4.3.3Additional lessons
Mutations and Genetic Variation — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 108 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Sources of Variation
Variation among individuals provides the raw material on which natural selection acts.
Mutation
- Mutation The only source of completely new alleles, producing differences such as coat colour, body length or flower colour.
Crossing over
- Crossing over In prophase I, homologous chromosomes exchange sections, creating new combinations of alleles on each chromosome.
Independent assortment
- Independent assortment Homologous pairs line up at random in metaphase I, so maternal and paternal chromosomes are shuffled into gametes.
Random fertilisation
- Random fertilisation Any sperm can fuse with any egg, producing a unique combination of alleles in each zygote.
| Process | When | Effect |
|---|---|---|
| Mutation | Any time | New alleles |
| Crossing over | Prophase I | New allele combinations on chromosomes |
| Independent assortment | Metaphase I | Different chromosome combinations in gametes |
| Fertilisation | Zygote formation | Unique 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
- Uses Tests can identify alleles linked to inherited disease, informing decisions about treatment, lifestyle and family planning.
Home kits
- Home kits Tests sold directly to customers may give results without counselling, which can cause worry or false reassurance.
Privacy
- Privacy Genetic information is sensitive and could be misused by insurers or employers if not protected.
Limits
- 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?
- Each pair can orient in two ways, independently of the others.
- Number of combinations = 2ⁿ with n = 23.
- 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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