Allele

Allele:

https://www.thoughtco.com/allele-a-genetics-definition-373460

An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent.

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Dominant and Recessive Alleles:

https://www.thoughtco.com/allele-a-genetics-definition-373460

Diploid organisms typically have two alleles for a trait. When allele pairs are the same, they are homozygous. When the alleles of a pair are heterozygous, the phenotype of one trait may be dominant and the other recessive. The dominant allele is expressed and the recessive allele is masked. This is known as complete genetic dominance. In heterozygous relationships where neither allele is dominant but both are completely expressed, the alleles are considered to be co-dominant. Co-dominance is exemplified in AB blood type inheritance. When one allele is not completely dominant over the other, the alleles are said to express incomplete dominance. Incomplete dominance is exhibited in pink flower color inheritance from red and white tulips.

Multiple Alleles:

https://www.thoughtco.com/allele-a-genetics-definition-373460

While most genes exist in two allele forms, some have multiple alleles for a trait. A common example of this in humans is ABO blood type. Human blood type is determined by the presence or absence of certain identifiers, called antigens, on the surface of red blood cells. Individuals with blood type A have A antigens on blood cell surfaces, those with type B have B antigens, and those with type O have no antigens. ABO blood types exist as three alleles, which are represented as (IA, IB, IO). These multiple alleles are passed from parent to offspring such that one allele is inherited from each parent. There are four phenotypes (A, B, AB, or O) and six possible genotypes for human ABO blood groups.

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