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The chart shows the inheritance of a condition like phenylketonuria as an autosomal recessive trait. Parents need to be heterozygous as two of their children are known to be sufferers of the disease. It cannot be recessive sex-linked inheritance because then the male parent would also be a sufferer.

1. It is due to mutation.
2. By introducing bone marrow cells producing ADA into cells at early embryonic stages, the genetic defect adenosine deaminase (ADA) deficiency may be cured permanently.
3. Sickle cell anaemia is caused by a change in a single base pair of DNA. Sickle cell anaemia is controlled by a single pair of alleles, HbA and HbS and the genotype HbS HbS show the diseased phenotype.
4. Sickle cell disease or sickle cell anaemia (or drepanocytosis), is a life-long blood disorder characterised by red blood cells that assume an abnormal, rigid, sickle shape.

Correct answer: Sickle cell anaemia is caused by a change in a single base pair of DNA. Sickle cell anaemia is controlled by a single pair of alleles, HbA and HbS and the genotype HbS HbS show the diseased phenotype.

Solution

The pedigree description fits an autosomal recessive trait, so affected individuals must be homozygous recessive while parents can be heterozygous carriers. The option about sickle cell anaemia correctly states it is caused by a single base-pair change and that the diseased phenotype appears in the homozygous HbS HbS genotype.

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