(a) Explain the following: (10×2=20 marks) (i) Haemophilia (ii) Red-green colour blindness (b) Describe various stages of…

(a) Haemophilia and red-green colour blindness

(i) Haemophilia: Nature: an X-linked recessive bleeding disorder in which blood fails to clot properly because a clotting factor is missing. Haemophilia A (classical) lacks Factor VIII, and haemophilia B (Christmas disease) lacks Factor IX. The genes lie on the X chromosome. Inheritance: a male has one X, so one defective allele (XʰY) causes the disease. A heterozygous female (XᴴXʰ) is a normal carrier, and only a homozygous female (XʰXʰ) is affected, which is rare and often lethal. Cross: a carrier mother and a normal father give daughters half normal and half carriers, and sons half normal and half haemophilic. An affected father and a normal mother give carrier daughters and normal sons, so there is no male-to-male transmission. Features: prolonged bleeding after injury, bleeding into joints and bruising. The disease passed to many royal families from Queen Victoria, a carrier. Treatment is by replacing the factor. (ii) Red-green colour blindness: Nature: an X-linked recessive defect of colour vision in which red and green cannot be told apart. The genes for the red- and green-sensitive cone pigments lie close together on the X chromosome (Xq28). Types: protanopia (red-blind), deuteranopia (green-blind), and the milder protanomaly and deuteranomaly. Inheritance: the pattern is the same as haemophilia. It is much commoner in males (about 8 per cent of men and 0·5 per cent of women), because the female needs two defective alleles. A colour-blind father passes the allele to his daughters, who become carriers. Detection: Ishihara plates.

(b) Stages of prophase of meiosis I in an animal cell

Prophase I is the longest and most complex phase, in which homologous chromosomes pair and exchange segments. It has five stages. Leptotene: chromosomes condense and appear as long thin threads, each already having two chromatids. The ends attach to the nuclear envelope. Zygotene: homologous chromosomes come together, gene by gene, in synapsis. The synaptonemal complex forms between them, and each pair is a bivalent. Pachytene: chromosomes become short and thick, and each bivalent shows four chromatids, a tetrad. Crossing over takes place between non-sister chromatids at recombination nodules, giving new combinations of genes. Diplotene: the synaptonemal complex dissolves and the homologues begin to separate, but remain joined at X-shaped chiasmata where crossing over has occurred. In the oocytes of human females this stage can last for years. Diakinesis: chromosomes reach maximum condensation, the chiasmata move to the ends (terminalisation), the nucleolus disappears and the nuclear envelope breaks down, and the spindle forms.

(c) Natural selection, with peppered moth and sickle-cell anaemia

Natural selection is the differential survival and reproduction of individuals whose heritable variations suit them better to the environment, so that favourable alleles become more common. Peppered moth, Biston betularia: Before industrialisation in England, the light speckled form (typica) was well hidden on lichen-covered bark, and the dark form (carbonaria) was rare because birds picked it out. Pollution killed the lichens and blackened tree trunks, so dark moths were hidden and light ones were eaten. By about 1900 the dark form made up nearly all moths in industrial areas. Kettlewell's release-recapture experiments in the 1950s showed that birds were the agent. After the Clean Air Act (1956) the light form increased again. It is a case of directional selection. Sickle-cell anaemia: It is caused by a point mutation in the β-globin gene, in which glutamic acid is replaced by valine, forming the allele HbS. Homozygotes (HbSHbS) have severe anaemia and die early, so the allele should be lost. Heterozygotes (HbAHbS) are healthy, and resist falciparum malaria. In malarial regions, such as Africa and tribal areas of central and southern India, heterozygotes are fitter than both homozygotes, so the HbS allele is maintained at high frequency. This is balancing selection (heterozygote advantage).