What are dominant and recessive alleles?

• Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.
• Alleles can be either dominant or recessive.
• Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the ‘brown eye’ allele to have brown eyes (although, with two copies you will still have brown eyes).
• If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
• Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What are sex-linked genes?

• Some genes are found on the sex chromosome, X.
• These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• This means females have two alleles for X-linked genes while males only have one.
• Some genetic diseases, are caused by sex linked genes, for example haemophilia.
• The allele for haemophilia is recessive so two copies are needed for a female to have the disease
• However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease.
• This means haemophilia is much more common in males than in females.

For example:

Functioning allele = H

Haemophilia allele = h

X^H X^H = healthy female

X^H X^h = carrier female

X^h X^h = haemophilia female

X^H Y = healthy male

X^h Y = haemophilia male