CBSE Class 10 Science 2022 · Paper 31/1/3 · Question 15

(a) The probability is $\frac{1}{2}$ (50%) for a male child and $\frac{1}{2}$ (50%) for a female child, a ratio of $1:1$. Justification: the mother (XX) produces only one kind of egg, each carrying an X chromosome, while the father (XY) produces two kinds of sperm in equal numbers, half carrying X and half carrying Y. An X-bearing sperm fertilising the egg gives XX, a girl, and a Y-bearing sperm gives XY, a boy; both kinds of sperm are equally likely to fertilise the egg, so each sex has an equal chance. (b) The female has the perfect pair of sex chromosomes (XX). Her gametes are all of the same kind: every egg carries one X chromosome. (c)(i) Turtles and snails. In some animals, such as many turtles, the temperature at which the fertilised eggs are kept decides whether the young developing in them become male or female. In snails, as NCERT describes, individuals can change sex, which shows that their sex is not genetically determined. OR (c)(ii) Father ($44+\text{XY}$) forms sperms $22+\text{X}$ and $22+\text{Y}$; mother ($44+\text{XX}$) forms eggs $22+\text{X}$ only. Egg $22+\text{X}$ with sperm $22+\text{X}$ $\to$ zygote $44+\text{XX}$ $\to$ girl; egg $22+\text{X}$ with sperm $22+\text{Y}$ $\to$ zygote $44+\text{XY}$ $\to$ boy. The flowchart is drawn in the figure.