Use the information below to answer questions (a)(i) and (a)(ii). Hb Are presents normal haemoglobin, HbS represents sickled haemoglobin. a. A female hetero...

(a)(i) Genetic cross

Let Hb^A represent the allele for normal haemoglobin and Hb^S represent the allele for sickle-cell haemoglobin.

Female heterozygote: Hb^AHb^S
Male sickler: Hb^SHb^S

The female produces gametes Hb^A and Hb^S, while the male produces only Hb^S gametes.

Therefore, the possible genotypes of the offspring are:

• Hb^AHb^S, heterozygous normal carrier or sickle-cell trait, 2 out of 4;
• Hb^SHb^S, sickler, 2 out of 4.

Genotypic ratio: Hb^AHb^S : Hb^SHb^S = 2:2 = 1:1.

(a)(ii) Phenotypic ratio

Normal carriers (non-sicklers) : sicklers = 1:1.

(b) Rhesus incompatibility and loss of the second pregnancy

A Rhesus-negative woman has no Rh antigen on her red blood cells. If her first foetus is Rhesus-positive, some foetal red blood cells may enter her circulation, especially at delivery when the placenta separates. The Rh antigen stimulates the mother to form anti-Rhesus antibodies. In a later pregnancy with a Rhesus-positive foetus, these antibodies cross the placenta and destroy the foetal red blood cells. The resulting severe haemolysis, called erythroblastosis foetalis, may cause death of the foetus and miscarriage.

(c) Features explained by Lamarck's use and disuse theory

1. The long neck of the giraffe, attributed to continual stretching to reach high leaves.
2. The webbed feet of ducks or geese, attributed to constant use in swimming.

(d) Vestigial structures in mammals

1. Vermiform appendix in humans.
2. Coccyx, or reduced tail vertebrae, in humans.
3. Pelvic and hind-limb bones in whales.