Sex Linked Inheritance in Human Beings

In humans, about 120 sex-linked genetic disorders are found. These are classified as X-linked, Y-linked, and X-Y-linked trits. The common sex-linked disorders that are mostly found in humans are mostly recessive - Color-blindness and Haemophilia.

Colour-blindness in human

It is a defect in which a person cannot distinguish between red, green or both the colours from other colours. The colour is perceived in cone cells of the retina of the eye. This cone cell contains three pigments which absorb a particular wavelength of light. That means one absorbs blue, second absorbs red, and the third absorbs green light. Human eye detects only three colours- red, green, and blue. But the brain mixes the signals coming from cone cells to develop the wide spectrum of colours that we perceive.

Each of the three pigments is encoded by a gene of the separate locus. The locus for the blue pigment is found on Y and those for green and red pigments lie on X-chromosomes. Human colour blindness is caused by defects of the red and green pigments; called as red-green colour blindness.Mutations produce this defect.

Color-blindness is a recessive and X-linked trait. That is why it is recessive to normal vision and only gets inherited through X-chromosome. Therefore, it shows criss-cross inheritance. Genes for this character is located on the X-chromosome but not on Y-chromosome. So, the trait is transferred from father(sufferer) to his daughters and carrier mother to her sons.

⇒CROSS#1⇒A cross between colorblind man with a normal colour vision woman

When a colorblind man (X^CY) marries a woman homozygous for normal colour vision (X⁺X⁺), all of the gametes produced by the woman will contain an allele for normal colour vision. Half of man's gametes will receive the X⁺ chromosome with colorblind allele (X^C), and the other half will receive the Y chromosome, which carries no alleles affecting colour vision (Y).

When an X^C bearing sperm unites with the X⁺ bearing egg, a heterozygous daughters with normal vision ( X⁺X^C) are produced. These are called carriers. In F₁ generation, the daughters and sons have normal colour vision.

When these F₁daughters (carrier) are married to a normal man, 50% of the sons are colorblind and remaining 50% of the sons are normal, while all the daughters are normal.

⇒CROSS#2⇒A cross between a colorblind woman with a normal colour vision male

When a colorblind woman (X^CX^C) marries a man with a normal colour vision (X⁺Y), in F₁ generation their daughters are normal visioned but are carries(X^CX⁺) whereas the sons are colorblind (X^CY).

When these F₁ daughters (X^CX⁺) marries with the colorblind man (X^CY), this the colorblind sons and daughters are produced in equal number. The cross is shown below:

Haemophilia (Bleeder's disease)

Haemophilia is called a royal disease and known as the most serious of all the diseases. A person suffering from this disease have the inability of their blood to clot normally even after a minor injury. It is due to the lack of a blood protein called clotting factor VIII and clotting factor IX.It was described by Otto in 1803, Hay in 1813), and Buels in 1815. Haemophilia is of two types -

• Haemophilia A - It is caused due to the deficiency of clotting factor VIII.
• Haemophilia B - It is caused due to the deficiency of clotting factor IX.

Haemophilia is a sex-linked trait and shows a criss-cross pattern of inheritance i:e the gene for the disease is inherited from mother to son and father to daughter (which is generally not possible due to the non-viability of haemophilic male). As such, haemophilia in females is very rare.

⇒CROSS#1⇒A cross between normal woman and haemophilic male:

If a normal woman (XX) marries with a haemophiliac male (X^hY), 2 normal sons and 2 carrier daughters are produced in F₁ generation. Haemophilia does not appear in any of the children. Haemophilia bearing X-chromosome of the male is directly passed into the daughters only. So, the daughter becomes the carrier (XX^h). Similarly, sons receice Y-chromosome from the father (which does not carry the gene for haemophilia) and normal X-chromosome from the mother. Therefore, the sons become normal.

⇒Cross#2⇒A cross between normal male and carrier woman:

If a normal male (XY) marries with a carrier woman (XX^h), four types of children XX, XX^h, X^hY, XY are produced. Among sons, 50% of them receive the defective gene for haemophilia (X^hY) suffers from the disease.However, among daughters, 50% of them receive the defective gene for haemophilia through X^h chromosome of their mother. Therefore, such daughter becomes carrier for haemophilia (XX^h).

⇒CROSS#3⇒A cross between haemophilic man and carrier female:

If a haemophilic man (X^hY) marries with a carrier woman (XX^h), 50% of the sons are normal and 50% sons are haemophilic. Similarly, 50% of the daughters are carriers and 50% daughters are haemophilic. For a female to have the disease herself, she must be homozygous for the recessive gene (X^hX^h), but this condition is usually fatal in utero(typically resulting in a natural abortion).

History of haemophilia

The history of the disease had its origin in the royal family of England. Queen Victoria (1819-1901) was a carrier of haemophilia which was transmitted to three of her nine children and to at least seven of her grand children. There was no history of haemophilia in her mother's or her father's families, so most probably, a mutation occurred prior to her conception in one of her partners or her natural father who was a haemophiliac. The transmission of this royal haemophilia was made to the son of last Czar of Russia and also the Prince of Spain probably from the Queen.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.