XX-XY chromosomes are primarily concerned with the determination of sex. These do carry some genes for other body characters. Such body characters whose genes are located on the sex chromosomes and follow sex during inheritance are known as ‘sex -linked characters. The genes governing the sex linked characters are called ‘sex- linked genes.’ The mode of inheritance by the sex-linked genes is described as sex-linked inheritance.
The genes present on the X-chromosome are called sex-linked genes or X-linked genes and the genes occur on Y-chromosome are known as Holandric genes. Holandric genes are present only in males. Certain characters are not sex-linked but are limited to one sex. These are described as sex-limited characters and sex-influenced characters.
One of the earliest known instance of sex-linked character is the Bleeder’s disease-Hemophilia’ found only in males in the royal family of Spain. In 1910, Thomas Hunt Morgan introduced the concept of sex-linked inheritance while working on Drosophila melanogaster.
Approximately twenty characters of man exhibit sex-linked inheritance. The most popular examples of sex-linked inheritance in Man are
1. Red-green colour blindness and 2. Hemophilia.
Red- green colour blindness
Some persons unable to distinguish certain colours are called color-blind people. The most common one is “Red-green colour blindness It has been described by Homer in 1876. The red blindness is termed as ‘protanopia’and the green blindness as deutaronopia.X-chromosome possesses a normal gene which controls the formation of colour sensitive cells in the retina. Its recessive allele fails to do its job properly and results in colour blindness. These alleles are present on the X-chromosome is evidenced by the examples.
When a normal woman is married to a colorblind man, all her sons and daughters have normal vision. But when her daughters are married with normal colour vision man, some colour-blind sons are formed. It means that a woman with normal colour vision whose father is colourblind gives birth to children of which about half of the sons are colourblind and other half are normal colour vision.
In other instance, if a colour-blind woman married to a normal colour vision men, all her sons are colour-blind where as all the daughters have normal colour vision. When these daughters having normal colour vision are married to colour-blind man, the colour-blind grand-sons and colourblind grand-daughters are produced with almost equal number of normal grandsons and grand-daughters It means that a colour-blind woman has sons all colour-blind and all daughters with normal colour vision. A colour-blind woman has always a colour-blind father and her mother is carrier. It is known as criss-cross inheritance.
The above results could easily be explained with the assumption that colour vision is sex-linked character and its gene is present on the Xchromosome. Y-chromosome lacks its allele. In man sex-chromosomes are X,Y while in woman these are X & X. It means that a man possesses only one gene for colour vision, whereas woman possesses two. The normal colour vision is dominant over colour-blindness. Therefore, woman will be colour-blind only when she possesses a gene for colour-blindness in both the X-chromosomes, whereas a man will be colour-blind even when this gene is present singly.
Always male receives its X-chromosome from mother (through ovum) and V-chromosome from father (through sperm), whereas female receives one K-chromosome from each parent (through ovum and sperm).
From the above results following conclusions may be observed.
(i) Colour-blindness is more common In males than in females.
(ii) Two recessive genes are required for the expression of ‘colorblindness in female, whereas only one gene gains expression In male.
(iii) Males are never carriers.
(iv) Colour-blind woman always have colour blind fathers and always produce colour-blind. sons.
(v) Colour-blind women pciuce colour-blind daughters only when their husbands are colour-blind.
(vi) Woman with normal colour Vision, whose fathers are colorblind, produce colour-blind and, normal sons in approximately equal proportion.