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  4. The recessive genes located on X-chromosomes of humans are always

Q. The recessive genes located on X-chromosomes of humans are always

 1810  226 AIPMTAIPMT 2004Principles of Inheritance and Variation Report Error

Solution:

The recessive genes located on X-chromosome of humans are always expressed in males e.g. colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene tor the nonnal vision is dominant. The normal gene and its recessive allele are carried by X-chromosomes. In females colour blindness appears only when both the sex chromosomes carry the recessive gene (XCXC). The females have nonnal vision but function as carrier if a single recessive gene for colour blindness is present (XXC). However, in human males the defect appears in the presence of a single recessive gene (XCY) because Y- chromosome of male does not carry any gene for colour vision.