Allele (alternative form of a gene) that will show in the (phenotype observed characteristics of an organism) only if its partner allele on the paired chromosome is identical. Such an allele will not show if its partner is of a different, dominant type, that is if the organism is heterozygous for a particular characteristic. Alleles for blue eyes in humans and for shortness in pea plants are recessive. Most mutant alleles are recessive and therefore are only rarely expressed (see haemophilia).
For every characteristic of a plant or animal that is inherited, there are two genes present in the cells determining this characteristic in all but a few examples. ‘Characteristic’ refers to traits such as ‘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 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 not expressed in this case will only be expressed when present in the homozygous state. This characteristic is said to be recessive. Sometimes the allele that produces the dominant characteristic is described as being a dominant allele and the one that tends to produce the recessive characteristic as being the recessive allele. This is not really the correct use of the terms dominant and 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 will have an alternative allele that tends to produce brown eye colour. In 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.
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