Biology • Year 12 • Module 5 • Lesson 13
Sources of Genetic Variation — Meiosis, Crossing Over, Fertilisation, Mutation
Lock in the four sources of variation, the difference between reshuffling existing alleles and creating new ones, and the precise vocabulary that separates them.
1. Label the concept map of variation sources
The concept map below organises the four sources of variation introduced in this lesson and tracks whether each produces new allele combinations or new alleles. Write the missing labels into boxes A–H. Each label is drawn from the lesson's Key Terms or Cards 1–5. 8 marks
- A — meiosis source that exchanges segments between homologous chromosomes _______________________
- B — meiosis source that randomly distributes whole chromosomes into gametes _______________________
- C — source that combines one gamete from each parent _______________________
- D — source that changes the DNA sequence itself _______________________
- E — outcome category produced by A, B and C _______________________
- F — outcome category produced only by D _______________________
- G — level at which A and B operate (cell / division stage) _______________________
- H — level at which D operates (molecule) _______________________
| Box | Your label |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F | |
| G | |
| H |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: genetic variation, allele, crossing over, independent assortment, random fertilisation, mutation, homologous chromosomes, gamete, recombinant chromosome, new allele combination. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | An alternative version of a gene. | |
| 2.2 | The exchange of corresponding segments between paired homologous chromosomes during meiosis. | |
| 2.3 | The random orientation and separation of homologous chromosome pairs in meiosis, distributing whole chromosomes into gametes. | |
| 2.4 | The chance fusion of one gamete with another, producing many possible zygote genotypes. | |
| 2.5 | A change in DNA sequence that can create a genuinely new allele. | |
| 2.6 | Differences in genetic makeup within or between individuals of a population. | |
| 2.7 | A pair of chromosomes carrying the same gene loci, one inherited from each parent. | |
| 2.8 | A haploid sex cell produced by meiosis that fuses during fertilisation. | |
| 2.9 | A chromosome carrying a new mix of maternal and paternal alleles after crossing over. | |
| 2.10 | The outcome of reshuffling existing alleles — not a new allele, but a new arrangement of alleles already in the population. |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the ruled line. 8 marks (1 for T/F, 1 for the correction where needed)
3.1 Crossing over creates a brand-new allele by changing the base sequence of DNA. T / F
3.2 Independent assortment changes the combination of whole chromosomes distributed into each gamete. T / F
3.3 Random fertilisation creates new alleles by combining gametes. T / F
3.4 Mutation is the only one of the four sources that can introduce a genuinely new allele into a population. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of crossing over as a source of genetic variation?
4.2 What is the function of independent assortment as a source of genetic variation?
4.3 What is the function of random fertilisation in increasing offspring variety?
4.4 What is the unique function of mutation that the other three sources cannot perform?
4.5 What is the function of genetic variation within a population whose environment is changing?
5. Cloze — variation in one paragraph
Complete the paragraph using terms from this word bank (each used once): crossing over, independent assortment, random fertilisation, mutation, new allele, new combinations, homologous, DNA sequence, reshuffle, gametes. 10 marks
During meiosis, (5.1) ______________ occurs between (5.2) ______________ chromosomes, producing chromosomes with (5.3) ______________ of existing alleles. Whole chromosomes are then randomly distributed into different (5.4) ______________ by (5.5) ______________, adding a second layer of reshuffling. After meiosis, (5.6) ______________ combines any one gamete from one parent with any one gamete from the other, generating many possible zygote genotypes. These three processes (5.7) ______________ alleles that already exist in the population — they do not invent new ones. By contrast, (5.8) ______________ is the only source that can produce a (5.9) ______________, because it changes the (5.10) ______________ itself.
Q1 — Labelled concept map
A: crossing over. B: independent assortment. C: random fertilisation. D: mutation. E: new allele combinations (reshuffling existing alleles). F: new alleles (genuinely novel sequence). G: meiosis (specifically prophase I for crossing over and metaphase I for independent assortment). H: the DNA molecule / DNA sequence (base level).
Q2 — Term–definition matches
2.1 allele • 2.2 crossing over • 2.3 independent assortment • 2.4 random fertilisation • 2.5 mutation • 2.6 genetic variation • 2.7 homologous chromosomes • 2.8 gamete • 2.9 recombinant chromosome • 2.10 new allele combination.
Q3 — True / false with correction
3.1 False. Correction: crossing over reshuffles existing alleles by exchanging chromosome segments — it does not change DNA base sequence. New alleles are created by mutation.
3.2 True.
3.3 False. Correction: random fertilisation produces new combinations of existing alleles by combining different gametes. It does not change DNA sequence and so does not create new alleles.
3.4 True.
Q4.1 — Function of crossing over
Crossing over exchanges corresponding segments between paired homologous chromosomes during meiosis. Its function as a source of variation is to produce recombinant chromosomes carrying new combinations of existing maternal and paternal alleles, so the gametes a parent produces are not all genetically the same.
Q4.2 — Function of independent assortment
When homologous chromosome pairs line up at metaphase I they orient randomly, so the maternal/paternal mix delivered to each gamete is different. Its function is to add a second layer of variation by reshuffling whole chromosomes, multiplying the number of distinct gamete types each parent can make.
Q4.3 — Function of random fertilisation
Any one of the genetically varied gametes from one parent can fuse with any one of the genetically varied gametes from the other parent. Its function is to greatly multiply the number of possible zygote genotypes, which is why siblings can share parents but still differ genetically.
Q4.4 — Unique function of mutation
Mutation is the only source listed that changes the DNA sequence itself. Its unique function is therefore to introduce genuinely new alleles into the population — alleles that did not previously exist — rather than merely rearranging alleles that already do.
Q4.5 — Function of genetic variation in a changing environment
Variation means individuals differ in their allele combinations. When the environment changes, at least some individuals are more likely to carry allele combinations suited to the new conditions, so the population as a whole is more likely to persist — variation is the raw material that natural selection acts on.
Q5 — Cloze paragraph
5.1 crossing over • 5.2 homologous • 5.3 new combinations • 5.4 gametes • 5.5 independent assortment • 5.6 random fertilisation • 5.7 reshuffle • 5.8 mutation • 5.9 new allele • 5.10 DNA sequence.