A pedigree is a family tree that follows one trait through the generations. In SPM Biology you read the chart, decide which pattern it supports, and quote the evidence in your answer.
This lesson belongs to inheritance. If the allele letters themselves are unclear, revise allele, genotype and phenotype terminology first.
How do you read the symbols?
A square is a male and a circle is a female. A shaded symbol shows the trait, and an unshaded symbol does not.
A horizontal line joins two parents, and a vertical line drops to their children. Roman numerals mark the generations (I, II, III) and Arabic numbers mark the people within each row, so the second person in the first generation is I-2.
What is the four-step reading routine?
Do these steps in order and write a short note at each one.
- Find a couple whose children differ from them.
- Ask which allele must be hidden in a parent.
- Test dominant against recessive with one couple.
- Check that sex does not explain the pattern by itself.
Step 1 matters most because the evidence sits in one couple and their children. Two unaffected parents with an affected child, or two affected parents with an unaffected child, tells you more than counting shaded symbols.
Worked example: an original invented pedigree
Take a made-up trait called “trait Q”. It is not a real condition, and the chart is only for practice.
Generation I: I-1 is an unaffected male and I-2 is an unaffected female. Generation II: II-1 is an unaffected son, II-2 is an affected daughter, and II-3 is an unaffected son.
Step 1. I-1 and I-2 are both unaffected, yet II-2 shows trait Q.
Step 2. II-2 has the trait, so she must carry the allele for it. That allele came from her parents, who do not show it.
Step 3. An allele that stays hidden in both parents must be recessive. If trait Q were dominant, a parent carrying it would show it.
Step 4. Could it be X-linked recessive instead? An affected daughter needs an affected father, because she must receive an X carrying the allele from him. I-1 is unaffected, so the trait is more likely on an autosome.
Using T for the dominant allele and t for the recessive one: I-1 and I-2 are both Tt, and II-2 is tt. The sound answer is: “The trait is recessive. Unaffected parents I-1 and I-2 have an affected daughter II-2, so both parents must carry a hidden recessive allele.”
Which mistake costs marks here?
A common slip is to write “the trait is dominant because more people are shaded.” The number of shaded symbols does not decide dominance. A rare dominant trait can appear in few people, and a recessive trait can appear in many.
| Student wrote | Why it fails | Better version |
|---|---|---|
| “Q is dominant because most of the family has it” | Counting shaded symbols is not evidence | “Q is recessive because I-1 and I-2 are unaffected but II-2 is affected” |
| “I-1 has the gene” | Everyone has the gene, but people carry different alleles | “I-1 must carry the recessive allele” |
| “II-1 is definitely Tt” | The chart allows TT or Tt | “II-1 is TT or Tt” |
How do you avoid personal claims?
Stay inside the chart. Say “the pedigree shows a pattern consistent with a recessive trait,” not “this family has a condition.”
Exam pedigrees use invented or textbook families. A real family’s questions belong with a doctor or a genetic counsellor, because a few symbols cannot settle what is true for a real person. Describe what the chart shows and stop there.
Useful sentence frames:
- “The trait is recessive because two unaffected parents, I-1 and I-2, have an affected child, II-2.”
- “II-1 could be TT or Tt, because the chart does not show which.”
- “The pattern is consistent with an autosomal trait, because an affected daughter has an unaffected father.”
Check yourself
In a new invented pedigree, III-1 and III-2 are both affected by trait R. Their child IV-1 is unaffected. What does this tell you about trait R?
Answer
Trait R is dominant. Two affected parents have an unaffected child, so both parents must carry a dominant allele and also a hidden recessive one.
Using R for the dominant allele and r for the recessive one, III-1 and III-2 are both Rr and IV-1 is rr. If R were recessive, both parents would be rr and every child would also be rr, so the unaffected child could not exist.
What to study next
Pedigrees depend on crosses, so practise completing monohybrid crosses next. Try parent genotypes yourself in the simple inheritance cross explorer, and note every wording slip in the mistake log.
If you want a teacher to check your pedigree sentences line by line, see online one-to-one Biology tuition or the one-hour trial class (from RM50).