CBSE Class 10 Science 2022 · Paper 31/1/1 · Question 14

(a) In a human male the two sex chromosomes are of different types — one X and one Y — and of different sizes: X is a normal-sized chromosome while Y is a much shorter one. Since the two members of the pair differ in type and in size, the male's pair of sex chromosomes is called a mismatched pair. (b) The female has a perfect pair of sex chromosomes (XX). The gametes (eggs) she produces are all of the same kind: every egg carries an X chromosome (22 + X). (c)(i) Two such animals are turtles and snails. In some reptiles, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young developing in them will be male or female (in many turtles, eggs kept warmer develop into females). In snails, individuals can change their sex, which shows that their sex is not genetically determined. (Crocodiles, whose sex is also decided by the incubation temperature of the eggs, are another accepted example.) OR (c)(ii) Flowchart (drawn in the figure): Father 44 + XY and Mother 44 + XX → gametes: sperms 22 + X or 22 + Y, in equal numbers, and eggs, all 22 + X → fertilisation: 22 + X sperm + 22 + X egg → zygote 44 + XX → girl; 22 + Y sperm + 22 + X egg → zygote 44 + XY → boy. There is an equal chance of a girl or a boy, and the sperm decides which.