Reading Kinship in DNA Bands

In the picture

1. 1The ostrich, standing along the top beside the emu and the kiwi.
2. 2The strip of coloured bands under the emu: the same stretch of DNA, read band by band.
3. 3The odd-coloured band in the ostrich’s strip, where the letter does not match the other two birds.
4. 4The high fork where the emu and kiwi lines meet: their most recent shared ancestor.
5. 5The low fork where the ostrich line joins: a far older shared ancestor.

Examples

• Human and chimpanzee DNA differs at roughly one letter in a hundred, placing our shared ancestor only a few million years back.
• A dog breed DNA test works the same way: the lab matches letter patterns against reference breeds instead of judging the dog by its looks.

Quiz: Reading Kinship in DNA Bands (Age 15)

1. 1

   You compare the same stretch of DNA in two species and find only a few letter differences. What does that tell you?

   • They split from a shared ancestor more recently than species with many differences.
   • They split from a shared ancestor far longer ago than species with many differences.
   • Their DNA is shorter, so there is simply less of it that could differ.
2. 2

   Birds A and B look almost identical, while B and C look nothing alike. In the same stretch of DNA, B differs from C at 4 letters and from A at 30. Which pair is more closely related?

   • B and C, because fewer letter differences means a more recent shared ancestor.
   • A and B, because species that look alike must be the closest relatives.
   • Neither, because body shape and DNA disagree, so no answer is possible.
3. 3

   Among three related species, one differs from the other two at many more DNA letters than those two differ from each other. Where does that species join the family tree?

   • At an older branching point, further back than the split between the other two.
   • At exactly the same branching point as the other two, since all three belong to one group.
   • At the most recent branching point, because its DNA is the most unusual.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.