4.7.5Additional lessons
Pedigree Charts — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 127 of 142
Part 1 of 6Reading a Pedigree
A pedigree traces a characteristic through a family so that you can infer its inheritance pattern.
Symbols
- Squares represent males and circles represent females; a shaded symbol means the person has the condition.
Lines
- A horizontal line joins two parents, and a vertical line links them to their children, who are shown in a row beneath.
Generations
- Generations are labelled with Roman numerals, and individuals within a generation are numbered from left to right.
Carriers
- Unshaded individuals may carry a recessive allele, which can only be deduced from their parents or children.
Part 2 of 6Clues for the Pattern
Clues for the Pattern
Recessive
- Two unaffected parents having an affected child proves that the condition is recessive and both parents are carriers.
Dominant
- Every affected person has an affected parent, and two affected parents may have an unaffected child.
Autosomal
- Males and females are affected in similar numbers, and an affected father with an unaffected daughter rules out X-linked dominance.
X-linked recessive
- Many more males are affected;
- the trait passes through carrier females, and an affected daughter needs an affected father.
Part 3 of 6Method for Answering
1
Look for unaffected parents with an affected child, which is the strongest clue that the allele is recessive.
2
Choose letters for the alleles and write the genotypes you can be certain of, starting with the affected individuals.
3
Test whether the pattern could be sex-linked by checking whether an affected daughter has an unaffected father.
4
Work out genotypes of the others from the parent and child genotypes, using the words must be or could be.
5
Calculate probabilities by using a Punnett grid for the relevant parents.
Part 4 of 6Related Ideas
Consanguinity
- Matings between close relatives are riskier because relatives are more likely to share the same recessive alleles.
Inductive reasoning
- General rules are drawn from many specific observations, such as repeated patterns across families.
Deductive reasoning
- A known rule is used to deduce a specific conclusion, such as the genotypes of two unaffected parents with an affected child.
Part 5 of 6Worked Example
Question
- Two unaffected parents have a daughter with albinism, a condition caused by the absence of melanin.
- State the inheritance of the condition, the parents' genotypes, and the probability that their next child is affected.
- Unaffected parents with an affected child means the allele must be recessive.
- An affected daughter has an unaffected father, which rules out X-linked recessive inheritance, so it is autosomal.
- Use A and athe daughter is aa and each parent must be Aa.
- Aa × Aa gives AA, Aa, aA, aa, so one in four children is aa.
Answer
- Autosomal recessive;
- both parents are Aa;
- the chance of the next child being affected is 25%.
Part 6 of 6Exam Checklist & Tips
4.7.5 – Key Points for the Exam
- Pedigree chart A diagram showing inheritance of a trait across generations of a family.
- Affected Showing the phenotype of the condition being studied.
- Carrier An unaffected person with one copy of a recessive allele.
- Consanguinity Reproduction between closely related individuals.
- Inductive reasoning Forming a general conclusion from specific observations.
Common Mistakes
- Calling every unaffected individual homozygous dominant; some are carriers.
- Concluding a condition is sex-linked just because it is seen in males; check an affected daughter.
- Mixing up the symbols for male and female or forgetting what shading means.
- Giving a definite genotype when the chart only allows a possibility.
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