dominance

In genetics, the masking of one allele (an alternative form of a gene) by another allele. For example, if a heterozygous person has one allele for blue eyes and one for brown eyes, his or her eye colour will be brown. The allele for blue eyes is described as recessive and the allele for brown eyes as dominant.

For every characteristic of a plant or animal that is inherited, there are always two genes present in the cells which determine the characteristic. By ‘characteristic’ is meant ‘height’ or ‘eye colour’ or ‘ability to make a particular enzyme’. If the two genes are identical (homozygous state) the characteristic seen in the organism is determined by either one of the two genes. However, one gene may be different from the other (heterozygous state). If so, the two genes are alleles - contrasting genes for a characteristic. In this case it is possible that one of them determines the characteristic seen and the other does not. The characteristic seen in this case is said to be dominant. The other allele, which does not contribute to the appearance of the organism in this case, will only be expressed and contribute to appearance when no dominant gene is present. This characteristic is said to be recessive.

An allele is one of two or more alternative forms of a gene. This is best explained with examples. A gene which tends to produce blue eyes in a person has an alternative allele that tends to produce brown eye colour. A plant that may be found in tall and short forms may have an allele that tends to produce tall plants though its alternative allele produces short plants. The gene for tallness and the gene for blue eyes are not alleles - they are not alternatives.