By the end of this lesson
- Read standard pedigree symbols.
- Infer genotypes in simple dominant/recessive pedigrees.
- State uncertainty where more than one genotype is possible.
BIG QUESTION
How can a family diagram reveal alleles that cannot be seen directly?
Relationships constrain which alleles each person could have inherited or passed on.
Pedigrees encode relationships and phenotype
Squares and circles traditionally represent male and female in exam pedigreeA family tree diagram showing how a characteristic is inherited through several generations. conventions; shading marks the phenotype being tracked. Horizontal and vertical lines show partnerships and descent.
Symbols describe the diagram’s recorded categories, not every aspect of a person’s biology or identity.
Recessive phenotypes reveal both alleles
In a simple autosomeAny chromosome that is not a sex chromosome; a human body cell has 22 pairs of autosomes.-recessive model, an affected individual is aa. Each parent of an aa child must have supplied a recessive allele.
An unaffected parent of an affected child is therefore usually inferred to be a heterozygous carrierA person who is heterozygous for a recessive allele, so does not show the condition but can pass the allele to offspring., assuming the stated model.
Dominant phenotypes can remain uncertain
An unaffected person in a simple dominant model is aa. Affected people may be AA or Aa unless offspring or parents provide further evidence.
Pedigrees support probabilities and inferences; small families may fit more than one inheritance model.
Pedigree evidence
Start with genotypes that are logically fixed.
| Observation | Simple recessive model | Inference |
|---|---|---|
| Affected person | aa. | Both recessive alleles present. |
| Unaffected parent with affected child | Usually Aa. | Parent supplied a while showing dominant phenotype. |
| Unaffected person | AA or Aa. | Cannot decide without more evidence. |
| Two unaffected parents, affected child | Both carriers. | Supports recessive inheritance. |
| Two affected recessive parents | aa × aa. | All children expected affected. |
Language that earns precision
- pedigree
- Diagram showing inheritance of a characteristic through a family.
- carrier
- Heterozygous individual carrying a recessive allele without the recessive phenotype.
- proband
- Individual through whom a family is first identified for investigation.
- autosomal
- Located on a non-sex chromosome.
Common misconceptions
Claim: Every unshaded person is homozygous dominant.
Correction: In a recessive model, unshaded people may be carriers.
Claim: A pedigree proves the molecular cause.
Correction: It supports an inheritance pattern; DNA evidence is needed for a specific variant.
Claim: One small family always identifies the pattern uniquely.
Correction: Several models may fit limited data.
Match the answer to the command word
Start with recessive phenotypes, then trace obligatory alleles.
Write both possible genotypes when evidence does not distinguish them.
Recall, apply, then justify
Two unaffected parents have a child with an autosomal-recessive condition. Infer all three genotypes.
Hint
The child must receive one recessive allele from each parent.
Suggested marking points
- The affected child has genotype aa.
- The first unaffected parent must be Aa.
- The second unaffected parent must be Aa.
Explain why an unaffected sibling in this family cannot automatically be labelled AA.
Hint
Which unaffected genotypes are possible?
Suggested marking points
- An unaffected sibling may be AA or Aa.
- The dominant phenotype does not distinguish homozygous dominant from a carrier.
Evaluate whether one pedigree with two unaffected parents and one affected child proves autosomal-recessive inheritance.
Planning hint
Support, alternatives and additional evidence.
What a strong design includes
- The pattern is consistent with two heterozygous carriers and an aa child.
- It supports a recessive model because unaffected parents can carry the allele.
- A small family provides limited statistical evidence.
- New mutation, incomplete penetrance, sex linkage or recording error could provide alternatives depending on context.
- More relatives and generations would strengthen inference.
- Molecular testing could identify the causal variant.
- The conclusion should therefore be stated as supported, not absolutely proven.