A pedigree shows who has a trait in each generation, so you can work out whether it is dominant or recessive. Every conclusion must come from the symbols, and it applies only to the fictional family in the question.
This lesson opens the set on inheritance evidence rather than family assumptions. It builds on completing monohybrid crosses.
What are the reading steps?
- Symbols: squares are males, circles are females, shading marks the trait.
- Key observation: find a couple whose offspring show something that the parents do not.
- Mode of inheritance: decide dominant or recessive from that observation.
- Genotypes: assign only those the evidence fixes.
Worked example with a fictional family
This family and its trait are invented for practice.
- Generation I: I-1 (male, unshaded) and I-2 (female, unshaded) are a couple.
- Generation II: II-1 (male, shaded) and II-2 (female, unshaded) are their children.
Question: Is the trait dominant or recessive? State the genotypes you can be sure of.
- Key observation: two unaffected parents (I-1 and I-2) have an affected son (II-1).
- Mode: the trait must be recessive. If it were dominant, at least one parent would show it.
- Genotypes: use A for the dominant (unaffected) allele and a for the recessive. II-1 shows the trait, so he is aa. Each parent gave him an a, so I-1 and I-2 are both Aa.
- II-2: she is unaffected, so she has at least one A, but she is AA or Aa. The evidence cannot settle which.
A written answer: “The trait is recessive, because two unaffected parents have an affected child. II-1 is aa, and both parents are Aa. II-2 is AA or Aa, since the diagram does not give enough information to decide.”
The mistake that loses marks
The common slip is to fill in II-2 as AA because she looks unaffected. The diagram cannot support that.
| Person | Claim | Supported? |
|---|---|---|
| II-1 | aa | Yes, shows the recessive trait |
| I-1, I-2 | Aa | Yes, each passed a to an aa child |
| II-2 | AA | No, could be AA or Aa |
Write “AA or Aa” and move on.
What a pedigree cannot do
A pedigree question is a method exercise about invented people. Its answer says nothing about any real person or family.
If a real genetic question worries you or your family, speak to a doctor. A genetic counsellor can advise using full medical and family information, which no textbook diagram has.
A routine for any pedigree
- Read the symbols and the key.
- Find a couple whose children show something the parents do not.
- State the mode of inheritance with one reason.
- Give genotypes only where the evidence fixes them.
The claim, evidence and explanation revision desk gives a frame for writing the reason.
Check yourself
A second fictional family: I-1 (affected) and I-2 (unaffected) have an unaffected daughter II-1 and an affected son II-2. The trait is recessive. State what you can say about I-2 and II-1.
Answer
II-2 is affected, so he is aa and received an a from each parent. I-2 is unaffected but passed an a, so she is Aa. I-1 is affected, so he is aa, and II-1 received an a from him, so II-1 is Aa.
What to study next
Continue with distinguishing probability for one offspring from an expected family count. For the wider skill of reading family diagrams, see reading pedigrees without making personal genetic claims.
If you want a teacher to go through a pedigree with you, see online one-to-one Biology tuition or the one-hour trial class (from RM50).