Most animals and flowering plants reproduce sexually. Two parents each make gametes, and when the nuclei of two gametes fuse the new cell, a zygote, has a mix of genes from both. That is why brothers and sisters are never exactly alike. Haploid and diploid nuclei, and the trade-offs against asexual reproduction, are Extended content.
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other. Fertilisation is the fusion of the nuclei of gametes. Extended: the nuclei of gametes are haploid (one set of chromosomes) and the nucleus of a zygote is diploid (two sets).
In the gametes tab the sperm and egg cell are drawn at the same scale. Press Release sperm. Many sperm swim to the egg, but only the first to arrive gets in, and its nucleus fuses with the egg nucleus.
Fertilisation is the fusion of the nuclei, not just the sperm touching or entering the egg. Write "the nucleus of the sperm fuses with the nucleus of the egg".
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the gametes tab, what happens to the jelly coat once the first sperm enters the egg?
Which event in the sim is fertilisation?
How many sperm fertilise the egg cell in the sim?
What is the cell formed at fertilisation called?
Extended. Body cells contain two sets of chromosomes, one set from each parent. If gametes had two sets too, the number would double every generation. So gametes carry only one set.
| Cell | Nucleus | Human example |
|---|---|---|
| Gamete (sperm, egg cell) | haploid: one set of chromosomes | 23 chromosomes |
| Zygote and body cells | diploid: two sets of chromosomes | 46 chromosomes (23 pairs) |
Extended. Variation is the big advantage of sexual reproduction, but it comes at a cost in time and effort. Compare the two methods for a species in the wild and for a farmer growing crops.
| Advantages of sexual reproduction | Disadvantages | |
|---|---|---|
| Population in the wild | variation, so some individuals may survive a new disease or a change in the environment; the species can adapt (natural selection); seeds can spread offspring far from the parent | slower; needs two parents (or a pollinator or the wind), so a lone individual cannot reproduce; energy spent finding a mate or making flowers and nectar |
| Crop production | new varieties can be bred with better yield or disease resistance; seeds are easy to store and transport | offspring vary, so good features of a parent may be lost; slower to reach harvest; growth from seed is less predictable |
Variation is an advantage for the population, not a guarantee for each individual. Write that some individuals are likely to survive a change, so the species is less likely to die out.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which statement about sexual reproduction is correct?
Extended: a cat has 38 chromosomes in its body cells. How many are in the nucleus of a cat egg cell?
Extended: why is it important that gamete nuclei are haploid?
Extended: which is a disadvantage of sexual reproduction for a population in the wild?
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