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The problem states that normal haemoglobin is Hb^A and abnormal haemoglobin (sickle-cell) is Hb^S. Sickle-cell anaemia is due to a recessive gene, meaning Hb^S is recessive to Hb^A.

Biology

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  1. Define the alleles and genotypes.

    The problem states that normal haemoglobin is HbA^A and abnormal haemoglobin (sickle-cell) is HbS^S. Sickle-cell anaemia is due to a recessive gene, meaning HbS^S is recessive to HbA^A.

    Genotypes and their corresponding phenotypes:

    • HbA^AHbA^A: Normal (no sickle-cell trait)
    • HbA^AHbS^S: Sickle-cell trait (carrier)
    • HbS^SHbS^S: Sickle-cell anaemia (affected)

    The cross is between a man who is heterozygous and a woman who is also heterozygous.
    Parental genotypes: Man (HbA^AHbS^S) ×\times Woman (HbA^AHbS^S)

    a) What are the possible phenotypes of the offspring of a man who is heterozygous and a woman who is also heterozygous?

  2. Perform the cross using a Punnett square.

    Gametes from each parent are HbA^A and HbS^S.

    GametesHbAHbSHbAHbAHbAHbAHbSHbSHbAHbSHbSHbS\begin{array}{|c|c|c|} \hline \text{Gametes} & \text{Hb}^A & \text{Hb}^S \\ \hline \text{Hb}^A & \text{Hb}^A\text{Hb}^A & \text{Hb}^A\text{Hb}^S \\ \hline \text{Hb}^S & \text{Hb}^A\text{Hb}^S & \text{Hb}^S\text{Hb}^S \\ \hline \end{array}

  3. Determine the phenotypes from the genotypes.

    • HbA^AHbA^A: Normal
    • HbA^AHbS^S: Sickle-cell trait
    • HbS^SHbS^S: Sickle-cell anaemia

    The possible phenotypes of the offspring are: Normal, Sickle-cell trait, Sickle-cell anaemia\boxed{\text{Normal, Sickle-cell trait, Sickle-cell anaemia}}

    b) What proportion of the offspring would have been:

    b) (i) Sickle-cell anaemia?
    Sickle-cell anaemia corresponds to the genotype HbS^SHbS^S.
    From the Punnett square, 1 out of 4 offspring has the genotype HbS^SHbS^S.

    The proportion of offspring with sickle-cell anaemia is: 14\boxed{\frac{1}{4}}

    b) (ii) Sickle-cell trait?
    Sickle-cell trait corresponds to the genotype HbA^AHbS^S.
    From the Punnett square, 2 out of 4 offspring have the genotype HbA^AHbS^S.

    The proportion of offspring with sickle-cell trait is: 24 or 12\boxed{\frac{2}{4} \text{ or } \frac{1}{2}}

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