Analysis of Pedigrees | Genetics

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The diagram below shows a pedigree of three generations in a family. Black circles/squares indicate persons with a genetic disorder. A square indicates a male and a circle indicates a female. The two males in generation 1 are siblings.

Q1. Looking only at the generation 2 offspring (of the two generation 1 brothers), what can you say about the gene(s) controlling the genetic disorder? Is the disorder caused by a gene that is dominant or recessive, autosomal or sex-linked?

Q2. What additional information do you gain from examining the generation 3 offspring?

A1. The gene is most likely dominant. If it is dominant, the gene may be either autosomal or sex-linked based on these data alone. There is a chance that the gene is recessive. However, for this to be true, the two brothers would both have to mate with a heterozygous female in order to produce the offspring in generation 2. This is much less likely but still a possibility.

A2. The mating between two affected individuals (lineage A – Generation 2) produces one unaffected male offspring. If the disorder were caused by an autosomal recessive gene, all of the offspring in this cross would be homozygous recessive and have the disorder. Because one male does not have it, the disorder must be caused by a dominant allele. Given the information in generations 2 and 3, it is likely that the allele is also sex-linked (since all daughters of affected males have the disorder and males only have the disorder if their mother had it). However, given that this is a relatively small population, there is still a possibility that the disorder is autosomal, dominant.