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4.3.3Additional lessons

Mutations and Genetic Variation — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 108 of 142

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Part 1 of 5Sources of Variation

4.3.3 Mutations and Genetic Variation

Variation among individuals provides the raw material on which natural selection acts.

Mutation
  • The only source of completely new alleles, producing differences such as coat colour, body length or flower colour.
Crossing over
  • In prophase I, homologous chromosomes exchange sections, creating new combinations of alleles on each chromosome.
Independent assortment
  • Homologous pairs line up at random in metaphase I, so maternal and paternal chromosomes are shuffled into gametes.
Random fertilisation
  • Any sperm can fuse with any egg, producing a unique combination of alleles in each zygote.
ProcessWhenEffect
MutationAny timeNew alleles
Crossing overProphase INew allele combinations on chromosomes
Independent assortmentMetaphase IDifferent chromosome combinations in gametes
FertilisationZygote formationUnique combination of two parents

Part 2 of 5How Variation Accumulates

Variation builds up through a sequence of events across generations.

1
A mutation in a germ cell creates a new allele in a gamete.
2
Crossing over and independent assortment combine this allele with different alleles during meiosis.
3
Random fertilisation brings the allele into a new genotype with alleles from the other parent.
4
Natural selection favours genotypes that increase survival and reproduction in the environment.
5
Over many generations, the frequency of the allele changes within the population.

Part 3 of 5Genetic Testing

Knowing a person's alleles can help medicine but raises ethical questions.

Genetic Testing
Uses
  • Tests can identify alleles linked to inherited disease, informing decisions about treatment, lifestyle and family planning.
Home kits
  • Tests sold directly to customers may give results without counselling, which can cause worry or false reassurance.
Privacy
  • Genetic information is sensitive and could be misused by insurers or employers if not protected.
Limits
  • Having a risk allele does not mean disease will develop, because environment and other genes also contribute.

Part 4 of 5Worked Example

Question Ignoring crossing over, how many different combinations of chromosomes can a human gamete contain, given n = 23 pairs of chromosomes?

  1. Each pair can orient in two ways, independently of the others.
  2. Number of combinations = 2ⁿ with n = 23.
  3. 2²³ = 8 388 608.

Answer A human can produce 8 388 608 chromosome combinations in gametes, even before crossing over is counted.

Part 5 of 5Exam Checklist & Tips

4.3.3 – Key Points for the Exam

  • Genetic variation Differences in alleles or genotypes among individuals of a population.
  • Crossing over Exchange of segments between non-sister chromatids of homologous chromosomes.
  • Independent assortment Random orientation of homologous pairs during meiosis I.
  • Allele frequency The proportion of copies of a gene in a population that are of a particular allele.

Common Mistakes

  • Saying meiosis creates new alleles; it shuffles existing alleles, and only mutation creates new ones.
  • Forgetting random fertilisation in a list of sources of variation.
  • Confusing crossing over, which is within a pair of homologues, with independent assortment, which is between pairs.
  • Assuming a genetic test result is a certain prediction.

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