Question

Long answer type question.

Explain cris-cross inheritance with suitable example.

Answer 1

Criss-cross inheritance can be explained with the help of two examples: colour blindness and haemophilia.

1. i. Colour blindness:
   1. A person suffering from colour blindness cannot differentiate between red and green colours. Both these colours appear grey to the colour blind person.
   2. It is caused due to recessive X-linked genes (X^c) which prevent the formation of colour-sensitive cells in the retina that are necessary for distinguishing red and green colours.
   3. Dominant X linked gene (X^C) is necessary for the formation of colour-sensitive cells in the retina of the eye.
   4. The homozygous recessive females (X^cX^c) and hemizygous recessive male (X^cY) are unable to distinguish between red and green colours. The frequency of colour blind women is much less than colour blind men.
   5. When a normal man marries a carrier woman, half of their sons may be colour blind, while the remaining half will have normal vision. All their daughters will have normal vision and half of them will be carriers for the disease.
   6. If a colour blind male (X^cY) marries a female with normal vision (X^CX^C), then all the offsprings will have normal vision. The sons will have normal vision but daughters will be carriers for the disease. The carriers have normal vision.
2. Haemophilia (Bleeder’s disease):
   1.  Haemophilia is an X-linked recessive disorder in which blood fails to clot or coagulates very slowly.
   2. The genes for normal clotting are dominant over the recessive genes for haemophilia.
   3. The person having the recessive gene for haemophilia is deficient in clotting factors (VIII or IX) in blood.
   4. Even minor injuries cause continuous bleeding, hence haemophilia is also called bleeder’s disease.
   5. The recessive gene for haemophilia is located on a nonhomologous region of X chromosome.
   6. As there is no corresponding allele on Y chromosome to suppress its expression, so men suffer from this disease.
   7. Women suffer only when both X chromosomes have recessive genes (alleles).
3. If a haemophilic male (X^hY) marries a female with the normal clotting of blood (XX), then all the offsprings will show normal clotting of blood. The sons will have normal clotting of blood, but daughters will be carriers for the disease. The carriers have normal clotting of blood.
   
4. When carrier woman (X^HX^h) marries a normal man (X^HY), then all the daughters will have normal clotting of blood but half of them will be carriers for the disease. Half the sons will be haemophilic while the remaining will have normal clotting of blood.