Difficulty: Intermediate | Prerequisites: Cell Division Foundations and Mitosis study notes (understand the cell cycle, chromosome vs. chromatid, diploid vs. haploid, and the phases of mitosis before tackling this material).
Meiosis is the form of cell division that produces gametes (sex cells), and it is the reason offspring are genetically unique rather than clones of their parents. Where mitosis copies, meiosis shuffles: it halves the chromosome number from diploid to haploid and introduces genetic variation through crossing over and independent assortment. This set of notes walks through both divisions of meiosis, compares them directly with mitosis, and covers the basic genetics that follow from sexual reproduction, including Punnett squares, genotypes, and phenotypes. You should already be comfortable with the mitosis phases and the vocabulary from the companion notes before working through this material.
Meiosis has two rounds of division (meiosis I and meiosis II) that turn one diploid cell into four haploid cells. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids (like mitosis). Crossing over in prophase I and independent assortment at metaphase I create genetic variation, which is why siblings from the same parents are not identical.
Crossing over (recombination)
The exchange of segments of DNA between non-sister chromatids of homologous chromosomes during prophase I. This creates new combinations of alleles on a chromosome that did not exist in either parent.
Think of it as swapping chapters between two editions of the same textbook, so you end up with a version that has some content from each.
Tetrad (bivalent)
The structure formed when a pair of homologous chromosomes (4 chromatids total) synapse during prophase I. The four chromatids are held together, and crossing over happens within this structure.
Independent assortment
The principle that homologous pairs orient randomly at the metaphase plate during meiosis I. Each pair's orientation is independent of every other pair, so the maternal and paternal chromosomes are shuffled into different combinations in the resulting gametes.
In simple terms, which version of chromosome 1 you get has no bearing on which version of chromosome 2 you get.
Recombinant pair
A chromatid that carries a new combination of alleles resulting from crossing over, different from either parental chromosome.
Genotype
The specific set of alleles an organism carries for a given gene. Written with letters: AA (homozygous dominant), Aa (heterozygous), or aa (homozygous recessive).
Phenotype
The observable trait that results from the genotype, influenced by dominance relationships between alleles. Two organisms can have different genotypes (AA and Aa) but the same phenotype if one allele is dominant.
Punnett square
A grid used to predict the genotype and phenotype ratios of offspring from a cross between two parents. Each parent's possible gametes are placed along one axis.
Homozygous
Having two identical alleles for a gene (AA or aa).
Heterozygous
Having two different alleles for a gene (Aa). If one allele is dominant, the phenotype matches the dominant trait.
Meiosis I is the reductive division. It takes a diploid cell (2n) and produces two haploid cells (n). The critical difference from mitosis: homologous chromosomes are separated, not sister chromatids.
Prophase I
Chromosomes condense, and homologous chromosomes find each other and pair up (synapsis), forming tetrads (bivalents).
Crossing over occurs: non-sister chromatids exchange segments of DNA, creating recombinant chromatids with new allele combinations.
This is the major source of genetic variation at the chromosomal level.
After crossing over, the sister chromatids within a homologous pair are no longer identical to each other.
The nuclear envelope breaks down and spindle fibres form.
Metaphase I
Homologous pairs (tetrads) line up at the metaphase plate. This is different from mitosis, where individual chromosomes line up.
Independent assortment happens here: each pair of homologues orients randomly, so the maternal and paternal chromosomes are distributed in different combinations.
For an organism with just 2 pairs of chromosomes, there are 2^2 = 4 possible arrangements. Humans (23 pairs) have 2^23 = over 8 million combinations from independent assortment alone.
Anaphase I
Homologous chromosomes separate and move to opposite poles.
Sister chromatids remain attached at their centromeres. This is the key distinction from anaphase in mitosis, where sister chromatids separate.
Each pole now receives one chromosome from each homologous pair.
Telophase I and Cytokinesis I
Chromosomes arrive at opposite poles. Each pole has a haploid set of chromosomes, though each chromosome still consists of two sister chromatids.
The cell divides into two daughter cells.
