Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.
Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
Alleles are dominant or recessive:
The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.
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