A cross between a normal couple resulted in a son who was haemophilic and a normal daughter. In course of time, when the daughter was married to a normal man, to their surprise, the grandson was also haemophilic.
(a) Represent this cross in the form of a pedigree chart. Give the genotypes of the daughter and her husband.
(b) Write the conclusion you draw of the inheritance pattern of this disease.

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Solution

(a) Image has been attached.
Genotype of daughter- $X X^{h}$
Genotype of husband- XY
(b) Haemophilia is a sex-linked recessive disease. Here, it is transmitted from the carrier female to the sons. From the above pedigree chart, it can be observed that the disease is being transmitted from the carrier female to her daughter (carrier) and son (affected). The carrier daughter transmits this disease to the grandson. This pattern of inheritance is called crisscross inheritance.

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