By the end of this lesson
- Identify sources of genetic variation in sexual reproduction.
- Explain random fertilisation.
- Relate variation to population diversity.
BIG QUESTION
Why are siblings usually genetically different despite having the same parents?
Meiosis creates varied gametes; random fertilisation combines them unpredictably.
Meiosis makes many gamete combinations
Independent assortment distributes maternal and paternal homologues randomly into gametes. Crossing over exchanges DNA between homologous chromosomes.
These processes make each gamete’s allele combination different.
Fertilisation is random
Any one sperm can fuse with an egg in random fertilisationAny male gamete can fuse with any female gamete, which adds to genetic variation among offspring.. Each possible sperm-egg pairing creates a different combination of parental alleles.
This is additional to variation already generated during meiosis.
Variation is raw material for selection
genetic variationDifferences between individuals of the same species caused by differences in their alleles. means individuals differ in inherited traits. If conditions change, some variants may improve survival or reproduction.
Variation does not guarantee advantage; usefulness depends on the environment.
Sources of inherited variation
Separate chromosome reshuffling from gamete fusion.
| Process | When it occurs | Effect |
|---|---|---|
| Crossing over | Meiosis I. | Recombines alleles within homologues. |
| Independent assortment | Meiosis I. | Distributes whole homologues randomly. |
| Random fertilisation | Gamete fusion. | Combines any sperm with any egg. |
| Mutation | Before/during DNA copying. | Creates new allele sequences. |
Language that earns precision
- recombination
- Production of new allele combinations, including through crossing over.
- random fertilisation
- Chance pairing of compatible gametes.
- genetic variation
- Inherited differences in DNA sequence or allele combination.
- population
- Members of one species living in the same area at the same time.
Common misconceptions
Claim: Random fertilisation creates new alleles.
Correction: It creates new combinations; mutation creates new allele sequences.
Claim: Siblings have exactly half the same DNA in every region.
Correction: They share ancestry but receive different recombined allele combinations.
Claim: Variation is always useful.
Correction: Its effect depends on the environment and trait.
Match the answer to the command word
Name process → new allele combinations → different gametes/offspring.
Mutation makes new alleles; meiosis/fertilisation reshuffle existing ones.
Recall, apply, then justify
Explain two ways sexual reproduction produces genetic variation.
Hint
Meiosis plus fertilisation.
Suggested marking points
- Independent assortment or crossing over in meiosis produces different gamete allele combinations.
- This makes gametes genetically varied.
- Random fertilisation allows different sperm and egg combinations.
- Offspring therefore inherit different combinations of parental alleles.
Explain why random fertilisation does not itself create a new allele.
Hint
Combination versus DNA sequence.
Suggested marking points
- It combines alleles already present in gametes.
- A new allele requires a change in DNA sequence/a mutation.
Estimate how independent assortment alone can create many gamete chromosome combinations in humans.
Planning hint
There are 23 homologous pairs, each with two possible directions.
What a strong design includes
- Each of 23 homologous pairs can assort in two ways.
- The number of combinations is 2 to the power of 23.
- This is about 8.4 million combinations per parent before crossing over.
- Crossing over increases the number further.
- Random fertilisation combines one set from each parent.
- The calculation assumes independent behaviour of each pair and ignores linkage.