If two alleles on a gene (or locus) on homologous chromosomes are the same, then the individual is homozygous for this trait. If the alleles are different, the individual is heterozygote for the trait. In heterozygote pairing, the dominant allele makes the trait of recessive allele not displayed.
Some pairs of genes have two dominant members, that is, there is no dominant or recessive relationship between them. For example, human ABO blood group system: people with AB blood group have one allele to determine A and one allele to determine B (people who have neither A nor B alleles are O blood groups). Most traits are determined by more than two alleles.
Extended data
Different alleles can lead to changes in genetic characteristics, such as hair color or blood type. Alleles can be divided into different categories by controlling the dominant and recessive relationships and genetic effects of relative traits.
In an individual, one form of allele (dominant) can be expressed much more than other forms (recessive). For example, the locus of human Rh blood group gene is in the third and fifth regions of the short arm of chromosome 1, and RH and RH located at the same locus of two chromosomes 1 are a pair of alleles.
Baidu encyclopedia-allele