4.3.2Additional lessons
Consequences of Gene Mutations — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 38 of 101
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 5Range of Effects
Whether a mutation matters depends on where it occurs and on the environment.
Harmful
- Harmful The new allele makes a faulty or non-functioning protein, as in cystic fibrosis where the CFTR protein fails to work properly.
Neutral
- Neutral The change has no effect on fitness, for example a silent substitution or a change in a non-coding region.
Beneficial
- Beneficial Rarely the change helps survival, such as bacterial resistance to an antibiotic in an environment containing the drug.
Context dependent
- Context dependent The same allele can be helpful or harmful depending on the environment, as sickle cell shows in malaria regions.
| Location | Possible consequence |
|---|---|
| Coding region | Altered amino acid sequence |
| Promoter or control region | Changed level of gene expression |
| Intron | Often none, unless splicing is affected |
| Gamete cell line | Allele can be inherited |
Part 2 of 5Following a Mutation Through Generations
1
A mutation arises in a gamete-producing cell, creating a new allele that was not present in either parent.
2
The offspring that inherits the allele shows a changed phenotype if the allele is dominant or in a homozygous state.
3
If the phenotype increases survival and reproduction, individuals with the allele leave more offspring.
4
Over many generations the allele frequency in the population rises through natural selection.
5
If the phenotype lowers fitness, the allele is selected against and usually stays rare.
Part 3 of 5Case Studies
Case Studies
Cystic fibrosis
- Cystic fibrosis A common mutation deletes three bases and so removes one phenylalanine from CFTR, causing thick mucus in lungs and gut.
Sickle cell
- Sickle cell A substitution changes one amino acid in haemoglobin, and heterozygotes gain protection against severe malaria.
Lactase persistence
- Lactase persistence Mutations near the lactase gene keep it active into adulthood,
- and are common in populations with long traditions of dairy farming.
Antibiotic resistance
- Antibiotic resistance Random mutations in bacteria that reduce drug effect are selected when antibiotics are present.
Part 4 of 5Worked Example
Question
- In a population of 10 000 newborns, 1 in 2 500 has a recessive disorder.
- How many affected babies are expected?
- Fraction affected is 1 ÷ 2 500 = 0.0004.
- Number affected = 10 000 × 0.0004 = 4.
Answer About 4 affected babies.
Part 5 of 5Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Harmful mutation A change that reduces the ability of an organism to survive or reproduce.
- Neutral mutation A change with no effect on fitness.
- Beneficial mutation A change that increases fitness in a particular environment.
- Heterozygote advantage Greater fitness of carriers of two different alleles than of either homozygote.
- Raw material of evolution New alleles created by mutation on which selection can act.
Common Mistakes
- Claiming that all mutations are harmful, when many are neutral and a few are beneficial.
- Saying that a mutation is beneficial in all environments, ignoring context.
- Stating that mutations occur because natural selection needs them.
- Confusing a carrier, who has one copy of the allele, with an affected person with two copies.
Last updated:
\n