How Meiosis gives Rise to Variation
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of meiosis. You can test your knowledge on this below.
What is the purpose of meiosis?
Meiosis produces genetically unique haploid gametes for sexual reproduction, ensuring genetic variation and maintaining chromosome number at fertilisation.
What are homologous chromosomes?
Homologous chromosomes are pairs of chromosomes, one from each parent, that have the same genes in the same loci but may carry different alleles.
At which stages of meiosis does genetic variation arise?
Genetic variation arises during crossing over in Prophase I and independent assortment in Metaphase I
Topic Explainer Video
Check out this @JoeDoesBiology video that explains independent aasortment and crossing over or read the full notes below. Once you've gone through the whole note, try out the practice questions!
How Does Meiosis Create Genetic Variation
Genetic variation in meiosis occurs due to two key processes:
- Crossing Over (Prophase I)
- Independent Assortment (Metaphase I & II)
These processes lead to offspring with unique genetic combinations, increasing variation within a population and enabling evolution by natural selection.
Crossing Over (Prophase I) – Recombination of Alleles
- During Prophase I, homologous chromosomes pair up to form bivalents.
- Sections of chromatids break and rejoin at points called chiasmata.
- This swaps alleles between maternal and paternal chromatids (also known as non-sister chromatids).
- As a result, the gametes contain new combinations of alleles that were not originally present in the parent.
Example: If a maternal chromosome carries the allele for blue eyes (b) and the paternal chromosome carries the allele for brown eyes (B), crossing over can mix these alleles with other genes, creating unique combinations of alleles in the gametes.
Independent Assortment (Metaphase I & II) – Random Combination of Chromosomes
- In Metaphase I, homologous chromosome pairs line up randomly at the equator.
- Each homologous pair is sorted into daughter cells independently of other pairs.
- This means different combinations of maternal and paternal chromosomes end up in different gametes.
- Metaphase II further randomises which sister chromatids end up in each gamete.
Random Fertilization – Increases Variation Further
Each gamete has a unique combination of alleles due to crossing over and independent assortment.
During fertilisation, any sperm can fertilise any egg, doubling genetic variation.
This means the genetic combination of offspring is completely unique.
Maths Tips
We can use the expression to calculate the possible number of different combinations of chromosomes following meiosis, without crossing over.
= the number of homologous pairs in the diploid parent cell
Example: If a species has three homologous pairs of chromosomes, there are possible combinations of chromosomes in gametes.
For humans, the number of possible combinations is .
A human egg and sperm each have possible chromosome combinations. When combined, the possible combinations in a zygote exceed .
= the number of homologous pairs in the diploid parent cell
This gives us the number of possible chromosome combinations in a zygote following fertilisation. This does not take crossing over into account so in reality the genetic variation would be much higher.
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Key Terms
Meiosis: A type of cell division that produces four genetically unique haploid gametes.
Haploid (n): A cell with single chromosomes (e.g., gametes).
Diploid (2n): A cell with homologous pairs of chromosomes (e.g., body cells).
Homologous Chromosomes: A pair of chromosomes (one from each parent) that carry the same genes in the same loci but may have different alleles.
Crossing Over: The exchange of genetic material between chromatids in Prophase I.
Chiasma (plural: Chiasmata): The site of crossing over between chromatids.
Independent Assortment: The random alignment and separation of homologous chromosomes in Metaphase I, creating new chromosome combinations.
Random Fertilisation: The process where any sperm can fertilise any egg, increasing genetic variation.
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Exam Tips
When explaining the results of crossing over in meiosis I use the following phrase:
Crossing over results in new combinations of alleles.
When explaining the results of independent assortment in meiosis I use the following phrase:
Independent assortment results in new combinations of maternal and paternal chromosomes.
If asked about the significance of meiosis in evolution, link genetic variation to natural selection and survival due to advantageous traits.
No answer provided.
Describe and explain the processes that occur during meiosis that increase genetic variation. (5 marks)
Homologous chromosomes pair up / bivalents form.
Crossing over occurs / chiasmata form.
Produces new combination of alleles.
Independent assortment / independent segregation.
Produces varying combinations of maternal and paternal chromosomes.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!