Meiosis and sources of variation: WACE Year 12 Biology
“Explain how crossing over, independent assortment and random fertilisation generate genetic variation”
Meiosis and sexual reproduction generate variation through three processes: crossing over swaps segments between homologous chromosomes in prophase I; independent assortment randomly orients each homologous pair at the equator so chromosomes are shuffled into gametes in many combinations; and random fertilisation combines two genetically unique gametes. Together these make each offspring genetically distinct.
What this dot point is asking
SCSA wants you to explain the specific mechanisms within meiosis that shuffle alleles, and to link them to random fertilisation, so that you can account for why sexually produced offspring differ from their parents and each other. A strong answer names the stage of meiosis where each process happens.
Why variation matters
Genetic variation within a population is the raw material for natural selection. Without it, a population could not adapt to a changing environment. Sexual reproduction is a powerful generator of variation, and meiosis is where most of it is created.
Crossing over
Early in meiosis (prophase I), homologous chromosomes pair up closely. Where they touch, at points called chiasmata, they exchange equivalent segments of DNA. This is crossing over.
Because the two homologues carry different alleles, crossing over produces recombinant chromosomes that carry new combinations of alleles not present in either parent chromosome. It happens many times along the chromosomes in every meiosis.
Crossing over recombines alleles that were previously linked on the same chromosome. This is why genes far apart on a chromosome are inherited more independently than genes close together: there is more chance of a crossover between them.
Independent assortment
In metaphase I, the homologous pairs line up at the equator of the cell. The orientation of each pair, which homologue faces which pole, is random and independent of every other pair.
When the pairs separate in anaphase I, each gamete receives a random mix of maternal and paternal chromosomes. In humans, with 23 pairs, this alone produces over 8 million (2 to the power of 23) possible combinations of chromosomes per gamete, before crossing over is even considered.
Random fertilisation
Any one of a vast number of genetically different sperm can fuse with any one of a vast number of genetically different eggs. This random fertilisation multiplies the variation already generated within each parent, producing an enormous number of possible offspring genotypes from a single pair of parents.
Count the possible combinations
Consider an organism with just 2 pairs of chromosomes. Independent assortment alone gives 2 to the power of 2, which is 4 different gamete types per parent. Random fertilisation between two such parents gives 4 times 4, which is 16 possible offspring combinations from independent assortment alone. Adding crossing over makes the true number far larger.
How the three processes combine
The three sources act in sequence and multiply together:
- Crossing over creates new allele combinations on each chromosome.
- Independent assortment shuffles whole chromosomes into many gamete combinations.
- Random fertilisation combines two unique gametes.
The result is that, except for identical twins, no two individuals produced by sexual reproduction are genetically the same.
Crossing over: exchange of segments between homologous chromosomes in prophase I. Independent assortment: random orientation of homologous pairs in metaphase I. Recombinant: a chromosome carrying a new combination of alleles. Random fertilisation: the chance fusion of any two gametes.
Linking variation to evolution
These mechanisms do not create new alleles; mutation does that. Instead, they reshuffle existing alleles into new combinations every generation. This constant reshuffling means a population holds far more variation than any single individual, giving natural selection a wide range of phenotypes to act on and helping the species survive environmental change.
- Saying meiosis creates new alleles
- Meiosis recombines existing alleles; only mutation creates new ones.
- Putting crossing over in metaphase
- Crossing over occurs in prophase I; independent assortment is set in metaphase I.
- Forgetting random fertilisation
- Variation comes from meiosis and the chance fusion of gametes; leaving out fertilisation understates the total.
- Confusing independent assortment with crossing over
- Assortment shuffles whole chromosomes; crossing over swaps segments within a homologous pair.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksExplain how crossing over and independent assortment during meiosis generate genetic variation in gametes, naming the stage at which each occurs.Show worked answer →
A 6 mark answer needs both processes, their stages and why they create variation.
- Crossing over (prophase I)
- Homologous chromosomes pair up and exchange equivalent segments at points called chiasmata. Because the two homologues carry different alleles, this produces recombinant chromosomes with new combinations of alleles not present on either original chromosome.
- Independent assortment (metaphase I)
- Each homologous pair lines up at the equator independently of every other pair, so maternal and paternal chromosomes are sorted into gametes in many different combinations. With pairs in humans this alone gives (over million) chromosome combinations per gamete.
- Why variation results
- Both processes shuffle existing alleles into new combinations, so the gametes (and therefore offspring) differ genetically.
Markers reward crossing over in prophase I, independent assortment in metaphase I, and the link of each to new allele or chromosome combinations.
WACE 2024-style4 marksAn organism has 3 pairs of homologous chromosomes. Calculate the number of genetically different gametes that can be produced by independent assortment alone, and explain why random fertilisation increases variation further.Show worked answer →
A 4 mark answer needs the calculation plus the fertilisation point.
Calculation. Independent assortment gives combinations where is the number of homologous pairs. With , the number of different gametes is .
Random fertilisation. Any one of the many genetically different gametes from one parent can combine with any one from the other parent at fertilisation. If each parent can make gamete types, fertilisation gives possible offspring combinations from independent assortment alone, multiplying the variation. Crossing over makes the true number even larger.
Markers reward and the explanation that random pairing of gametes multiplies the possible combinations.