19 10, American geneticist T.H. Morgan proved that genes located on the same chromosome are linkage groups. Eukaryotic cells are mostly diploid cells, and the chromosomes in the cells exist in pairs and are homologous chromosomes. But each germ cell has only one chromosome, so it is a haploid cell. Every gene in diploid cells also exists in pairs, and each pair of genes is located in the same position on the chromosomes from both parents, which is called a locus. A pair of genes at the same locus of a pair of homologous chromosomes are called a pair of alleles. For example, the tall stem gene and short stem gene of pea are alleles. If a diploid organism has a pair of different alleles, it is a heterozygote of the gene, and vice versa. If only one allele in a pair of heterozygotes can express a trait and the other allele cannot, the former is called a dominant gene and the latter is called a recessive gene. If a pair of alleles are expressed at the same time, it is called codominance.

For individuals, a pair of homologous chromosomes has only one pair of alleles at one locus. However, in a biological population, when a locus has more than two alleles, it is called multiple alleles. For example, there are three alleles that determine human ABO blood group system, namely A, B and I. As far as everyone is concerned, these three multiple alleles are only possible with 1 or 2, thus showing a specific blood group. Here a and b are dominant to I, a and b are co-dominant, and I is recessive.