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Family pedigrees and probability
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Family pedigrees show how a characteristic is passed down. Genetic diagrams let you predict the chances for the next child.
Reading a pedigree
Squares are males and circles are females. Shaded symbols show people with the characteristic.
A horizontal line joins parents; their children hang below.
Clue for a recessive allele: two parents without the characteristic have a child with it. Both parents must be heterozygous.
A horizontal line joins parents; their children hang below.
Clue for a recessive allele: two parents without the characteristic have a child with it. Both parents must be heterozygous.
Predicting outcomes
Use a genetic diagram (Punnett square).
Probability is a chance, not a promise
Each child is a separate event. If two carriers already have an affected child, the chance for the next child is still 1 in 4.
In a large number of offspring, the ratios get closer to the predicted ones.
In a large number of offspring, the ratios get closer to the predicted ones.
Two parents are both Tt for a recessive condition (tt). What is the probability that their child has the condition?
- Gametes: T or t from each parent.
- Offspring: TT, Tt, Tt, tt.
- One in four is tt.
Answer: 1/4 (25%)
In a pedigree, squares are males, circles females, shaded means affected. Two unaffected parents with an affected child means the allele is recessive and both are heterozygous. Aa × Aa gives 25% recessive phenotype (3 : 1); Aa × aa gives 50% (1 : 1).
The interactive lesson includes the diagrams for this topic.
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