SEX LINKED INHERITANCE

 

The characters are controlled by genes. The genes are inherited through the chromosomes. The SEX LINKED INHERITANCEchromosomes are two types. These are autosomes and sex chromosomes. The autosomes are identical in both male and female organisms. But sex chromosomes which determinate sex are different in male and females.

The genes that lie on sex chromosomes are called sex linked genes. The traits that are controlled by the genes of sex chromosomes are called sex linked traits.

SEX LINKED INHERITANCE-DEFINITION

The transmission of the characters through the sex chromosomes T.H. Morgan to the progeny is called sex linked inheritance.

The sex linked inheritance was first observed by T.H. Morgan in Drosophila. The sex linked character observed by him is the eye colour.

SEX LINKAGE-EXAMPLES I

The sex chromosomes of Drosophila and human being are 'X' and *Y

A.Drosophila:

Sex linked trait is eye colour, transmitted through 'X' chromosome. Such eye colour controlling gene is absent on Y chromosome.

B.Human being:

Blood clotting and vision genes lie on 'X' chromosome. Such blood clotting and vision controlling genes are absent on 'Y' chromosome.

'Y' linked trait in male human being is hypertrichosis.

C. Wyandotte Fowl:

The sex chromosomes of birds are 'Z' and 'W. The feather colour controlling gene lies on 'Z' chromosome.

3. SEX LINKAGE-DROSOPHILA

Morgan first observed the sexlinked trait, eye colour in Drosophila. The eye colour in fruit fly (Drosophila) contains '2' phenotypes. They are red eye colour and white eye colour. The dominant red eye colour gene is represented as 'R' . Whereas white eye colour gene is represented as “r”. The eye colour controlling genes 'R' and r lie on 'x' chromosome.

A. Crossing of 'P' generation:

Morgan crossed pure red eyed female( xR XR) with white eyed male (x y), He got all red eyed progeny in 'Ft'.

Red eyed female produced only one kind of gametes( xR ). All female gametes carry red eyed gene through 'X' chromosome to the progeny. White eyed male produced two kinds of sperms. They are xr and Y type. In, F1, all female progeny is heterozygous.

SEX LINKAGE-DROSOPHILA

B. Crossing of 'F1 female and male:

When heterozygous red eyed female (xR Xr) is crossed with red eyed male (xR y) in' F2' homozygous red eyed, carrier red eyed females, red eyed and white eyed males were produced.

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The transmission of recessive white eye coloured gene from grand father to grand son through the daughter is called criss cross inheritance

Normally white eye coloured male flies are more than the number of white eyed female flies. It is because one recessive white allele is enough to cause white eyes in males. The white eye coloured female flies are rare. It is because female requires two white eye colour alleles, inherited from both the parents.

4. SEX LINKAGE IN MAN

A. COLOUR BLINDNESS:

The character vision contains two forms. These are colour vision and colour blind. The colour vision gene 'C is dominant over colour blind gene 'c'. These two genes are carried through 'X' chromosome. Colour blind ness is the inability of identification of red and green colours. Both appear as grey in colour.

COLOUR BLINDNESS

The different genotypes of the character vision are In a family, the 'X' chromosome of sons is inherited from mother. Father 'X' chromosome is inherited to daughters only. Daughters 'X' chromosomes are inherited from both the parents. Mother 'X' chromosomes are inherited to both daughters and sons.

i. Progeny produced from normal colour vision female and colour blind male:

When normal colour vision female marries a colour blind male, all the off springs are phenotypically colour vision. But the daughters are heterozygous or carriers for colour blind gene.

COLOUR BLINDNESS-1

ii. Progeny produced from carrier female and normal male:

When carrier female marries a colour vision male, half of the sons are colour blind and remaining half of the sons are normal. Where as half of the daughters are carriers and remaining half of the daughters are normal.

COLOUR BLINDNESS-2

iii. Progeny produced from colour blind female and normal male:

When colour blind female marries a normal colour vision male, all the sons produced are colour blind and all the daughters are carriers.

color blind

vi. Progeny produced from carrier female and color blind male:

When carrier female marries a colour blind man, 50% of the daughters and 50% of the sons are colour blind.

color blind -inheritance

B. HAEMOPHILIA:

The two contrasting of forms of the character blood are blood clotting and haemophilia (Bleeder's disease). The dominant blood clotting gene is represented as 'H'. Where as recessive haemophilic gene is represented as 'h'. Both 'H' and 'h' genes are carried"through 'X' chromosome.

The genotypes of the character blood are

HAEMOPHILIA

i. Progeny produced from normal female and hemophilic male:

When normal blood clotting female marries a hemophilic man, all the off springs produced are phenotypically blood clotting. But the daughters are heterozygous carriers. ii. Progeny produced from haemophilic female and blood clotting male:

When haemophilic female marries a normal blood clotting male, all the sons are haemophilic and daughters are carriers.

5. Y' CHROMOSOMAL INHERITANCE

In the human beings 'y' sex chromosome is present only in males. So, 'y' linked traits are found only in males. Such characters are called holandric characters. The y linked halandric character found in male human being is hyper trichosis (long bushy hair on ear lobes)

6. WYANDOTTE FOWL - FEATHER COLOUR

The character feather colour contains two contrasting forms. These are silver colour and golden yellow colour. The dominant silver feather colour gene'S' is dominant over golden yellow feather colour gene's'. The sex chromosomes in birds are 'Z' and 'w'. The sex chromosomal nature of female fowl is 'Zw'. The sex chromosomal nature of male fowl is 'ZZ'. The feather colour genes' S' and's' lie on 'Z' chromosome.

HAEMOPHILIA

When sliver feather coloured hen is crossed with golden yellow coloured male , the female progeny produced is golden yellow coloured and the male progeny is sliver feather coloured.

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