Chromosomes, meiosis and gametes — Edexcel International AS Level Biology
Gene loci and linkage, how meiosis produces genetic variation, specialised mammalian gametes, and fertilisation in mammals and flowering plants.
Revision notes
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A locus is the location of a gene on a chromosome. Genes at different loci on the same chromosome are linked, which means they tend to be inherited together because they stay on the same chromosome during meiosis. Linkage is broken only if crossing over separates the alleles, and genes that are closer together are separated less often.
Meiosis is a reduction division that produces four haploid cells, which are genetically different from each other and from the parent cell. Crossing over between chromatids of homologous chromosomes in prophase I exchanges alleles, and the random orientation of homologous pairs in metaphase I (independent assortment) gives different combinations of chromosomes in each gamete.
Mammalian gametes are specialised for fertilisation. A sperm has a flagellum for swimming, many mitochondria in the midpiece to make ATP, and an acrosome containing digestive enzymes. An egg cell is large with food stores in its cytoplasm, has a zona pellucida around it, and cortical granules just under its surface membrane.
In mammalian fertilisation, a sperm binds to the zona pellucida and undergoes the acrosome reaction, releasing enzymes that digest a path through it. The sperm and egg membranes fuse and the sperm nucleus enters. The cortical reaction then hardens the zona pellucida to block other sperm, and the two haploid nuclei fuse to form a diploid zygote.
In a flowering plant, a pollen grain lands on a stigma and grows a pollen tube down the style towards an ovule, attracted by chemicals. The tube enters the ovule through the micropyle and its two male nuclei are released. One fuses with the egg nucleus to form the zygote and the other fuses with the polar nuclei to form the endosperm.
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Quick check questions
Define the term locus.
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The location of a gene on a chromosome.
In which stage of meiosis does crossing over occur?
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Prophase I.
Why is the zona pellucida hardened after fertilisation?
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To prevent other sperm from entering, which would cause polyspermy.
What do the two male nuclei do in a flowering plant?
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One fuses with the egg nucleus to form the zygote; the other fuses with the polar nuclei to form the endosperm.
Why does a sperm have many mitochondria?
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To make ATP by aerobic respiration for the movement of the flagellum.
Frequently asked questions
How does meiosis produce genetic variation?
Meiosis produces genetically different gametes in two ways. Crossing over in prophase I swaps alleles between non-sister chromatids of homologous chromosomes. Independent assortment in metaphase I means the maternal and paternal chromosomes of each pair separate into gametes in random combinations.
What is the acrosome reaction?
The acrosome reaction happens when a sperm head binds to the zona pellucida. The acrosome membrane fuses with the sperm cell surface membrane and releases digestive enzymes by exocytosis. These enzymes digest a path through the zona pellucida so that the sperm can reach the egg cell surface membrane.
What is the cortical reaction and why does it happen?
The cortical reaction happens after a sperm fuses with the egg. Cortical granules release enzymes that harden the zona pellucida, so that no further sperm can bind. This prevents polyspermy, which would give a zygote with too many chromosomes that could not develop normally.
What is double fertilisation in flowering plants?
Double fertilisation is when the two male nuclei from the pollen tube each fuse with a different nucleus in the ovule. One fuses with the egg nucleus to form the diploid zygote and the other fuses with the polar nuclei to form the triploid endosperm nucleus.
What does linkage mean in genetics?
Genes are linked when they are at different loci on the same chromosome. They tend to be inherited together because the chromosome is passed on as a unit during meiosis. Crossing over can separate linked alleles, and it does so less often when the loci are closer together.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026