CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 18

(a) Homologous structures are organs of different organisms that have the same basic structure (and a similar origin) but perform different functions. Example: the forelimbs of a frog, a lizard, a bird and a human being (for instance, the wing of a bird and the arm of a human being) have the same basic design of bones but are used for different functions. (b) Flow chart of sex determination in human beings: Parents: Father (XY) × Mother (XX) Gametes: sperms with X and sperms with Y (in equal numbers); eggs, all with X Zygotes: X sperm + X egg → XX (girl); Y sperm + X egg → XY (boy) Justification: the mother always contributes an X chromosome, so the sex of the child depends only on whether the sperm that fertilises the egg carries X or Y. Both kinds of sperm are formed in equal numbers, and which one fertilises the egg is purely a matter of chance, so a girl and a boy are equally likely. Hence the sex of a newborn child is a matter of chance, and neither parent can be held responsible for it.