Explain why two children from the same biological parents can have different characteristics from each other.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Each human body cell contains 23 pairs of chromosomes. During sexual reproduction, each parent passes on one chromosome from each pair to their offspring.
Model answer (5 marks)
Each child receives one chromosome from each parent for every pair, so the 23 chromosomes from the mother and 23 from the father are mixed.
During meiosis, the chromosomes are shuffled and paired randomly, producing gametes that contain different combinations of chromosomes.
When fertilisation occurs, the sperm and egg combine at random, giving a zygote with a unique set of chromosomes.
Because each chromosome carries genes (alleles) that determine traits, the random combination of alleles inherited by each child is different.
Thus two children from the same parents inherit different combinations of alleles, leading to differences in their characteristics.
During meiosis, the chromosomes are shuffled and paired randomly, producing gametes that contain different combinations of chromosomes.
When fertilisation occurs, the sperm and egg combine at random, giving a zygote with a unique set of chromosomes.
Because each chromosome carries genes (alleles) that determine traits, the random combination of alleles inherited by each child is different.
Thus two children from the same parents inherit different combinations of alleles, leading to differences in their characteristics.
Examiner tips
- Use the word ‘random’ to show fertilisation is not predetermined.
- Explain meiosis as the source of variation – mention chromosome shuffling.
- Show that each child gets a unique set of alleles – link to traits.
- Use the exact terms: chromosomes, genes, alleles, gametes, fertilisation.
Common mistakes
- Saying the parents give the same chromosomes to both children – ignores random assortment.
- Confusing meiosis with mitosis – not mentioning chromosome shuffling.
- Using vague terms like ‘different genes’ without linking to alleles and traits.
Mark scheme (5 marks)
- Chromosomes are inherited from both parents (one from each parent per pair)
- Each chromosome carries genes / alleles that determine characteristics
- Gametes are formed by a process that produces cells with different combinations of chromosomes
- Fertilisation is random, so different combinations of alleles are inherited each time
- Each child therefore inherits a unique / different combination of alleles, producing different characteristics
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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