Pedigree Analysis

Standard symbols

Step 1: is the trait dominant or recessive?

• If two unaffected parents have an affected child, the trait must be recessive, two carriers (heterozygous, unaffected) can each pass on a hidden recessive allele.
• If an affected parent has all affected children whenever the other parent is affected, or if the trait skips no generations, dominant inheritance is more likely.
• A recessive trait can 'skip' a generation because carriers show no symptoms; a dominant trait cannot skip a generation, since at least one parent must show it for a child to be affected.

Step 2: is the trait autosomal or X-linked?

• If the trait is recessive and affects roughly equal numbers of males and females, it is likely autosomal recessive.
• If the trait is recessive and affects far more males than females, and an affected father never passes it to his sons, it is likely X-linked recessive (sons get their X only from their mother).
• If every daughter of an affected father is also affected, this points toward X-linked dominant inheritance, which is much rarer than the other patterns.

Step 3: deducing genotypes

Once the pattern is established, work through the pedigree individual by individual. Affected individuals for a recessive trait must be homozygous recessive.

Unaffected individuals with an affected child must be heterozygous carriers. Individuals with no information can only be described as 'dominant phenotype, genotype unknown' until more data narrows it down.

How it is examined

SPM questions typically give a pedigree diagram and ask you to state whether the trait is dominant or recessive, whether it is autosomal or sex-linked, and to give the genotype of specific individuals with reasoning.

Worked problem 1 (easy): two carriers in a pedigree

A pedigree shows two unaffected parents who have a child affected by an autosomal recessive condition (recessive allele a, dominant allele A). Work out the genotypes of the parents and the chance that their next child is affected.

An affected child is a a, so it received one a allele from each parent; since both parents are unaffected, each must be a heterozygous carrier, A a. The cross A a x A a gives offspring of 1 AA : 2 Aa : 1 aa, so 3 in 4 children are unaffected and 1 in 4 is affected.

Worked problem 2 (medium): an X-linked recessive trait

In colour blindness, the normal allele X^B is dominant over the colour-blind allele X^b, and the gene is on the X chromosome. A carrier mother (X^B X^b) has children with an unaffected father (X^B Y).

Work out the expected phenotypes of the children.

The mother forms X^B and X^b gametes; the father forms X^B and Y gametes. Daughters receive the father's X^B, so none are colour blind although half are carriers (X^B X^b).

Sons receive the father's Y and either the mother's X^B or X^b, so the sons are 1 unaffected (X^B Y) : 1 colour blind (X^b Y).

Worked problem 3 (SPM-level): an affected father in the pedigree

A pedigree shows a colour-blind man (X^b Y) who has children with a woman who is a carrier for colour blindness (X^B X^b). Work out the expected phenotypes of their sons and daughters.

The mother forms X^B and X^b gametes; the father forms X^b and Y gametes. The daughters are 1 carrier but unaffected (X^B X^b) : 1 colour blind (X^b X^b), and the sons are 1 unaffected (X^B Y) : 1 colour blind (X^b Y).

This is the situation in which a colour-blind daughter can appear, because the father contributes an X^b allele to every daughter.

Practice problems

1. In a pedigree, two unaffected parents have a son with an autosomal recessive condition. State the genotypes of both parents and the chance that their next child is affected.
2. A woman who is a carrier for haemophilia (X^H X^h) has children with an unaffected man (X^H Y). State the expected phenotypes of their sons.
3. A pedigree shows that an affected father with an X-linked recessive condition never passes it to his sons, but all of his daughters are carriers. Explain why, in terms of the X and Y chromosomes he passes on.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions