Step 1: Read the pedigree:
Squares are males and circles are females. Filled symbols are affected people.
The first pair of parents is a normal female and a normal male. Both are unaffected.
They have five children: an affected son, an affected daughter and three unaffected children (two sons and a daughter).
The next generation has one affected son and two unaffected daughters, born to an unaffected couple.
Step 2: Work out the mode of inheritance:
Two unaffected parents have an affected daughter. So the trait is recessive and both parents are carriers.
For an X-linked recessive trait, an affected daughter needs an affected father, because she must get one X from him.
Here the father is unaffected, so the trait cannot be X-linked. It must be autosomal recessive.
Step 3: Check each option:
(A) Sickle cell anaemia is autosomal recessive (a single base change in the beta globin gene on chromosome 11). It fits the pedigree. So (A) is correct.
(B) Haemophilia is X-linked recessive. An affected daughter from an unaffected father is not possible. So (B) is wrong.
(C) Colour blindness is also X-linked recessive. It fails for the same reason. So (C) is wrong.
(D) Down's syndrome is a chromosomal disorder (trisomy 21), not a Mendelian trait shown on a pedigree like this. So (D) is wrong.
Final Answer:
The pedigree shows an autosomal recessive disorder, which is sickle cell anaemia. This is option (A).
