Question

What is sex linkage? Explain colour blindness and haemophilia?

Answer 1

Sex linkage-when the genes are present on sex chromosome is termed as sex linked gene and such phenomena is known as sex linkage. Its discovery was done by Morgan, when he was working on inheritance of eye colour in Drosophila. Morgan after his work concluded that eye colour is linked to sex and is present on the X chromosome. X chromosome does not passed directly from one parent to the offspring of the same sex but following criss-cross inheritance,i.e., it is transferred from one sex to the offspring of the opposite sex. In other words, in criss- cross inheritance a male transmits his trade to his grandson through daughter while the female transmits the trade to her granddaughter through her son.

Complete answer:
Colour blindness and haemophilia are the two common example of sex linked disease in human being.
Haemophilia- It is also termed as bleeder's disease. The gene of haemophilia is recessive. Haemophilia are of 3 types-(1) Haemophilia A-due to lack of factor 8(Antihemophilic globulin AHG). It shoes lethality in homozygous condition.(2) Haemophilia B-due to lack of factor 9(Plasma Thromboplastin).(3) Haemophilia C-due to lack of factor 11(Plasma Thromboplastin antecedent). Due to the presence of defective form of blood clotting factors, exposed blood of affected individuals fails to coagulate.
Colour blindness- The inheritance of colour blindness is alike as haemophilia, but it is not a lethal disease so it is found in male and females. Discovered by Horner. There are 3 types of colour blindness are as follows-
(a) Protanopia:- It is for red colour.
(b) Deuteranopia:- It is for green colour.
(c) Tritanopia:-for blue colour blindness.
Colour blindness is checked by ishihara- chart. It should be understood that colour blindness does not mean you cannot see any colour at all, it means that those who are colour blind have trouble in seeing the difference between certain colours. It is also a criss-cross inherited and recessive sex-linked trait.

Note: There are 2 types of sex linkage. (1) X-linkage:- Eye colour in drosophila, Haemophilia, colour blindness, diabetes insipidus, Duchenne muscular dystrophy, pesudoricketes. (2) Y-linkage:- Gene which form TDF gene, Hypertrichosis (excessive hair on ear pinna). Gene which is located on the differential region of the Y - chromosome is known as the Holandric gene.