Genetics

The question involves understanding the genotype-phenotype relationship in sickle cell anaemia. Let's explore each option:

• Option A: Hb^AHb^A represents normal haemoglobin. Individuals with this genotype exhibit a normal phenotype, not a diseased one. This makes option A incorrect as normal phenotype is not a disease.
• Option B: Hb^AHb^S is associated with a carrier state, often manifesting the sickle cell trait but typically not the disease, except in situations of extreme exertion or low oxygen. However, for the purpose of genotype-phenotype questions in exams, it can be considered as diseased phenotype since individuals might show symptoms under certain conditions (partly correct).
• Option C: Hb^SHb^S leads to the production of abnormal haemoglobin, causing sickle-shaped cells, which results in sickle cell disease. This is correctly categorized as a diseased phenotype.
• Option D: Hb^SHb^A shows a heterozygous carrier state, typically asymptomatic. While correct, carriers show no disease, they do show sickle cells under low oxygen conditions, classifying them as a carrier of the disease.

Thus, the correctly matched pairs about sickle cell anaemia are (B), (C) and (D).

To determine the phenotypic ratio of the offspring when a heterozygous tall plant (Tt) is crossed with a dwarf plant (tt), we'll use a Punnett square.
In this cross:

• The heterozygous tall plant (Tt) can contribute either a 'T' or a 't' allele.
• The dwarf plant (tt) can only contribute a 't' allele.

The possible offspring combinations are:

From the Punnett square, we can see:

• Two Tt (tall) offspring
• Two tt (dwarf) offspring

Thus, the phenotypic ratio is 1 Tall : 1 Dwarf.