Biology · Introductory biology · Worked example
Read a pedigree to find the inheritance pattern
Two unaffected parents have an affected daughter and an unaffected son. Assuming full penetrance, decide which inheritance pattern fits, give each parent’s genotype, and find the chance that another child is affected.
Dominant or recessive?
An affected child of two unaffected parents rules out a dominant allele: with full penetrance, a parent carrying it would show the trait. The allele is recessive, and the daughter received a copy from each parent.
Autosomal or X-linked?
If the allele were X-linked recessive, an affected daughter would have received an X carrying it from her father, and he would show the trait. He does not, so the gene is autosomal.
Rule out the rest
Y-linked traits appear only in males, and mitochondrial traits pass from an affected mother to all her children. Neither fits an affected daughter of unaffected parents.
Assign genotypes
The daughter is aa. Each parent must carry one a while showing the dominant phenotype, so both are Aa carriers.
Predict the next child
Aa × Aa gives aa with probability ¼, whatever happened with earlier children.
Result
Autosomal recessive: both parents are carriers (Aa) and the daughter is aa. Each further child has a ¼ chance of being affected.
Your turn
What is the probability that the unaffected son is a carrier?
Show the answer and explanation
2/3.
Among the children of Aa × Aa, AA, Aa, aA and aa are equally likely. The son is unaffected, which leaves AA, Aa and aA, and two of those three carry a. So the probability is (1/2)/(3/4) = 2/3, not 1/2.
Keep exploring
The pedigree tool opens with the parents and their affected daughter under autosomal recessive inheritance, and shows both parents as Aa. Choose another inheritance pattern to see it reported as a contradiction.
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