CBSE Class 10 Science 2022 · Paper 31/4/2 · Question 15

(a) The X chromosome is normal-sized, while the Y chromosome is short (smaller than X). The mismatched pair of sex chromosomes in humans is XY. (b) Apart from the two sex chromosomes, a human body cell has 44 chromosomes (22 pairs of autosomes). The father has the mismatched pair (XY); the mother has a perfect pair (XX). (c)(i) Example 1: in some animals, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young become male or female. Example 2: in some animals an individual changes its sex during its life; the slipper limpet, a sea snail, starts life as a male and later becomes a female. In both, the sex is set by the environment or changes during life rather than being fixed by inherited sex chromosomes, so the sex of an individual is not always determined genetically. OR (c)(ii) Flow chart: Father (44 + XY) × Mother (44 + XX) → gametes: sperms 22 + X or 22 + Y, in equal numbers; eggs all 22 + X → fertilisation: egg 22 + X with sperm 22 + X gives a 44 + XX zygote, a girl; egg 22 + X with sperm 22 + Y gives a 44 + XY zygote, a boy → girls : boys $=1:1$. The sex of the child is decided by the sperm, that is, by the chromosome inherited from the father (see the flow chart).