Each daughter cell is now haploid (n), but each chromosome is still in its replicated (two-chromatid) form.
Result of meiosis I: Two haploid cells, each containing one chromosome from every homologous pair, with each chromosome still made of two (now potentially recombinant) sister chromatids.
Meiosis II is mechanically very similar to mitosis. The difference is that it starts with haploid cells and there is no DNA replication between meiosis I and meiosis II.
Prophase II
Chromosomes condense again (if they decondensed after meiosis I).
New spindle fibres form.
No crossing over occurs here; that was exclusive to prophase I.
Metaphase II
Individual chromosomes (each still consisting of two sister chromatids) line up at the metaphase plate, just as in mitosis.
Anaphase II
Sister chromatids separate at the centromere and move to opposite poles.
This is the same event as anaphase in mitosis.
Telophase II and Cytokinesis II
Nuclear envelopes reform around each set of chromosomes.
The two cells from meiosis I each divide again, producing a total of four haploid daughter cells.
Each daughter cell contains a single set of unreplicated chromosomes.
Result of meiosis (overall): Four genetically unique haploid cells from one diploid parent cell. These become gametes (sperm or eggs).
Real-world application: Meiosis is why siblings look different from each other despite having the same parents. The combination of crossing over and independent assortment means every gamete is essentially one of a kind. In plants, there is an additional haploid multicellular stage (the gametophyte) between meiosis and fertilisation, which does not occur in animals.
Feature | Mitosis | Meiosis |
|---|---|---|
Number of divisions | 1 | 2 (meiosis I and II) |
Daughter cells produced | 2 | 4 |
Daughter cell ploidy | Diploid (2n) | Haploid (n) |
Genetically identical to parent? | Yes | No |
Crossing over? | No | Yes (prophase I) |
Homologous pairing? | No | Yes (prophase I) |
Independent assortment? | No | Yes (metaphase I) |
What separates in the main division? | Sister chromatids | Homologous chromosomes (meiosis I), then sister chromatids (meiosis II) |
Purpose | Growth, repair | Production of gametes |
The tutoring notes frame meiosis as "just like mitosis, but" with three key additions: (1) crossing over occurs in prophase I, forming tetrads that split into recombinant pairs; (2) sister chromatids end up different from each other because of crossing over; (3) there is a second division (meiosis II) that separates chromatids, producing haploid cells.
Once meiosis produces haploid gametes, fertilisation brings two gametes together to restore the diploid number. The alleles each parent contributes determine the offspring's genotype and phenotype.
Monohybrid cross (one gene, two alleles)
Consider two heterozygous parents: Aa x Aa.
Each parent can produce gametes carrying either A or a.
The Punnett square yields: AA, Aa, Aa, aa.
Genotype ratio: 1 AA : 2 Aa : 1 aa (3 different genotypes).
Phenotype ratio: 3 dominant : 1 recessive (2 different phenotypes), assuming complete dominance.
Dihybrid cross (two genes)
Consider AaBb x AaBb.
Each parent produces four types of gametes: AB, Ab, aB, ab.
A 4x4 Punnett square gives 16 possible combinations.
The classic phenotype ratio for a dihybrid cross with complete dominance is 9:3:3:1.
The tutoring notes give a simpler example: AaBB x AaBb, which produces offspring including AABB, aaBB, and AaBb among others, because one parent is homozygous for the B allele.
Key points from the source material:
Diploid to haploid is achieved by meiosis.
Haploid to diploid is achieved by fertilisation, which combines two haploid gametes.
The number of distinct genotypes and phenotypes depends on the dominance relationships between alleles. With complete dominance of A over a, AA and Aa look the same (same phenotype) but are genetically different (different genotypes).
Students often think meiosis I and meiosis II are the same event repeated. They are not. Meiosis I separates homologous chromosomes (reductive division); meiosis II separates sister chromatids (equational division, like mitosis). Confusing which structure separates in which division is one of the most common exam errors.
Students sometimes believe crossing over makes all four chromatids in a tetrad different. In practice, crossing over exchanges segments between two of the four chromatids (the two non-sister chromatids involved). The other two may remain unchanged, depending on where and how many crossovers occur.
