Sex-Linked Inheritance

Characters carried on the X chromosome are said to be sex-linked.

Common examples of sex-linked traits are;

• Colour blindness.
• Red-green colour blindness.
• Haemophilia.
• Male pattern baldness.
• Adrenoleukodystrophy (ALD).

A person with red-green colour blindness cannot see red, instead, red and green objects appear green. A typical test for red-green colour blindness is a number made out of dots. If you can see the number then you have normal vision, with red-green colour blindness, the number is invisible.

If a sex-linked trait is due to a recessive mutation, a female will express the phenotype, only if she carries two mutated genes (X-X), if she carries only one mutated X-linked gene, she will be a carrier.

Males inherit only one sex-linked gene.

A male will express the character if he inherits a mutated X-linked gene. Males are not carriers. It is uncommon for a female to express a sex-linked condition because, in order to express it or be affected, she must have inherited a mutant gene from both parents.

Inheritance of Color Blindness – Carrier Mother and Father with Color Blindness:

Colour blindness is controlled by a recessive gene, let c represent the colour blindness gene and C represent the gene for normal vision.

1. X^C X^c – Carrier female 
2. X^C Y – Boy, Normal
3. X^c X^c – Girl – colour blind 
4. X^c Y – Boy – colour blind

A Colour blind person cannot distinguish near colours. It is an abnormality of the gene that controls the production of cone cells in the retina of the eye.

Inheritance of Haemophilia – Carrier Mother and Father without Haemophilia:

Haemophilia is a rare condition that affects the blood’s ability to clot. This can lead to spontaneous bleeding as well as bleeding following injuries or surgery. Blood contains many proteins called clotting factors that can help to stop bleeding. It’s usually inherited. Most people who have it are male. Let h represent the Haemophilia gene and N represent the gene without haemophilia.

1. X^N X^N – Daughter (does not carry the haemophilia gene)
2. X^N X^h – Daughter (carrier for haemophilia gene)
3. X^NY – Son without haemophilia
4. X^h Y – Son with haemophilia

• Probability of having a child with haemophilia = 25%
• Probability of having a son with haemophilia = 50%, as the other son will not suffer from this disorder.
• Probability of having a daughter with haemophilia = 0%

Example:

A haemophiliac man marries a healthy woman. What is the probability that they will have any haemophiliac children?

Let h represent the Haemophilia gene and N represent the gene without haemophilia.

1. X^N X^h – Daughter (carrier for haemophilia gene)
2. X^N X^h – Daughter (carrier for haemophilia gene)
3. X^NY – Son without haemophilia
4. X^NY – Son without haemophilia

Probability = 0%