Meiosis creates variation in two ways, and fertilisation adds a third. Each one is a separate process that needs its own sentence in an answer.
This lesson belongs to cell division and uses the stages from sequencing meiosis.
Where does each source of variation come from?
For each source, name the stage, the process and the effect.
| Source | Stage | Effect |
|---|---|---|
| Crossing over | Prophase I | Sections of chromatids are exchanged, creating new allele combinations |
| Independent assortment | Metaphase I and anaphase I | Each pair orients at random, so gametes carry different mixes of chromosomes |
| Random fertilisation | After meiosis | Any sperm can fuse with any egg, so offspring differ further |
Worked example: counting combinations
Question. A cell has 3 pairs of homologous chromosomes. How many different combinations of chromosomes can independent assortment produce in a gamete? (2 marks)
Each pair can send either its maternal or its paternal chromosome to a gamete. That gives 2 choices for each of 3 pairs.
Combinations = 2 × 2 × 2 = 2³ = 8.
For humans with 23 pairs, the number is 2²³, which is 8 388 608. That is before crossing over, so the real number is larger.
Worked example: write a full explanation
Question. Explain how meiosis makes siblings from the same parents genetically different. (3 marks)
During prophase I, crossing over swaps sections between homologous chromatids, so new combinations of alleles form. In metaphase I, each pair lines up in a random orientation, so every gamete receives a different mix of maternal and paternal chromosomes. Each gamete is therefore different, and random fertilisation joins two of them by chance.
Each sentence names a stage, says what happens, and links to the effect.
The mistake that costs marks
The slip is to write “crossing over makes the chromosomes different” without saying which chromosomes and when. Marks need the stage and the exchange of sections between homologous chromosomes.
Another slip is to place crossing over in meiosis II. It happens in prophase I only.
Check yourself
A cell has 4 pairs of homologous chromosomes. Calculate the number of chromosome combinations possible in gametes from independent assortment alone.
Answer
Each pair has 2 choices, so the combinations are 2⁴ = 16.
Crossing over is not included in this figure, so the real number of different gametes is greater.
What to study next
Practise everything in the cell division practice set. To recheck your numbers, go back to comparing chromosome and DNA changes. If you would like a teacher to sharpen your wording, see online one-to-one Biology tuition.