What is an allele? What are dominant and recessive alleles?

The nucleus of a cell contains DNA molecules, which code for proteins. In humans, DNA is arranged into 46 linear, double-stranded molecules called chromosomes. Chromosomes of similar length are paired up – forming 23 pairs of chromosomes.

 

In a pair of chromosomes, each chromosome contains the same genes in (roughly) the same places. (A gene is a sequence of DNA that encodes for a protein, which controls a particular trait of a person.) For example, chromosomes A and B are together in a pair. The middle of chromosome A has genes for eye colour. The middle of chromosome B also has genes for eye colour.

 

However, genes can vary slightly. (eg. A gene can code for blue eye colour or brown eye colour.) A different version of the same gene is called an allele. In a pair of chromosomes, a person can either have two different alleles (heterozygous) or two of the same (homozygous).

 

Some alleles can be expressed if the person only has one copy of it. These are called ‘dominant’ alleles. For example, if a person has one allele for brown eyes and one allele for blue eyes, they will have brown eyes. (If they have two alleles for brown eyes, they will also have brown eyes.)

 

Some alleles can only be expressed if the person has two copies of it. These are called ‘recessive’ alleles. For example, if a person has two alleles for blue eyes, they will have blue eyes.

 

Basically:

 

Dominant allele + Dominant allele = Dominant trait (Homozygous dominant)

 

Dominant allele + Recessive allele = Dominant trait (Heterozygous dominant)

 

Recessive allele + Recessive allele = Recessive trait (Heterozygous recessive)