Genetic variation comes from mutation, from meiosis making different gametes, and from random fertilisation. Mutation is the only one that creates a new allele. The other two shuffle alleles into new combinations.
Exam questions ask you to name a source and explain how it causes differences between offspring.
This lesson follows the two kinds of variation, so the difference between genes and environment should already be clear.
What does each source actually do?
- Mutation is a random change in the genetic material. It can produce a new allele. Most mutations change nothing you can see, and only some are passed on, so the effect depends on the case.
- Meiosis produces gametes with half the chromosomes. Because chromosome pairs separate randomly, each gamete gets a different mix of the parent’s chromosomes.
- Random fertilisation means that which male gamete meets which female gamete is down to chance. Each pairing gives the offspring a different combination of alleles.
Use the order “new alleles, then mixing, then chance pairing” when you write your answer. It keeps the three ideas apart.
Worked example
A fictional organism has 3 pairs of chromosomes. Use a simple model that ignores any swapping of pieces between chromosomes.
Step 1, gametes from one parent: for each chromosome pair, a gamete receives one of two chromosomes. With 3 pairs, the number of different gametes is 2 × 2 × 2 = 2³ = 8.
Step 2, fertilisation: any of the 8 male gametes can join any of the 8 female gametes. The number of different pairings is 8 × 8 = 64.
Step 3, check: 2³ = 8, and 8² = 64. Both can be reached another way: 2⁶ = 64, because there are 6 independent choices in total.
Step 4, say what it means: even with only 3 pairs and no mutation, two parents can give 64 different chromosome combinations. Real organisms have more chromosome pairs, so the possible number is far larger. That is why siblings from the same parents differ.
This is a model with assumptions, not a count of real individuals. The real number is larger still because pieces can also swap between chromosomes during meiosis.
The mistake to watch for
Mistaken answer: “Sexual reproduction makes new alleles, so the offspring are different.”
Sexual reproduction mixes the alleles the parents already carry. A new allele only appears by mutation. The correction: “Mutation can produce a new allele. Meiosis and random fertilisation combine existing alleles in new ways.”
Another slip is writing that a height difference between two plant cuttings grown in different places is “genetic”. Cuttings from one parent share the same alleles, so a difference in height points to the environment.
Check yourself
All organisms are invented.
1. An organism has 4 pairs of chromosomes. Using the simple model above, how many different gametes can one parent make?
Show answer
Each pair gives 2 choices, so 2 × 2 × 2 × 2 = 2⁴ = 16 gametes.
2. Name the source of genetic variation described: “A gamete from each parent joins, and which gametes meet is down to chance.”
Show answer
Random fertilisation.
3. A fictional fern makes spores by mitosis in one parent. The offspring vary in height. Give one reason that is not genetic.
Show answer
The offspring share the same alleles, so the difference is likely environmental, for example different amounts of light, water or minerals in the soil.
Where this leads next
Variation only matters for evolution if some forms survive better than others. Move on to trace natural selection across generations to see how that works. The probability tree and counting board can help you practise counting combinations.
Students sometimes get the counting right and lose the marks in the sentence that explains it. A teacher in online one-to-one Biology tuition can help you practise writing that explanation so each link is visible.