Q4. In the F of RRYY × rryy, what fraction of the plants are RrYy, and what fraction are true-breeding for both characters?
RrYy fills 4 of the 16 squares: 4/16 = ¼. The true-breeding (homozygous for both genes) plants are RRYY, RRyy, rrYY and rryy, 1 square each: 4/16 = ¼.
Q5. An F plant RrYy is crossed with a wrinkled green plant (rryy). What phenotypes do you expect, and in what ratio?
The F makes RY, Ry, rY and ry, ¼ each; rryy makes only ry. Offspring: RrYy round yellow, Rryy round green, rrYy wrinkled yellow and rryy wrinkled green, ¼ each, a 1 : 1 : 1 : 1 ratio (our working from NCERT page 64).
Q6. Give the reasons NCERT lists for the fruit fly being suitable for Morgan’s studies.
It grows on a simple synthetic medium in the laboratory; its life cycle takes about two weeks; a single mating gives a large number of progeny; males and females are easily told apart; and it has many hereditary variations visible with low power microscopes (NCERT page 67).
Q7. Why did Mendel’s work remain unrecognised till 1900?
Communication was poor, so it was not widely publicised; his discrete, non-blending factors were not accepted as an explanation for the apparently continuous variation in nature; his mathematical approach was new and unacceptable to many biologists; and he had no physical proof of what factors were (NCERT pages 64–65).
Q8. In Morgan’s Cross B (genes w and m), 37.2 per cent of the offspring were recombinant. What percentage were parental types, and which pair, y and w or w and m, is more tightly linked?
100 − 37.2 = 62.8 per cent were parental. The pair y and w, with only 1.3 per cent recombination, is more tightly linked (NCERT pages 67–68).
Q9. Name the three scientists who rediscovered Mendel’s results in 1900.
de Vries, Correns and von Tschermak (NCERT page 65).