Sex is determined by different factors in various species. However, in human beings, it is determined genetically. Which amongst the following option(s) is/are correct for human beings? (i) Gamete carrying X chromosome from female parent. (ii) Gamete carrying X chromosome from male parent. (iii) Gamete carrying Y chromosome from male parent.

In human beings, women have a matched pair of sex chromosomes (XX), while men have a mismatched pair (XY). As the NCERT chapter explains: 'All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father.'

Since every gamete produced by the female parent carries an X chromosome (she has no other option, being XX), the X-chromosome-bearing gamete from the mother is constant and cannot decide the sex of the child — this makes statement (i) irrelevant to sex determination even though it is factually true.

The father, being XY, produces two types of gametes: half carrying the X chromosome and half carrying the Y chromosome. It is this variable paternal contribution that decides the sex of the child — 'A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.' Hence statements (ii) and (iii), both describing the two possible paternal gametes, are the ones that correctly account for how sex is determined.

Why the other options are incorrect:

• (B) '(i) only' is wrong because the maternal X-bearing gamete is invariant across all children and therefore cannot explain why some children are boys and others girls.
• (C) '(i) and (iii)' incorrectly includes the constant maternal contribution alongside only one of the two variable paternal gametes, giving an incomplete and inconsistent explanation.
• (D) '(iii) only' captures just one of the two paternal possibilities (Y-bearing gamete, giving a boy) but omits the X-bearing paternal gamete (giving a girl), so it does not fully explain sex determination.