Reading pedigrees
A pedigree question is a logic puzzle with four possible answers: autosomal or sex-linked, dominant or recessive. This page gives the symbols, the tests that eliminate each option, a worked example, and how to get carrier probabilities out of a chart.
The symbols
The idea: squares are male, circles are female, shaded means affected. Generations are numbered with Roman numerals from the top, individuals left to right.
| Symbol | Means |
|---|---|
| Square | Male |
| Circle | Female |
| Shaded | Shows the trait |
| Unshaded | Does not show the trait — may still be a carrier |
| Horizontal line between two symbols | A mating |
| Vertical line down to a sibling bar | Their children, oldest on the left |
The symbols are on the Biology 30 data pages, so you do not have to memorize them — but the reasoning below is not, and that is where the marks are.
Four tests, in order
The idea: settle dominant or recessive first, then decide whether it sits on an autosome or the X.
- Two unaffected parents with an affected child → recessive. The child has two copies of an allele neither parent shows, so both parents are carriers. A trait that skips generations is recessive.
- Two affected parents with an unaffected child → dominant. Both parents must be heterozygous, and the unaffected child got the recessive allele twice.
- Then test sex linkage, for a recessive trait: an affected female must have an affected father, because she needs a recessive allele on each X and one X comes from him. An affected daughter with an unaffected father rules out X-linked recessive — it is autosomal.
- And for a dominant trait: an affected father passes his X to every daughter, so with X-linked dominant all his daughters are affected and his sons are unaffected unless the mother passes it. An affected father with an unaffected daughter rules out X-linked dominant.
Supporting clues, not proof
- Far more affected males than females suggests X-linked recessive, which is why red-green colour blindness and hemophilia show that pattern.
- The trait appears in every generation suggests dominant; skipping generations suggests recessive.
- Numbers in a small family prove nothing on their own — always back a claim with one of the four tests.
A worked pedigree
The idea: name the mode of inheritance, then assign genotypes from the affected individuals outwards.
- Recessive or dominant? Two unaffected parents (I-1, I-2) have an affected child (II-2), so the trait is recessive and both parents are carriers.
- Autosomal or X-linked? II-2 is an affected female. If the trait were X-linked recessive she would need Xa from her father, so he would be affected — and he is not. So it is autosomal recessive.
- Genotypes: II-2 is aa. I-1 and I-2 are both Aa. II-1 and II-3 are unaffected, so each is AA or Aa.
- State the reasoning, not just the conclusion — a written-response question asks you to justify, and the justification is the mark.
Carrier probabilities
The idea: knowing someone is unaffected removes one box from the Punnett square, which changes the odds.
Worked: what is the chance II-3 is a carrier?
Her parents are Aa × Aa, so the boxes are AA, Aa, Aa, aa. She is unaffected, so aa is out, leaving three equally likely boxes, two of which are Aa.
The probability is 2/3. The instinctive answer of 1/2 forgets that being unaffected is information.
Worked: what is the chance the next child is an affected boy?
Affected is 1/4, and a boy is 1/2, and the events are independent: 1/4 × 1/2 = 1/8.
Traps
The idea: most wrong pedigree answers come from four habits.
- Guessing from the number of affected males. It is a hint; the four tests are proof.
- Writing X-linked genotypes without the chromosome. Use XAXa and XaY, never Aa and aY.
- Assuming an unaffected person is homozygous dominant. For a recessive trait they may be a carrier, and often the question is about exactly that.
- Forgetting that marrying into the family brings new alleles. An unrelated partner is usually assumed homozygous dominant unless the chart shows otherwise — say that you are assuming it.