12th Class Biology Principle Of Inheritance And Variation Sex Linked Inheritance

Sex chromosomes of some animals and man besides having genes for sex character also possess gene for non sexual (somatic) characters. These genes for non sexual characters being linked with sex chromosomes are carried with them from one generation to the other. Such non-sexual (somatic) characters linked with sex chromosomes are called sex linked characters or traits, genes for such characters are called sex linked genes and the inheritance of such characters is called sex linked inheritance. The concept of sex-linked inheritance was introduced by T. H. Morgan in 1910, while working on Drosophila melanogaster.

Genes for sex linked characters occur in both segments of X and Y chromosomes. Many sex linked characters (About 120) are found in man. Such characters are mostly recessive.

Types of sex linked inheritance

(1) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. Alleles of these genes do not occur on Y chromosome. Genes of such characters are transferred from father to his daughter and from his daughter to her sons in F₂ generation. This is known as Cris-cross inheritance. As the genes for most sex linked characters are located in X chromosome, they are called X-linked characters e.g., colour blindness and haemophilia in man and eye colour in Drosophila.

Sex linked inheritance in Drosophila : Drosophila melanogaster has XX and XY sex chromosomes in the female and male respectively. Its eye colour is sex linked.

Allele of the eye colour gene is located in the X chromosome, and there is no corresponding allele in the Y chromosome. The male expresses a sex-linked recessive trait even if it has a single gene for it, whereas the female expresses such a trait only if it has two genes for it. The normal eye colour is red and is dominant over the mutant white eye colour. The following crosses illustrate the inheritance of X-linked eye colour in Drosophila.

Sex linked inheritance in man : Colour blindness and Haemophila are the two main sex linked or X-linked disease are found in man.

Colour blindness : Person unable to distinguish certain colours are called colour blind. Several types of colour blindness are known but the most common one is ‘red-green colour blindness’. It has been described by Horner (1876).

The red blindness is called protanopia and the green blindness deutoranopia. X-chromosome possesses a normal gene which control 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 in X chromosome.

Inheritance of colourblindness

Haemophilia : In haemophilia the blood fails to clot when exposed to air and even a small skin injury results in continuous bleeding and can lead to death from loss of blood.

It is also called bleeder’s disease, first studied by John Cotto in 1803. The most famous pedigree of haemophilia was discovered by Haldane in the royal families of Europe. The pedigree started from Queen Victoria in the last century. In a patient of haemophilia blood is deficient due to lack necessary substrate, thromboplastin. It is of two types :

Haemophilia-A : Characterized by lack of antihaemophilic globulin (Factor VIII). About four-fifths of the cases of haemophilia are of this type.

Haemophilia-B : ‘Christmas disease’ (after the family in which it was first described in detail) results from a defect in Plasma Thromboplastic Component (PTC or Factor IX).

Like colour blindness, haemophilia is a well known disorder which is sex-linked recessive condition. The recessive X-linked gene for haemophilia shows characteristic Criss-cross inheritance like the gene for colour blindness. Its single gene in man results in disease haemophilia, whereas a woman needs two such genes for the same.

(2) Holandric or Y-linked traits : Genes for these characters are located on non-homologous segment of Y chromosome. Alleles of these genes do not occur on X chromosome. Such characters are inherited straight from father to son or male to male e.g. hypertrichosis of ears in man.

(3) XY-linked inheritance : The genes which occur in homologous sections of X and Y-chromosomes are called XY-linked genes and they have inheritance like the autosomal genes. e.g., Xeroderesia pigmentosa, Nephritis.

(4) Sex-influenced traits : The autosomal traits in which the dominant expression depends on the sex hormones of the individual are called sex-influenced traits. These traits differ from the sex limited traits which are expressed in only one sex. e.g., Baldness in man, Length of index finger.

(5) Sex limited traits : Traits or characters which develop only in one sex are called sex-limited characters. They are produced and controlled by the genes which may be located on autosomes in only one sex. Such genes are responsible for secondary sexual characters as well as primary sexual characters. They are inherited according to Mendel’s laws. e.g., Moustaches and beards in human males, breast in human females, milk secretion in human females.