sequences - A, B, or O. A person inherits one allele from each parent and the combination of these two alleles determines the individual's blood type. For example, possible combinations can be AA, BB, OO (homozygous), or AB, AO, BO (heterozygous).
Each parent contributes one allele for each gene, and these alleles can be identical or different, resulting in organisms being homozygous or heterozygous for a trait. This variation in allele combinations leads to the diversity of traits observed within a species. The principles of how these alleles interact and express a particular trait were first described by Gregor Mendel and form the basis of Mendelian genetics.