Reproduction: sexual & asexual

Year 9 answers

Fluency

Gamete: a haploid sex cell (sperm or egg). Zygote: the diploid cell formed when two gametes fuse. Haploid ( $n$ ): one set of chromosomes. Diploid ( $2n$ ): two sets.
E.g. (i) binary fission in E. coli; (ii) budding in hydra or yeast; (iii) vegetative runners in strawberry plants; (iv) spore formation in fungi.
(a) 46 (diploid), (b) 23 (haploid), (c) 46 (diploid).
Male: stamen (anther + filament). Female: carpel or pistil (stigma + style + ovary).
Meiosis produces gametes. Mitosis produces body cells for growth and repair.
Advantages: fast, no mate needed, energy-efficient. Disadvantages: no genetic variation, vulnerable to disease and environmental change.

Reasoning

Fungal spores are produced by meiosis in the sexual stage of many fungi (sexual), but many fungi also release asexual spores formed by mitosis. Without more detail both answers are possible; most school contexts treat mushroom spores as products of sexual reproduction because meiosis forms them.
Each parent contributes half of the chromosomes; meiosis shuffles alleles via crossing over and random assortment, and fertilisation randomly pairs one sperm with one egg. The result is a unique combination.
Grafting keeps the desired characteristics (flavour, size, disease resistance) because the offspring are clones of the parent tree. Seed offspring would vary unpredictably.
The variable population — genetic variation means some frogs are likely to have alleles that give resistance to the new bacterium; clones all have the same susceptibility.
Meiosis halves the chromosome number (2n -> n) when gametes form; fertilisation restores the diploid number (n + n -> 2n). Together they keep the species’ chromosome number constant.

Problem solving

Number of divisions: 4 h / 0.5 h = 8. Population: $2^8 = 256$ cells.
Stable conditions favour fast asexual reproduction (exploit resources quickly). In droughts, variation from sexual reproduction increases the chance some offspring tolerate the stress.
Only one sperm fertilises the egg; sperm must compete and most are lost, so large numbers raise the chance of fertilisation. Eggs carry the resources for early development, which is costly, so fewer are made.
Pollination transfers pollen (male gametes) from one variety’s anther to the other’s stigma. Fertilisation occurs when the pollen tube delivers a sperm nucleus to an egg inside the ovule, forming a zygote. The ovule develops into a seed that combines the parents’ genes.

Reasoning

In good conditions, asexual reproduction rapidly increases population size. When conditions worsen (cold, food shortage), a switch to sexual reproduction produces varied offspring (often resting eggs) more likely to survive the challenge. This bet-hedging balances growth with resilience.
Female workers come from fertilised eggs with two sets of chromosomes (sexual). Males come from unfertilised eggs that develop by parthenogenesis — a form of asexual reproduction — so drones have only the queen’s genes.
(i) No genetic variation, so the population is vulnerable to novel disease or environmental change. (ii) Accumulation of genetic errors cannot be masked by a second allele. Also early ageing issues (as observed in Dolly) and low genetic diversity reducing adaptability.
Self-pollination still involves meiosis (producing pollen and eggs) and fertilisation (pollen + ovule), so offspring receive gametes from two gamete-producing events even if they come from the same plant. Variation is limited but genetic recombination still occurs.