Different forms of a gene are known as alleles. Alleles are classified as dominant or recessive based on the features they influence.
Since human cells contain two copies of each chromosome, each gene exists in two forms. These variants of a gene are known as alleles. Alleles can be either recessive or dominant.
Even if a person carries just one copy of the gene, the dominant allele's effect will manifest (also known as being heterozygous). For instance, the gene for brown eyes is dominant, thus only one copy of this allele is required to have brown eyes (although, with two copies you will still have brown eyes).
When both alleles are dominant, the phenomenon is known as codominance. The resultant trait is the outcome of both alleles being equally expressed. This is illustrated by the blood group AB, which results from the codominance of the A and B dominant alleles.
Recessive alleles only manifest if a person carries two copies of the allele (also known as being homozygous). For instance, the allele for blue eyes is recessive, hence to have blue eyes, two copies of the 'blue eye' allele are required.
Some genes are located on the X chromosome, which is the sex chromosome. These genes are transmitted by the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
Males have one X chromosome and one Y chromosome, whereas females have two X chromosomes (XX) (XY). This indicates that females have two alleles for X-linked genes, while males have just one.
Some genetic illnesses, such as haemophilia, are caused by genes associated with sex. Two copies of the recessive haemophilia allele are required for a female to develop the illness.
Due to the fact that males only have one X chromosome, they only require one copy of the haemophilia allele to be affected by the illness. This indicates that haemophilia is far more prevalent in men than in women.
Functioning allele = H
Haemophilia allele = h
XH XH = healthy female
XH Xh = carrier female
Xh Xh = haemophilia female
XH Y = healthy male
Xh Y = haemophilia male