Difficulty: Introductory | Prerequisites: Cell cycle and mitosis notes (see companion doc). You should understand how a cell divides before learning why meiosis exists and how chromosome numbers are maintained.
Once you know how cells divide by mitosis, the next question is: how do organisms that reproduce sexually keep their chromosome number stable across generations? The answer is meiosis, a special form of cell division that halves the chromosome number to produce gametes (sperm and eggs). This topic introduces the vocabulary you need to talk about chromosomes precisely: homologous pairs, haploid vs diploid, ploidy, alleles, and karyotypes. These concepts form the foundation for everything in genetics, from Mendelian inheritance to population-level allele frequencies.
Chromosomes come in matched pairs called homologs. Diploid organisms (like humans, 2n = 46) have two sets; haploid cells (like gametes, n = 23) have one. Meiosis is the cell division that reduces ploidy from 2n to n so that fertilisation restores the full count. Different versions of the same gene are called alleles, and a population can carry many alleles of any given gene.
Homologous chromosomes (homologs)
A pair of chromosomes, one from each parent, that are the same size and shape and carry the same genes at the same positions. Think of them as two editions of the same book: same chapters in the same order, but the exact wording (alleles) may differ.
Homologous pair
The two homologs considered together. In a diploid cell, every chromosome type is present as a homologous pair.
Gene
A segment of DNA found at a specific location (locus) on a chromosome that influences a trait.
Allele
A particular version of a gene. In simple terms, if a gene is the "question" (e.g., what colour are the flowers?), an allele is one possible "answer" (e.g., purple, white).
Diploid (2n)
A cell or organism that has two complete sets of chromosomes, one inherited from each parent. Humans are diploid: 2n = 46.
Haploid (n)
A cell or organism that has only one set of chromosomes, with no homologous pairs. Human gametes are haploid: n = 23.
Ploidy
The number of complete chromosome sets in a cell. Diploid = 2n, haploid = n, triploid = 3n, tetraploid = 4n, hexaploid = 6n.
Polyploid
Having more than two complete sets of chromosomes. Common in plants (e.g., many crop species are tetraploid or hexaploid).
Karyotype
A visual display of an organism's chromosomes, arranged by number and type. Used to identify chromosome abnormalities.
Meiosis
A type of nuclear division that reduces the chromosome number by half, producing haploid gametes (sperm and eggs) from diploid parent cells.
Gametes
Reproductive cells (sperm and eggs) that are haploid. When two gametes fuse at fertilisation, the diploid chromosome number is restored.
Fertilisation
The fusion of a sperm and an egg to form a diploid zygote.
Chromosomes that match in size, shape, and gene order are called homologous chromosomes (homologs).
Homologs carry the same genes at the same positions (loci), but the specific alleles at those loci can differ. One homolog comes from the mother (maternal), the other from the father (paternal).
A gene is a defined segment of DNA at a fixed locus on a chromosome. An allele is one version of that gene. A population may carry many different alleles of the same gene, but a diploid individual has at most two (one per homolog).
Alleles on the two homologs can be identical or different. This is the basis for concepts like homozygous (same allele on both) and heterozygous (different alleles), which you will meet in Mendelian genetics.
Diploid (2n): Two copies of each chromosome type, one from each parent. Humans: 2n = 46 (23 pairs).
Haploid (n): One copy of each chromosome type, no homologous pairs. Human gametes: n = 23.
The letter n represents the number of distinct chromosome types in a species. In humans, n = 23. Different sex chromosomes (X and Y) count as a single type within n.
Bacteria, archaea, and many algae and fungi are haploid in their normal state: one chromosome of each type, one allele of each gene.
Ploidy is the number of complete chromosome sets a cell contains.
Diploid cells are designated 2n. Haploid cells are designated n (the "1" is implied).
Stating ploidy is the same as stating how many copies of each chromosome type are present.
Many plants are polyploid, meaning they carry more than two sets. Common forms: triploid (3n), tetraploid (4n), hexaploid (6n). Wheat, for example, is hexaploid.
August Weismann reasoned that if sperm and egg each contributed a full set of chromosomes, the chromosome number would double every generation. For it to stay constant, gametes must contain half the usual number.
Meiosis is the nuclear division that achieves this halving: it takes a diploid cell (2n) and produces haploid cells (n).
When a haploid sperm (n) fuses with a haploid egg (n) at fertilisation, the resulting zygote is diploid (2n) again. This is how the species' chromosome number stays stable across generations.
