This is the pedigree of a family movement of the gene for haemophilia. Explain the pattern of inheritance of the disease in the family.

Explanation

In the given pedigree chart of a family the male is the affected individual in the parent generation. He marries a female, who is phenotypically normal. They produce two daughters which are also phenotypically normal. The first daughter, marries a normal man and produces a male progeny, which is affected by haemophilia. The second daughter who is also a carrier (XX") murries a normal man and produces a daughter who is phenotypically normal or may be carrier `(X X X X^h)`. The above pedigree analysis shows that the daughters are carriers of haemophilia `(X X^h)` and passes their `X//X^h` chromosome to their son and daughter. The males always receive X-chromosome from their mother and Y-chromosome from their father. The carrier daughter `(XX^h)` receivès one normal X-chromosome from their mother and the other Xh chromosome from their father. Since the second daughter is also phenotypically normal but carrier `(XX^h)`. It shows that their mother is normal (XX).
This pattern of inheritance of sex-linked diseases is called criss-cross inheritance i.e. male (Father) transfers the sex-linked characters to his grandsons through his daughters.