Why does the amount of DNA NOT keep doubling during sexual reproduction in animals?
Because both gametes get halved before fertilisation to form the zygote
Gametes (sperm and egg) are produced by meiosis, a special cell division that halves the chromosome number, so each gamete carries only half the DNA (the haploid amount) before fertilisation.
When two such halved gametes fuse during fertilisation, their DNA adds back up to the full (diploid) amount in the zygote. Because the halving happens first, the offspring ends up with the same DNA content as the parents instead of double.
The options saying gametes are halved or doubled "after fertilisation" are wrong: once fertilisation occurs, the zygote is already formed, so adjusting DNA afterwards could not keep generations stable.
The option saying gametes are "doubled before fertilisation" is wrong because doubling, not halving, would make DNA increase every generation.
Hence, the DNA stays constant because both gametes get halved before fertilisation to form the zygote.
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