Fertilisation doubles the chromosome number every time it happens. Something has to halve it again, or each generation would carry twice as many chromosomes as the last. That job belongs to meiosis — and while it is halving, it also shuffles the deck so that no two gametes are alike.
📘 What you need to know
Meiosis produces cells that are genetically different from each other, and is used to make gametes.
The parent nucleus is diploid (2n). Meiosis involves two rounds of division, meiosis I and meiosis II, producing four haploid (n) nuclei.
The chromosome number halves in meiosis I, which is why it is called the reduction division. There is no replication between the two divisions.
In prophase I, homologous chromosomes pair up to form a bivalent (the pairing is synapsis), and crossing over exchanges alleles between non-sister chromatids at points called chiasmata.
In metaphase I, the orientation of each bivalent is random, and bivalents assort independently of one another.
The number of possible chromosome combinations from random assortment is 2n, where n is the number of homologous pairs. For humans, 223 = 8 388 608.
Non-disjunction is the failure of chromosomes to separate correctly, producing gametes with an abnormal chromosome number. Down syndrome is trisomy 21 — 47 chromosomes.
Why halving is essential
Sexual reproduction works by fertilisation: the nuclei of two gametes fuse to form a zygote. That fusion doubles the chromosome number each time it happens.
So a sexual life cycle must contain a halving step somewhere, or the number would keep climbing generation after generation. In animals, the halving happens during the production of gametes, by meiosis.
The balance in a sexual life cycle
meiosis halves (2n → n) • fertilisation doubles (n + n = 2n)
Two divisions, one replication
Before meiosis I, the chromosomes replicate, exactly as they would before mitosis. Then:
Meiosis I splits up the homologous pairs, producing two haploid nuclei. At this point each chromosome still consists of two chromatids.
Between meiosis I and meiosis II there is no further replication.
Meiosis II separates the chromatids of each chromosome, producing four haploid nuclei. Now each chromosome is a single chromatid.
Notice where the “2n” becomes “n”: at the first division. Meiosis II is a separation of chromatids, much like mitosis.
Crossing over
During prophase I, homologous chromosomes pair up and lie very close together. A pair like this is a bivalent, and the pairing process is called synapsis. Each pair consists of one maternal and one paternal chromosome, and since replication has already happened, a bivalent contains four DNA molecules.
Two non-sister chromatids — one from each homologous chromosome — then form a junction. At that junction the chromatids break and rejoin. The crossing points are called chiasmata.
Because the break happens at exactly the same position on both chromatids, whole genes are exchanged. Non-sister chromatids are homologous but not genetically identical, so some of the alleles swapped over will be different versions. The result is chromatids carrying completely new combinations of alleles that were not present in the parent cell — recombinant chromosomes.
Only the two inner, non-sister chromatids exchange material. The outer two are left as they were.
Why it matters. Those recombinant chromatids end up in different gametes, so crossing over is a major source of genetic variation. That variation is what allows a species to evolve and adapt as its environment changes.
Random orientation of bivalents
At metaphase I, bivalents line up at the cell equator. Spindle microtubules grow out from the poles and attach to the centromeres, and each of the two homologous chromosomes in a bivalent is attached to a different pole.
Which homologue faces which pole is completely random. On top of that, bivalents assort independently: the orientation of one bivalent never influences another. So every bivalent is an independent coin flip, and the combinations multiply.
Possible combinations from random assortment
number of combinations = 2n (n = number of homologous pairs)
For humans, 46 chromosomes means 23 homologous pairs, so 223 = 8 388 608 possible chromosome combinations — before crossing over has added anything at all.
Non-disjunction and Down syndrome
Non-disjunction is what happens when chromosomes fail to separate correctly during meiosis. It can occur in anaphase I or anaphase II, and produces gametes with an abnormal number of chromosomes — one extra copy of a particular chromosome, or none at all.
If such a gamete is fertilised, the zygote has the wrong chromosome number, which is a chromosome abnormality.
Down syndrome
Down syndrome, or trisomy 21, is the best-known example. Non-disjunction of the 21st pair of homologous chromosomes occurs in anaphase I.
Individuals have a total of 47 chromosomes, with three copies of chromosome 21.
Effects vary between individuals, but common features include physical growth delays and reduced intellectual ability. Problems with sight or hearing may also occur.
Other trisomies include Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18).
The risk of chromosomal abnormality increases with age. The mother’s age matters particularly here, because non-disjunction is more likely in older ova.
Karyotyping of foetal cells — obtained by amniocentesis or chorionic villus sampling — can identify these abnormalities.
Be careful and respectful with your wording here. Describe the features of a syndrome factually; avoid language that treats people with a condition as a problem. Examiners notice, and so does everyone else.
Worked examples
WORKED EXAMPLE
Counting the combinations
An organism has 8 chromosomes in its diploid cells. Calculate the number of possible chromosome combinations in its gametes resulting from random assortment.
Step 1: Find the number of homologous pairs8 ÷ 2 = 4 pairs, so n = 4
Step 2: Apply the formula2n = 24 = 1616 possible combinationsThis ignores crossing over, which multiplies the variation much further.
WORKED EXAMPLE
A gamete with too many chromosomes
A human gamete is formed with 24 chromosomes instead of 23. Explain how this happened and state the chromosome number of a zygote formed with a normal gamete.
Step 1: Name the causeNon-disjunction — a pair of chromosomes failed to separate in anaphase I (or chromatids in anaphase II).
Step 2: Explain the extra copy
Both members of one pair went into the same gamete, giving it one extra chromosome.
Step 3: Fertilise it24 + 23 = 47 chromosomesNon-disjunction; the zygote would have 47 chromosomes (a trisomy)The matching gamete from that division would have only 22.
WORKED EXAMPLE
Why gametes differ
Explain two ways in which meiosis produces gametes that are genetically different from one another.
Way 1: crossing over
In prophase I, non-sister chromatids exchange sections at chiasmata, producing new combinations of alleles on a chromosome.
Way 2: random orientation
In metaphase I, each bivalent lines up randomly and assorts independently, so which homologue goes to which pole varies.
Combined effect
Together they mean the chance of two identical gametes is vanishingly small.
Crossing over and random orientation of bivalentsRandom fertilisation adds a third source, but that happens after meiosis.
💡 Exam tip
The halving happens in meiosis I. Say so explicitly — it is a favourite question.
State that there is no replication between the divisions. Many students assume there is.
Use non-sister chromatids when describing crossing over. “Chromosomes swap” is too vague.
Give both variation mechanisms together: crossing over and random orientation.
Learn the numbers for Down syndrome: chromosome 21, three copies, 47 in total.
For 2n, remember n is the number of pairs, not the number of chromosomes.
⚠ Common mix-up
Saying the chromosome number halves in meiosis II. It halves in meiosis I.
Thinking sister chromatids cross over. They are identical, so nothing would change. It is non-sister chromatids.
Using n as the total chromosome number in 2n. For humans n is 23, not 46.
Confusing bivalent and chromatid. A bivalent is a pair of homologous chromosomes, so four chromatids.
Saying non-disjunction is a gene mutation. It is a chromosome abnormality — the base sequence is unchanged.
Describing meiosis as producing four cells directly. It produces four haploid nuclei; cytokinesis makes the cells.
Up next: Cell Cycle — we have covered both kinds of nuclear division, so now we zoom out to the whole cycle: what happens between divisions, and what decides when a cell is allowed to divide at all.
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