Q4. A haemophilic man marries a woman who is not a carrier. What are the chances that (a) a son and (b) a daughter is haemophilic, and (c) a daughter is a carrier?
In NCERT’s simple recessive model, the man passes his Y to sons and his X, with the haemophilia allele, to daughters; the woman passes a normal X to all children. (a) 0; (b) 0; (c) every daughter is a carrier (our working from NCERT page 74). Beyond the NCERT text: carriers sometimes have some symptoms of haemophilia (MedlinePlus, Hemophilia; public domain).
Q5. For the classic full-chromosome forms, name the disorder and its chromosome number in each: (a) trisomy of chromosome 21, (b) an extra X in a male, (c) one X missing in a female.
(a) Down’s syndrome, 47; (b) Klinefelter’s syndrome, 47, XXY; (c) Turner’s syndrome, 45, X0 (NCERT pages 75–76; the total of 47 for Down’s syndrome is on page 78).
Q6. Distinguish aneuploidy from polyploidy, with the cause of each.
Aneuploidy is the gain or loss of a chromosome(s), caused by failure of segregation of chromatids during the cell division cycle; polyploidy is an increase in a whole set of chromosomes, caused by failure of cytokinesis after telophase (NCERT page 75). Beyond the NCERT text: failed cytokinesis alone can at first produce a binucleate cell (our reasoning).
Q7. Why is a haemophilic female extremely rare?
Because her mother must be at least a carrier and her father must be haemophilic (NCERT page 74). Beyond the NCERT text: a female with the gene change on one X can occasionally have haemophilia when her other X is missing or inactive, and carriers sometimes have some symptoms (MedlinePlus, Hemophilia; public domain).
Q8. Assume full penetrance, no new mutations and ordinary XX/XY inheritance. In a family, two parents without a disorder have an affected daughter and an unaffected son. Is the disorder dominant or recessive, and is it autosomal or X-linked?
Recessive: an affected child of two unaffected parents means both parents carry the allele without showing it. Autosomal: a daughter shows an X-linked recessive trait only if her father is affected (NCERT page 74), and he is not. Thus autosomal recessive is the supported model under these assumptions, and both parents are carriers (NCERT pages 73–74, Figure 4.14).
Q9. An individual has Down’s syndrome with 46 chromosomes. Does the count rule out the diagnosis?
No. NCERT’s cause is an extra chromosome 21, giving 47 in all (NCERT pages 76 and 78). Beyond the NCERT text: that full trisomy causes most cases, but in translocation Down’s syndrome extra chromosome-21 material is attached to another chromosome (MedlinePlus Genetics), so it adds no chromosome to the count and the total can stay 46 (our reasoning).