A chromosome is a long thread of DNA found in the nucleus. A gene is a short section of that DNA that codes for one protein. An allele is one version of a gene.
Students often swap these words in exam answers, and each swap costs a mark.
This lesson follows comparing mitosis and meiosis, because chromosome pairs explain why meiosis must halve the number.
How do the three ideas fit together?
Think of a library shelf. The chromosome is a whole shelf of books. A gene is one book on that shelf, for example the book about flower colour.
An allele is one edition of that book, so the purple edition and the white edition are two alleles of the same gene.
A body cell carries chromosomes in pairs, with one chromosome of each pair from each parent. The two chromosomes of a pair carry the same genes in the same places. They may carry the same allele or different alleles of a gene.
How to sort the words, step by step
- Ask “is it a structure or a version?” A chromosome is a structure. A gene is a length of DNA on it. An allele is a version of the gene.
- Ask “how many per cell?” A body cell has pairs of chromosomes, so two alleles of each gene. A gamete has one of each pair, so one allele.
- Ask “where is it?” Chromosomes are in the nucleus. Genes are on the chromosomes. Alleles occupy the same position on each chromosome of a pair.
Worked example
A fictional plant has 6 chromosomes in a body cell. Flower colour is controlled by one gene with alleles R (purple) and r (white). One plant has the alleles Rr.
Question: how many chromosomes are in a pollen grain, and how many alleles of the flower-colour gene does it carry?
Step 1: the body cell has 6 chromosomes, which is 3 pairs.
Step 2: a gamete receives one chromosome from each pair, so it has 6 ÷ 2 = 3 chromosomes.
Step 3: the two alleles R and r are on the same pair of chromosomes. A gamete receives one chromosome of that pair, so it carries one allele, either R or r.
Answer: 3 chromosomes, carrying one allele of the flower-colour gene. Half the pollen grains carry R and half carry r.
Check: 3 pairs is 6 chromosomes, and taking one from each pair gives 3. The count matches.
The mistake to watch for
A common error is to say that an allele is a smaller gene, or that a gene is a type of chromosome.
Mistaken answer: “An allele is a short piece of chromosome, like a small gene.”
This treats an allele as a length of DNA, when it is actually a version of a gene.
The correction is to use the word “version” for an allele. A gene is a section of DNA. Alleles are the different forms that section can take, and they sit in the same position on the chromosome pair.
Check yourself
1. Complete the sentence: “Purple and white flower colour are different ______ of the same ______.”
Show answer
alleles, gene.
2. A fictional animal has 10 chromosomes in a body cell. State how many pairs there are and how many chromosomes a gamete has.
Show answer
10 ÷ 2 = 5 pairs. A gamete has one from each pair, so 5 chromosomes.
3. A body cell has the alleles Tt for a gene. How many alleles of this gene does a gamete carry, and which could they be?
Show answer
A gamete carries one allele, either T or t.
Where this leads next
Once the words are clear, use them to fill a cross in constructing a simple inheritance grid. The inheritance model board shows which alleles go into which gametes in a fictional model.
If these words still swap under pressure, online one-to-one Biology tuition gives you a teacher who can ask follow-up questions on the spot.