All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
Answer the sub-parts based on the given passage about human chromosomes and sex determination.
Generated by claude-sonnet-4-6 · 2026-06-15 06:54 · grounding stimulus
Model Answer
(a) A zygote has 46 chromosomes (44 autosomes + 2 sex chromosomes). Each gamete (sperm or egg) has 23 chromosomes.
(b) In some reptiles like turtles, the temperature at which eggs are incubated determines whether the offspring will be male or female. There are no sex chromosomes involved; sex is determined by environmental factors.
(c)
$$
\underbrace{\text{Mother (44+XX)}}_{\text{Female}} \quad \underbrace{\text{Father (44+XY)}}_{\text{Male}}
$$
$$\downarrow \quad\quad\quad\quad \downarrow$$
$$\text{Eggs: 22+X} \quad \text{Sperms: 22+X \ or \ 22+Y}$$
$$\downarrow$$
$$\text{22+X + 22+X} = \textbf{Girl} \quad \text{22+X + 22+Y} = \textbf{Boy}$$
Since the mother always contributes X, the sex of the child depends on whether the sperm carries X or Y — which is a matter of chance.
Source: Heredity and Evolution, Sex Determination
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Explanation
- (a) Remember: gametes are haploid (23), zygote is diploid (46).
- (b) This is a direct factbook point — temperature-dependent sex determination in reptiles.
- (c) The examiner wants a flowchart, not a paragraph. Show both parents, their gametes, and the two possible combinations. The key conclusion is that the father's sperm (X or Y) determines the child's sex, not the mother, so no parent is "responsible."