4.7.3Additional lessons
Inheriting Alleles — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 125 of 142
Part 1 of 6Autosomal Recessive Inheritance
Pattern
- The trait appears only in homozygous recessive individuals, so it can skip generations.
Carriers
- Heterozygous people have a normal phenotype but can pass the allele to their children.
Cystic fibrosis
- A recessive allele of the CFTR gene causes thick mucus in the lungs and digestive system.
Carrier parents
- If both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier and 25% of being unaffected non-carrier.
Part 2 of 6Autosomal Dominant Inheritance
Autosomal Dominant Inheritance
Pattern
- An affected person usually has at least one affected parent, and the trait appears in every generation.
Huntington's disease
- It is caused by a dominant allele, and symptoms typically appear in adulthood.
Heterozygous parent
- If one parent is heterozygous and the other is unaffected, each child has a 50% chance of inheriting the allele.
Carriers
- There are no unaffected carriers, because a single copy of the dominant allele is enough for the phenotype.
Part 3 of 6ABO Blood Groups
Three alleles
- The gene has alleles Iᴬ, Iᴮ and i, so more than two alleles exist in the population, though each person has only two.
Dominance
- Iᴬ and Iᴮ are codominant with each other, and both are dominant over i.
Phenotypes
- Genotypes IᴬIᴬ or Iᴬi give group A, IᴮIᴮ or Iᴮi give group B, IᴬIᴮ gives AB and ii gives O.
Biological basis
- The alleles code for enzymes that add different sugar groups to antigens on red blood cells.
| Blood group | Possible genotypes |
|---|---|
| A | IᴬIᴬ, Iᴬi |
| B | IᴮIᴮ, Iᴮi |
| AB | IᴬIᴮ |
| O | ii |
Part 4 of 6Predicting Outcomes
1
Decide from the information whether the trait is dominant, recessive or codominant, and assign letters.
2
Deduce each parent's genotype from the phenotype and from the phenotypes of their other children.
3
Write the possible gametes, remembering that heterozygotes give two types in equal numbers.
4
Complete a Punnett grid and read off the genotype and phenotype probabilities.
5
State the results as ratios, fractions or percentages and apply them as chances for each child.
Part 5 of 6Worked Example
Question
- A man with blood group A (genotype Iᴬi) and a woman with blood group B (genotype Iᴮi) have a child.
- Give the possible blood groups and the probability of each.
| Type | Description |
|---|---|
| Gametes from the father | Iᴬ or i, in equal numbers. |
| Gametes from the mother | Iᴮ or i, in equal numbers. |
| Grid | IᴬIᴮ, Iᴬi, Iᴮi and ii, each with probability 1/4. |
| Convert genotypes into phenotypes | AB, A, B and O. |
Answer Each of the groups AB, A, B and O has a 25% chance.
Part 6 of 6Exam Checklist & Tips
4.7.3 – Key Points for the Exam
- Autosome A chromosome that is not a sex chromosome.
- Carrier A heterozygous individual who has one copy of a recessive allele.
- Multiple alleles More than two alleles of a gene exist in a population.
- Codominance Both alleles are expressed in the heterozygote.
- Cystic fibrosis A recessive genetic disease caused by a faulty CFTR allele.
Common Mistakes
- Believing that a carrier couple's next child must be unaffected because their first was affected;
- each child is independent.
- Giving a person three alleles of the ABO gene; each person has only two.
- Assuming that blood group O is dominant; the i allele is recessive.
- Describing Huntington's disease as recessive.
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