allele

One of two or more alternative forms of a gene at a given position (locus) on a chromosome, caused by a difference in the sequence of DNA. This is best explained with examples. A gene which controls eye colour in humans may have two alternative forms – an allele that can produce blue eyes, and an allele that produces brown eyes. In a plant that occurs in tall and short forms, there may be an allele that tends to produce tall plants and an alternative allele that produces short plants. The gene for tallness and the gene for bluel eyes do not represent a pair of alleles – they are not alternatives.

The individual genes that form a pair of alleles are located at exactly the same point along a chromosome. Organisms with two sets of chromosomes (diploids), such as animals and plants, have chromosomes that are found as matching pairs in the nucleus of each cell. This means that there will always be two genes for a characteristic in a cell. If the same allele is present twice, the organism is said to be homozygous for this characteristic. If, however, one chromosome contains one allele and the other chromosome a contrasting allele, the organism is said to be heterozygous.

In a heterozygous organism the appearance of the organism (phenotype) may be determined by one allele and not the other. The allele that determines the phenotype is said to be dominantly expressed; it shows dominance over other alleles. The expression of the other allele is described as being recessive.