What is a pedigree?
A pedigree is a drawing of a family over many generations. It follows one trait, for example a disease or a feature like attached earlobes. We use it because we cannot do crossing experiments with people the way Mendel did with peas.
A trait is a feature that can be passed from parent to child. Each person has two copies of a gene, one from each parent. We write the two copies as letters, like A and a.
Pedigree symbols
- Square: male. Circle: female.
- Dark (filled) shape: the person has the trait. Light (empty) shape: does not have it.
- Horizontal line between a square and a circle: the two are a couple (parents).
- Vertical line down: leads to their children. Children are drawn in a row, oldest on the left.
- Roman numerals I, II, III: generations. Numbers 1, 2, 3: people in that generation. So II-3 is the third person in the second generation.
Recessive trait: how to spot it
A recessive trait only shows when a person has two copies of the gene (aa). A person with one copy (Aa) looks normal. They are a carrier: they carry the gene but do not show the trait.
Clue: two parents without the trait have a child with the trait. That is only possible if both parents are Aa. The trait can skip a generation. Two Aa parents have a 1 in 4 (25%) chance of an aa child each time.
Dominant trait: how to spot it
A dominant trait shows even with one copy (Aa or AA). So everyone who has the trait has at least one parent with the trait.
Clue: the trait appears in every generation and never skips. Two parents with the trait can still have a child without it, if both are Aa (the child is aa). Someone without the trait is always aa.
Step by step: reading any pedigree
- Look for two unaffected parents with an affected child. If you find one, the trait is recessive.
- If every affected child has an affected parent and the trait is in every generation, it is probably dominant.
- Write the genes: an affected person with a recessive trait is aa; an unaffected parent of an aa child is Aa.
Try it: draw your own family tree for one trait, such as attached earlobes. Use a square or circle for each person and colour the ones who have it. Then decide if it looks dominant or recessive.
Some traits are sex-linked (on the X chromosome), such as colour blindness, which shows more often in males. This lesson uses traits on non-sex chromosomes.
Key formulas and definitions
- Key terms: pedigree, trait, gene copy (allele), carrier, dominant, recessive
- Aa × Aa gives 1 AA : 2 Aa : 1 aa (3 normal : 1 with the trait for a recessive trait)
- Aa × aa gives 1 Aa : 1 aa (half the children get a dominant trait)
- Square = male, circle = female, dark = has the trait
Worked examples
1. Two parents without a trait have a daughter who has it. Is the trait dominant or recessive? What are the parents' genes?
Recessive. The daughter is aa. She got one a from each parent, but the parents look normal, so both are Aa (carriers).
2. In a pedigree, a father with the trait and a mother without it have 4 children. 2 have the trait. Which kind of trait fits best?
Dominant fits: Aa × aa gives half Aa (trait) and half aa (no trait). The affected father is Aa, the mother aa.
3. Two carriers (Aa × Aa) of a recessive disease plan 4 children. How many are expected to have the disease?
The chance of aa is 1 in 4, so about 1 of 4 children. It is only a chance, so a real family can differ.
4. In a pedigree, II-2 is a circle that is filled in. Say in words who II-2 is.
II-2 is the second person in the second generation. A circle means a female. Filled means she has the trait.
5. A trait is seen in a grandmother, her son and her granddaughter. Which kind of trait is likely? Why?
Dominant. It shows in all three generations with no skip, and each affected person has an affected parent.
6. A couple are both aa for a recessive trait. Could they have a child without the trait?
No. Both can only pass a, so every child is aa and has the trait.
Common mistakes
- Thinking a shaded shape means 'dead'. In pedigrees, dark means 'has the trait'.
- Saying two unaffected parents cannot have an affected child. For a recessive trait they can, if both are carriers (Aa).
- Thinking a carrier shows the trait. A carrier (Aa) looks normal for a recessive trait.
- Mixing up the two lines: the line between parents is horizontal, the line to children goes down.