Students frequently assume that Aa x Aa gives a 50/50 split of phenotypes. The phenotype ratio is 3:1 (with complete dominance), not 1:1. The 1:1 ratio comes from a different cross (Aa x aa).
Students confuse genotype and phenotype. Two organisms can look the same (same phenotype) but carry different alleles (different genotype). AA and Aa both show the dominant phenotype.
⚠️ Be able to draw chromosomes at every stage of meiosis for a given diploid number (2n = 4 is the standard exam setup). Know what the cell looks like at the end of meiosis I vs. the end of meiosis II.
⚠️ Know exactly what separates in meiosis I (homologous chromosomes) vs. meiosis II (sister chromatids). This is the single most tested distinction.
⚠️ Be prepared to explain two sources of genetic variation in meiosis: crossing over (prophase I) and independent assortment (metaphase I).
⚠️ Punnett squares for monohybrid and dihybrid crosses are high-yield. Know the expected genotype and phenotype ratios (1:2:1 genotype, 3:1 phenotype for a monohybrid cross with complete dominance).
⚠️ Understand the ploidy transitions: diploid to haploid via meiosis, haploid to diploid via fertilisation.
True or False: Meiosis I separates sister chromatids.
False. Meiosis I separates homologous chromosomes. Sister chromatids separate in meiosis II.
Fill in the blank: The exchange of DNA segments between non-sister chromatids during prophase I is called ______.
Crossing over (recombination).
True or False: Independent assortment means the orientation of one homologous pair at metaphase I determines the orientation of all other pairs.
False. Each pair orients independently, which is the whole point.
Fill in the blank: A cross of Aa x Aa produces a phenotype ratio of ______ (assuming complete dominance).
3:1.
True or False: At the end of meiosis II, each daughter cell is diploid.
False. Each daughter cell is haploid.
Q: Explain why the four daughter cells produced by meiosis are genetically different from each other and from the parent cell.
A: Two mechanisms introduce variation. Crossing over in prophase I swaps DNA segments between homologous chromosomes, creating recombinant chromatids. Independent assortment in metaphase I randomly distributes maternal and paternal chromosomes into daughter cells. Together, these ensure each of the four haploid products carries a unique combination of alleles.
Q: A cell with 2n = 4 undergoes meiosis. How many chromosomes does each daughter cell have at the end of meiosis I? At the end of meiosis II?
A: At the end of meiosis I, each cell has 2 chromosomes (n = 2), each still consisting of two sister chromatids. At the end of meiosis II, each cell has 2 chromosomes, each consisting of a single chromatid.
Q: In a cross of Aa x Aa, what fraction of offspring are expected to be homozygous recessive (aa)?
A: 1/4 (25%). The Punnett square yields 1 AA : 2 Aa : 1 aa.
Q: What is the key structural difference between what lines up at the metaphase plate in mitosis vs. meiosis I?
A: In mitosis, individual chromosomes (each made of sister chromatids) line up. In meiosis I, homologous pairs (tetrads of four chromatids) line up together. This is why meiosis I separates homologues while mitosis separates sister chromatids.
Q: If an organism is AaBb and undergoes meiosis, list the four types of gametes it can produce (ignoring crossing over).
A: AB, Ab, aB, ab. Independent assortment of the two gene pairs gives these four combinations in equal proportion.
Meiosis connects directly to Mendelian genetics and inheritance patterns. The law of segregation (alleles separate during meiosis I) and the law of independent assortment (genes on different chromosomes sort independently) are both physical consequences of what happens during meiosis.
Crossing over links to the concept of genetic linkage: genes on the same chromosome do not assort independently unless recombination separates them, and the frequency of recombination between two genes reflects how far apart they sit on the chromosome (the basis of genetic mapping).
The ploidy transitions covered here (diploid to haploid to diploid) also connect to life cycle diversity in biology: animals have a dominant diploid stage, while plants alternate between a diploid sporophyte and a haploid gametophyte.
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