Meiosis, loci and linkage

Loci and linkage

The locus of a gene is its position on a chromosome. Homologous chromosomes carry the same genes at the same loci, but possibly different alleles.

Genes on the same chromosome are linked: they tend to be inherited together, unless crossing over separates them.

Meiosis

Meiosis has two divisions and produces four haploid cells, each genetically different. In humans, 46 chromosomes (diploid) become 23 (haploid) in gametes, so fertilisation restores 46.

In meiosis I homologous chromosomes pair up and are separated; in meiosis II sister chromatids are separated.

How meiosis creates variation

• Independent assortment (metaphase I): each homologous pair lines up at the equator independently of the others, so maternal and paternal chromosomes are shuffled. With n pairs there are 2ⁿ possible combinations: 2²³ (over 8 million) in humans.
• Crossing over (prophase I): non-sister chromatids of homologous chromosomes wrap round each other and exchange sections at points called chiasmata. This makes new combinations of alleles on each chromosome and can separate linked genes.

Random fertilisation adds further variation.

Worked example

An organism has a diploid number of 8. How many chromosome combinations are possible in its gametes from independent assortment alone?

1. Diploid 8 means 4 homologous pairs.
2. Combinations = 2⁴

Answer: 16

Check yourself

1. An organism has a diploid number of 10. How many combinations of chromosomes are possible in its gametes from independent assortment?
   Show answer

   32. 5 pairs, so 2⁵ = 32.

2. How many chromosomes are in a human gamete?
   Show answer

   23. Gametes are haploid: 23.

3. What is a locus?
   • A version of a gene
   • A pair of homologous chromosomes
   • The point where spindle fibres attach
   • The position of a gene on a chromosome
   Show answer

   The position of a gene on a chromosome. Locus = place.

4. Two genes are linked. What does this mean?
   • They are on the same chromosome and tend to be inherited together
   • They are on different chromosomes
   • They always code for the same protein
   • They are always separated in meiosis
   Show answer

   They are on the same chromosome and tend to be inherited together. Linked genes share a chromosome.

5. When does independent assortment happen?
   • Prophase II
   • Telophase II
   • Metaphase I, as homologous pairs line up randomly
   • During fertilisation
   Show answer

   Metaphase I, as homologous pairs line up randomly. Pairs line up independently at the equator in metaphase I.

6. Crossing over involves an exchange between
   • non-sister chromatids of homologous chromosomes
   • sister chromatids of one chromosome
   • two non-homologous chromosomes only
   • a chromosome and a plasmid
   Show answer

   non-sister chromatids of homologous chromosomes. Sections swap between chromatids of the paired homologues.

7. What does meiosis produce?
   • Two identical diploid cells
   • Four identical diploid cells
   • Four genetically different haploid cells
   • Two haploid cells identical to the parent
   Show answer

   Four genetically different haploid cells. Two divisions halve the chromosome number and shuffle alleles.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.