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  Section: Genetics » Sex Linked, Sex Influenced and Sex Limited Traits
 
 
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Sex linkage in human beings

 
     
 
Content
Sex Linked, Sex Influenced and Sex Limited Traits
Sex linked traits 
Sex linkage in Drosophila
Sex linked lethals in Drosophila
Sex linkage in human beings
Sex linkage in poultry
Non-disjunction of sex chromosomes in Drosophila
Sex Influenced Traits (horned character in sheep)
Sex Limited Traits (cock feathering in poultry and baldness in man).
In human beings, there are 46 chromosomes (23 pairs) present in each somatic cell. In female individuals, there are 22 pairs of autosomes and one pair of X-chromosomes (22 pair + XX) and in male individuals, there are 22 pairs of autosomes and one X and one Y chromosome (22 pairs + XY). Since female will produce only one type of gametes, gametes from male individual will determine the sex of the progeny (Fig. 16.8). Sex linked characters in human beings will, therefore, follow the same pattern as in Drosophila. Two examples of sex linked characters in human beings namely colour blindness and the disease hemophilia would be discussed in this section.

Determination of sex by Y-chromosome in human beings.
Fig. 16.8. Determination of sex by Y-chromosome in human beings.
 
Inheritance of sex linked colour blindness in human beings.
Fig. 16.9. Inheritance of sex linked colour blindness in human beings.


Colour blindness. A particular traits in human beings renders them unable to differentiate between red colour and green colour. The gene for this red green colour blindness is located on X-chromosome. Colour blindness is recessive to normal vision so that if colour blind man marries a girl who is normal (homozygous) for this character, sons will be normal, but daughters will be heterozygous (normal phenotype), which means that these daughters would be carriers of this trait. If such a carrier girl marries a colour blind man, 50% of female progeny and 50% of male progeny would be colour blind (Fig. 16.9).

Hemophilia. Hemophilia is another popular example of sex linked inheritance in human beings. It is recessive character and is, therefore, masked in heterozygous condition. Individuals suffering with this disease lack a factor responsible for clotting of blood. Consequently, even a minor cut may cause prolonged bleeding leading to death. Since it is a recessive character, a lady may carry the disease and would transmit the disease to 50% of her sons, even if the father is normal (Fig. 16.10).
 
Inheritance of the disease hemophilia in human beings.
Fig. 16.10. Inheritance of the disease hemophilia in human beings.

 
     
 
 
     




     
 
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