4.8.5Additional lessons
Pedigree Charts — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 72 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.
Section 1 of 5Reading a Pedigree
Symbols
- Symbols Squares stand for males and circles for females, and shaded symbols show people who have the characteristic.
Lines
- Lines A horizontal line joins a couple, and a vertical line leads down to their children, who are usually drawn left to right in birth order.
Generations
- Generations Generations are labelled with Roman numerals and individuals with numbers, so 'II-3' is the third person in the second generation.
Purpose
- Purpose Pedigrees allow genotypes to be deduced when direct genetic testing is not available.
Section 2 of 5Deducing the Pattern of Inheritance
Deducing the Pattern of Inheritance
Recessive clue
- Recessive clue Two unaffected parents with an affected child show that the condition is recessive and both parents are heterozygous.
Dominant clue
- Dominant clue Two affected parents with an unaffected child show that the condition is dominant and both parents are heterozygous.
Autosomal clue
- Autosomal clue An affected daughter whose father is unaffected cannot be X-linked recessive,
- because such a daughter must receive an affected X from her father.
X-linked clue
- X-linked clue Many more affected males than females, with the allele passing through unaffected carrier mothers, suggests X-linked recessive inheritance.
| Observation | Conclusion |
|---|---|
| Unaffected parents, affected child | Recessive |
| Affected parents, unaffected child | Dominant |
| Affected mother, all sons affected | Likely X-linked recessive |
Section 3 of 5Method for Pedigree Questions
1
Decide whether the allele is dominant or recessive by looking for a couple who have a child with a different phenotype from both of them.
2
Assign letters to the alleles and write down the genotype of every person who can be certain, starting with affected recessive individuals (aa).
3
Use parents and children to work out the unknown genotypes, because an aa child must have received an a allele from each parent.
4
Calculate probabilities from the genotypes of the parents using a Punnett grid.
5
Give the final answer with a reason referring to the specific individuals shown on the chart.
Section 4 of 5Worked Example
Question
- Two unaffected parents have an affected daughter with a recessive autosomal condition.
- Their next child is unaffected.
- What is the probability that this child is a carrier?
- The affected daughter is aa, so both parents passed on an a allele and each parent is Aa.
- The cross Aa × Aa gives 1 AA : 2 Aa : 1 aa.
- The child is unaffected, so aa is excluded and only AA and Aa remain: 3 out of 4 outcomes.
- Of those three, two are Aa, so the probability is 2/3.
Answer The unaffected child has a 2/3 (about 67%) probability of being a carrier.
Section 5 of 5Exam Checklist & Tips
4.8.5 – Key Points for the Exam
- Pedigree A diagram showing the inheritance of a characteristic through several generations of a family.
- Autosomal Carried on a chromosome that is not a sex chromosome.
- Affected Showing the phenotype under study, drawn as a shaded symbol.
- Carrier An unaffected individual heterozygous for a recessive allele.
Common Mistakes
- Giving the answer 1/4 for an unaffected child, forgetting that the aa possibility has been excluded.
- Calling a trait dominant just because it appears in every generation, without checking the parents of unaffected children.
- Mixing up the symbols for males and females.
- Ignoring the question's wording about which individual is being asked about.
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