Q4. Explain, with an example each, (a) co-dominance and (b) incomplete dominance. (NCERT Exercise 13)
(a) In co-dominance the F resembles both parents: in the ABO blood groups, IAIB red blood cells carry both the A and the B sugar (NCERT page 61). (b) In incomplete dominance the F is in between the parents: red (RR) × white (rr) snapdragons give pink (Rr) flowers, and the F is 1 red : 2 pink : 1 white (NCERT page 60).
Q5. A pink snapdragon (Rr) is crossed with a white one (rr). What colours do the offspring have, and in what ratio?
Rr gives R and r gametes, ½ each; rr gives only r. Offspring: ½ Rr pink and ½ rr white, 1 pink : 1 white.
Q6. How many genotypes and how many phenotypes does the ABO gene give, and which genotypes give blood group A?
Six genotypes and four phenotypes (A, B, AB, O). Group A comes from IAIA and IAi (NCERT Table 4.2, page 61).
Q7. Why does NCERT advise writing T and t for tall and dwarf, not T and d?
With the capital and small form of one letter you can see at once that the two are alleles of the same gene and which is dominant; with T and d you may not remember whether they are alleles of the same gene (NCERT page 56).
Q8. For the pea starch gene, are Bb seeds round or wrinkled, and what size are their starch grains? What does this show about dominance?
Bb seeds are round, like BB, but their starch grains are of intermediate size. For seed shape B is dominant; for grain size the alleles show incomplete dominance. So dominance depends on the phenotype examined (NCERT page 62).
Q9. NCERT asks what Mendel would have got had he self-pollinated a tall F plant. Answer it, and say what fraction of the tall F plants are TT.
A TT plant gives only tall offspring; a Tt plant gives 3 tall : 1 dwarf. Among the F, TT is ¼ and Tt ½, so of the tall plants (¾) the TT ones are ¼ ÷ ¾ = ⅓ and the Tt ones ⅔.
Q10. A tall plant of unknown genotype is test-crossed with tt. Four offspring are all tall. Does this prove that the tested plant was TT?
No. For Tt × tt, each offspring has probability ½ of being tall, so four tall offspring have probability (½) = 1/16. A finite all-tall sample supports but does not prove TT when the genotype is unknown.