Friday, September 25, 2009

Sex Linked Inheritance

A form of inheritance in which the transmission of the genetic material is correlated with the sex of the parents is called sex linked inheritance or any genetic trait which is transmitted through sex chromosomes is called sex linked inheritance.

Some genes of the body character are located on x chromosomes of the sex. Inheritance of such genes is known as sex linked inheritance e.g. red and white eye colour in Drosophila is a sex linked character. Similarly light yellow and bright yellow colour of the body of Drosophila is also sex linked character. In case of chickens the feathers may not be normal in structure and the condition is known as barred feathers. This is also a sex linked character. In human beings colour blindness (recessive) and haemophilia disease (recessive) are also sex linked.

Drosophila melanogaster (Black bellied dew lover) is a common fruit fly which can be seen hovering around over ripe fruits.

T. H. Morgan, a noble prize winner in 1933 first selected this fly as his experiments animal. He noted that the male and female Drosophila have differences in the chromosomes.

This shows that there are three pairs of chromosomes which are some in male as well as female fly.

These are called as autosomes but the differences lies in 4th pair. The female has both the chromosomes of the 4th pair similar and rod shaped. On the other hand male has both the chromosomes different from one another. One chromosome is rod shaped and other is hook shaped. This fourth pair of chromosomes has been designed as sex chromosomes because this pair is going to decide the sex in Drosophila. More over the rod shaped sex chromosomes, two of the female and one of the male, which are alike are called x chromosomes. The unlike sex chromosome is called Y. Drosophila individual getting XX will be a female and that receiving XY will be a female.

CROSS I

When red eyed female (XX) is crossed with white eyed male (XY), F1 generation shows all red flies, female as well as male. F2 generation shows red eyed and white eyed flies in ration of 3: 1 (all females are red but ½ male red and ½ male white eyed). Again female flies were of two types. One producing only red type offspring and another producing half red eyed and other half white eyed offsprings.

CROSS II

When red eyed male (XY) is crossed with white eyed female (XX) results are different from Cross I.

In F1 both types of flies were produced that is red eyed and white eyed. More over all the red eyed are females and all the white eyed males. In F2 generation again red and white eyed appeared in equal ratio and the result become half of male were red eyed and half white eyed. Similar in the case with female flies.

T. H. Morgan on the basis of results obtained from his experiments concluded that eye colour in Drosophila is present in X chromosome, and Y chromosome carries no allele for eye colour.

SIX LINKED INHERITANCE IN MAN

In human beings male has XY sex chromosomes while female has XX chromosomes. Total number of chromosomes is 46.

(a) Colour blindness: It is sex linked inheritance found in human beings. Person suffering from colour blindness have difficulty in distinguishing red from green. It is common trait and more common in males. It is because Y chromosome is inert for this trait, only one gene of this trait will render a colour blind man. While on the other hand a woman must have two genes for this trait to because colour blind. This trait of colour blindness can easily be detected by using special charts made up of a number of coloured dots so arranged what colour blind person see different pattern than other person do.

Possible results of various crosses are as under

(b) Haemophilia: It is sex linked inheritance found in human beings. It is a defect in which the blood fails to clot after external or internal injury, or it clots very slowly. Persons with extreme cases can be bleeding to death from even a small cut. Thus it is very serious or evens a lethal defect.Haemophilia is comparatively very rare in woman. The reason is the same. Y chromosome is inert for this trait. A man needs only one gene controlling this trait, while a woman needs two genes. This occurs very rarely because few haemophilic male survive and reproduce.

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