SEX LINKED INHERITANCE

·         The chromosomes present in the diploid cells of the majority of the sexually reproducing animals are of two types: autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex.

·         Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia.

·         Such genes which are always associated with sex chromosomes are called sex-linked genes.

·         In man and Drosophila the sex chromosomes (X and Y) are unequal in size and shape, X being larger and rod shaped whereas Y is small and slightly curved.

·         In birds and butterflies the sex chromosomes (Z and W) are also unequal in shape and size, Z being larger than W.

·         In Mendelian pattern of inheritance, the genes for contrasting characters were located on autosomes but not on the sex chromosomes.

·         Secondly, the result of reciprocal cross is same as normal cross which is not the case with sex linked inheritance.

·          There are three types of sex-linked genes depending upon their association with particular chromosome.

·         They are as follows:

·         (i) The genes which are located on X-chromosomes are called X-linked genes or sex linked genes.

·         (ii) The genes which are located on Y chromosomes are called Y-linked genes or holandric genes.

·         (iii) Certain genes are found to occur in both X and Y chromosomes. Such genes are called incomplete sex-linked genes.

·         Characteristics of Sex Linked Inheritance:

·         (a) It is a criss-cross inheritance as the father passes its sex-linked character to his daughter who in turn passes it to the grandson.

·         (b) Daughter does not express the recessive trait but act as carrier in the heterozygous condition.

·         (c) Female homozygous for recessive trait expresses the trait.

·         (d) Any recessive gene borne by the X chromosome of male is immediately expressed as Y chromosome has no allele to counteract.

Sex Linked inheritance in Man:

In man about fifty six sex-linked genes have been reported, the most common examples are:

1. Red green colour blindness.

2. Haemophilia.

1. Red Green Colour Blindness:

·         Colour blindness is an example of sex linked character.

·         Those who suffer from red green colour blindness cannot distinguish between red and green colour.

·         The gene for this defect is located on X chromosome. It was first studied by Horner (1876).

·         Colour blindness is recessive to normal vision.

(i) Normal Woman and Colour Blind Man:

·         When a normal woman is married to a colour blind man, their children (daughters and sons) have normal colour vision.

·         But when their daughters were married to normal man, 50% of their sons are colour blind and the remaining 50% are normal, while the daughters were all normal.

(ii) Colour Blind Woman and Normal Man:

·         If a colour blind woman marries a normal man, their daughters are normal but all their sons are colour-blind.

·         When these F1 daughters are married to colour blind men, colour blind sons and daughters are born in equal number.

2. Haemophilia (Bleeder’s Disease):

·         Haemophila is another popular example of sex linked inheritance in human beings.

·         It is caused by a mutant gene (h) present in X chromosome and recessive to normal gene and is, therefore, suppressed in heterozygous condition.

·         Individuals suffering from this disease lack a factor responsible for clotting of blood.

·          So in the absence of blood clotting substance, a minor cut or injury may cause prolonged bleeding leading to death.

·          This disease in man is generally restricted to male members.

·         If a haemophilic man marries a normal woman, the daughter are all carriers (phenotypically normal but carries haemophilic gene in one on her X chromosome) but sons are normal.

·         Such a carrier daughter, when marries a normal man transmits the haemophilic gene to half of her son.

·         A haemophilic woman is produced only if a carrier woman is married to a haemophilic man.

SEX LINKED INHERITANCE OF HAEMOPHILIA

·         Haemophilia is also called ‘Royal disease’ as it is found in certain royal families of Europe.

·         Apparently the gene for haemophilia (h) arose as a mutation in a reproductive cell which produced Queen Victoria of England.