Topic 17.1 · Core + Supplement
Chromosomes, Genes and Proteins
Almost every mark lost early in this chapter comes from one habit: treating chromosome, gene and allele as three names for the same thing, or nesting one inside another in a way Cambridge’s own definitions don’t support. Get the levels right here and the rest of the chapter — mitosis, meiosis, monohybrid crosses — stops needing new vocabulary.
Chromosomes, DNA, genes and alleles
A chromosome is a structure made of DNA, found in the nucleus, that carries genetic information in the form of genes. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene. Those three definitions describe three different levels, not three synonyms: a chromosome contains many genes, and a gene can exist as more than one allele.
It’s worth being precise about how those levels connect, because a slightly wrong version of the chain is one of the most common ways this topic loses marks. Chromosomes are made of DNA. Particular lengths of that DNA are genes. A gene can exist in alternative forms called alleles. Notice what that description does not say: it never treats an allele as a smaller structure sitting inside a gene, the way a gene sits inside a chromosome. An allele isn’t a component of a gene — it’s a whole alternative version of it. Two individuals can carry different alleles of the same gene, and the gene is still doing the same job (coding for the same protein); it’s the exact genetic information in that length of DNA that differs.
This distinction is what the rest of the chapter is built on. When you reach monohybrid inheritance, dominant and recessive alleles, homozygous and heterozygous genotypes — every one of those ideas assumes you already know that an allele is a version of a gene, not a piece of one.
Inheritance of sex in humans
Humans have sex chromosomes called X and Y. A typical female has two X chromosomes, written XX. A typical male has one X and one Y, written XY. Every egg cell carries an X chromosome — there is no variation on the mother’s side. Sperm cells, by contrast, carry either an X chromosome or a Y chromosome, produced in roughly equal numbers.
| Egg | Sperm | Offspring |
|---|---|---|
| X | X | XX (female) |
| X | Y | XY (male) |
Because the egg only ever contributes X, it’s the sperm’s contribution that actually decides the outcome — an X-bearing sperm gives XX, a Y-bearing sperm gives XY. That gives each fertilisation an expected probability of approximately 1:1, or 50% for each combination. An examiner will expect you to phrase this as a probability, not a rule: a 1:1 ratio describes the long-run expectation across many fertilisations, and it does not mean that any one small family has to contain equal numbers of sons and daughters. Cambridge specifically wants human sex inheritance described in terms of X and Y chromosomes, so “the father determines the sex because his sperm carries the variable chromosome” is worth stating explicitly rather than leaving implied.
Don’t confuse this with sex linkage, which is a separate idea: sex determination asks whether a zygote is XX or XY, while sex linkage asks where a particular gene for some other characteristic is located, and what that location does to how the characteristic is inherited.
Haploid and diploid nuclei
A haploid nucleus contains one set of chromosomes, usually written n. A diploid nucleus contains two sets, written 2n. In a diploid cell, chromosomes exist as pairs — one of each type of chromosome, not one chromosome overall. A human diploid cell contains 23 pairs of chromosomes, which is 46 chromosomes in total, not 23. That “pairs versus total” distinction is worth stating explicitly in an answer, since a bare “humans have 23 chromosomes” is not accurate for a diploid body cell.
| Haploid | Diploid | |
|---|---|---|
| Sets of chromosomes | one | two |
| Notation | n | 2n |
| Typical cell type | gamete nuclei | most body-cell nuclei |
Human gametes are haploid, each containing 23 chromosomes. This is exactly the condition that meiosis produces: a reduction division that halves the diploid chromosome number to haploid. When two haploid gamete nuclei fuse at fertilisation, the diploid number is restored rather than doubling with every generation — which is precisely why gametes have to be haploid in the first place.
What this sets up next
This page covers what genetic information is and how it is organised. The next step is what a cell does with it: how the base sequence in a gene actually determines a protein’s shape, and how a gene’s information gets out of the nucleus and into a working protein. That mechanism, along with why not every cell expresses every gene it carries, is covered on the protein synthesis and gene expression page.