(a) List four characters which can be transmitted from parent to offspring in man.
(b) State two differences between mitosis and meiosis.
(c) Explain briefly the importance of meiosis and fertilization in the reproduction of organisms.
(d) In a monohybrid cross between a pure breeding plant that produces blue flowers and a pure breeding plant that produces white flowers, the F1 generation produced only blue flowers. By means of labelled cross diagrams, state the type of flowers you would expect if the Fl generation is (i) self-pollinated; (ii) cross-pollinated with a pure breeding plant that produces white flowers. Give reasons for your answers in (d) (i) and (ii).
(a) Four characters transmitted from parent to offspring in man
- Blood group
- Eye colour
- Skin colour
- Height / stature
(Others: hair type/colour, ability to roll the tongue, sex.)
(b) Two differences between mitosis and meiosis
| Mitosis | Meiosis |
|---|
| Produces two daughter cells | Produces four daughter cells |
| Daughter cells are diploid (2n), same number as parent | Daughter cells are haploid (n), half the parent number |
(c) Importance of meiosis and fertilization
Meiosis halves the chromosome number to produce haploid gametes and, through crossing over and independent assortment, introduces variation among the offspring. Fertilization then restores the full diploid chromosome number when two gametes fuse, and combines genes from two parents, giving new gene combinations. Together they keep the chromosome number constant from generation to generation while ensuring genetic variation.
(d) Monohybrid cross
Let B = allele for blue flowers (dominant) and b = allele for white flowers (recessive). Parents: BB (blue) × bb (white). All F1 = Bb (blue).
(i) F1 self-pollinated: Bb × Bb
Offspring: 3 blue (BB, Bb, Bb) : 1 white (bb), a ratio of 3 blue : 1 white. Reason: the heterozygotes each carry a hidden recessive b, so a quarter of the offspring are homozygous recessive and show white.
(ii) F1 cross-pollinated with pure white (Bb × bb)
Offspring: 2 blue (Bb) : 2 white (bb), a ratio of 1 blue : 1 white (a test cross). Reason: half the gametes of the heterozygote carry b, so half the offspring are homozygous recessive white.