A karyotype is a photograph or diagram of all the chromosomes in a cell, arranged by size and type.
It is used to identify the organism's chromosome number, ploidy, sex chromosomes, and any structural or numerical abnormalities.
Key relationship to memorise:
2n = the diploid number (total chromosomes in a somatic cell)
n = the haploid number (number of distinct chromosome types)
For humans: n = 23, so 2n = 46.
After meiosis: each gamete has n chromosomes. After fertilisation: the zygote has 2n chromosomes.
Visual to know: a karyotype diagram showing 23 pairs of chromosomes arranged by size, with one pair of sex chromosomes (XX or XY).
Karyotyping is used in clinical genetics to diagnose chromosomal disorders such as Down syndrome (trisomy 21, where there are three copies of chromosome 21 instead of two). Prenatal karyotyping from amniocentesis samples can detect these conditions before birth.
Polyploidy is commercially important in agriculture. Many crop plants are polyploid, and breeders sometimes induce polyploidy deliberately to produce larger fruits or more vigorous plants. Seedless watermelons, for instance, are triploid.
Students often confuse "homologous chromosomes" with "sister chromatids." Homologs are the two chromosomes of the same type (one from each parent). Sister chromatids are the two identical copies of a single chromosome produced during S phase. They are different things.
Students sometimes think haploid means "half the DNA." It means one set of chromosomes rather than two. The amount of DNA in a haploid cell depends on where it is in the cell cycle (before or after S phase).
Students often assume diploid organisms always have an even chromosome number. That is true by definition (2n), but the value of n itself can be odd or even depending on the species.
Students sometimes think all organisms are diploid. Bacteria, archaea, and many fungi and algae spend most or all of their life cycle as haploid organisms.
⚠️ Be able to define and distinguish: gene vs allele, homologous chromosomes vs sister chromatids, haploid vs diploid.
⚠️ Know that ploidy refers to the number of chromosome sets, and be comfortable converting between n and 2n (e.g., if n = 23, then 2n = 46).
⚠️ Understand why meiosis is necessary: to halve the chromosome number so fertilisation restores it.
⚠️ Know that diploid organisms have two alleles per gene (one on each homolog) and that these can be the same or different.
⚠️ Be able to identify what "polyploid" means and give an example (plants, crop species).
True or false: Homologous chromosomes are identical copies of each other. (False. They carry the same genes but may have different alleles.)
Fill in the blank: Human somatic cells are ________ with a chromosome number of 2n = 46. (Diploid.)
True or false: A haploid organism has homologous pairs. (False. Haploid means one set, no pairs.)
Fill in the blank: The type of cell division that produces gametes is called ________. (Meiosis.)
True or false: A tetraploid organism has four copies of each chromosome type. (True.)
Q: What is the difference between a gene and an allele?
A: A gene is a segment of DNA at a specific locus on a chromosome that influences a trait. An allele is one particular version of that gene. A single gene can have many different alleles in a population.
Q: A diploid organism has 2n = 14. How many chromosomes are in its gametes?
A: n = 7. Meiosis halves the chromosome number, so each gamete has 7 chromosomes.
Q: Why did Weismann argue that a special type of cell division must exist to produce gametes?
A: Because if both sperm and egg contributed a full (2n) set of chromosomes, the chromosome number would double with every generation. A division that halves the number (meiosis) is necessary so that fertilisation restores the original 2n.
Q: What is the difference between homologous chromosomes and sister chromatids?
A: Homologous chromosomes are the two chromosomes of the same type in a diploid cell, one from each parent. Sister chromatids are the two identical copies of one chromosome, joined at the centromere after DNA replication.
Q: A plant species has 6n = 42. What is its haploid number, and what term describes its ploidy?
A: n = 7, and the organism is hexaploid (six sets of chromosomes).
This material connects directly to Mendelian genetics: once you understand that diploid organisms carry two alleles per gene (one on each homolog), you can start predicting inheritance patterns with Punnett squares and understanding dominant vs recessive traits.
It also links back to the cell cycle: meiosis is a modified version of mitotic division, so understanding mitosis first makes the differences (two rounds of division, crossing over, independent assortment of homologs) much easier to follow.
Population genetics builds on allele frequencies: how common each allele is in a population, how those frequencies change over time, and what forces (selection, drift, migration, mutation) drive that change.
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