Two Peas in a Pod? A Case of Questionable Twins

The constant stares, innuendoes, and inquiries from nosey neighbors became more than the new parents could stand. As hard as she tried, Hannah could not protect her family from the town gossip.

“We’ve lived in this town for a year now. When I walk with the kids, people stare at me and think: ‘It’s true, those kids really exist.’ People I have never met look in the buggy and say: ‘They’re twins? How is that possible?’”

After a lot of debate, Hannah, 33, and Nathan, 40, decided to have their family genetically tested using gel electrophoresis, also called DNA profiling. The results of four different single locus autorads are shown in Figure 1. Four different locations on their genome were analyzed for the number of VNTRs (Variable Number Tandem Repeats) found in relation to a sample of VNTRs of known (standard) lengths and each other. Strands of similar length are pulled similar distances through a gel by an electrical current resulting in “bands” that can be read.

Questions

1. What does each “band” represent? Account for the two bands shown by each individual. Why is it necessary to run more than a single gel?
2. What conclusions regarding paternity can be drawn from the gel results? Which scenarios listed in Part I, Question 3, are no longer likely in light of this evidence? Which scenarios remain possible? Why?

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