Sexual reproduction — two gamete nuclei fuse
Two linked elements, and a complete answer needs both: the nuclei of two gametes (sex cells), one from each parent, fuse; and the resulting zygote carries a mixture of both parents' genetic information, so the offspring are genetically different from each other and from both parents. That second element is what separates sexual from asexual reproduction, where a single parent gives genetically identical offspring.
Species — interbreed AND fertile offspring
A species is a group of organisms that can interbreed (reproduce sexually with one another) and whose offspring are themselves fertile, able to reproduce in turn. Both elements are load-bearing: two different species can sometimes interbreed, but if the offspring cannot itself reproduce, the parents are not the same species. This fertility test is the standard way of deciding whether two organisms belong to one species or to two.
(Extended) Haploid gametes, diploid zygote
A gamete nucleus is haploid: one complete set of chromosomes, unpaired. When two gamete nuclei fuse at fertilisation the zygote nucleus is diploid: two complete sets, one from each parent, now arranged in homologous pairs. Meiosis halves the number to make gametes and fertilisation doubles it again, so the chromosome number stays constant between generations while every zygote receives genetic information from two parents.
Drawn from real examiner reports.
Chromosome numbers are not the answer
Weak answers say only that the zygote has more chromosomes than the gamete, or give bare numbers such as 23 and 46. Full credit needs the vocabulary: the gamete nucleus is haploid — one complete, unpaired set — and the zygote nucleus is diploid — two complete sets in pairs, one set from each parent. Numbers alone do not score.
Flagged Jun 2022 P41 Q1c · Jun 2023 P42 Q10a — candidates merely compared chromosome numbers.
Zygote to embryo is mitosis
Once the zygote has formed, it develops into an embryo by repeated mitosis, not meiosis. Mitosis gives two genetically identical diploid daughter cells each time, which is exactly what a growing embryo needs — every new cell keeps the zygote's full diploid set. Meiosis only happens again much later, when that individual makes its own gametes.
Flagged Nov 2023 P42 Q1b — zygote-to-embryo division given as meiosis.
"DNA gets mixed up" is not a mechanism
Asked why sexual reproduction gives genetically different offspring, name the mechanism: each gamete nucleus carries its own combination of alleles, and fusing gamete nuclei from two separate individuals puts two different sets of genetic information into every zygote. "Reproduction shuffles genes", with no mention of two parents' gametes fusing, earns nothing.
Mitosis and meiosis statements get swapped
Mitosis produces genetically identical diploid cells; meiosis produces genetically different haploid gametes. Candidates routinely attach each statement to the wrong process, then credit mitosis with the variation. The safe chain: meiosis makes the haploid gametes, fertilisation makes the diploid zygote, mitosis grows the embryo.
Flagged Jun 2022 P41 Q4b · P42 Q1c (digest B16, reused at the B15.2 gamete/zygote level) — statements for the two are frequently swapped.
Interbreeding alone is not a species
The species test has two halves. Two organisms that breed together are only the same species if the offspring produced is itself fertile. A horse and a donkey interbreed and produce a mule, but the mule cannot reproduce, so horses and donkeys are two different species. Concluding "same species" from "they can breed together" alone loses the mark.
A gamete is not yet a zygote
A sperm cell, or an egg cell before fertilisation, is a gamete: haploid, from one parent. The zygote exists only once the two gamete nuclei have fused, and it is a single diploid cell — not yet an embryo. Answers that define a zygote as "an embryo", or that call a fertilised egg a gamete, are marked wrong on the definition alone.
Asexual reproduction gives no variation
Sexual reproduction needs two parents and gamete fusion, and gives genetically different offspring. Asexual reproduction needs one parent, no gamete fusion, and gives clones — identical to that parent. Describing sexual reproduction as needing one parent, or crediting asexual reproduction with variation, reverses the comparison.
Do not restate variation three times
A four-mark "discuss" needs four distinct ideas. "Produces variation", then "offspring are not identical", then "there is genetic diversity" is one point written three ways and scores once. Count your distinct points against the tariff before moving on.
(Extended) Bank two advantages
For a changing environment: genetically varied offspring mean some are more likely to suit the new conditions, so the population survives; and variation spreads disease resistance, since not every individual is equally susceptible.
(Extended) Bank two disadvantages
Two parents must meet and transfer gametes, often by pollination, costing time and energy and slower than asexual reproduction; and a successful parental combination is not passed on unchanged, since each offspring is a genetic mixture, not a copy.
Tie each point to the named scenario
"Discuss" asks for both sides, clearly labelled advantage then disadvantage. Each one must be tied to the population and the environmental change named in the question — scientifically true but generic statements are the commonest way to lose marks here.
Define a species.
(a) Define sexual reproduction. (2 marks) (b) Define a species. (2 marks)
