Chapter 11, Problem 9P

In 2015, an international team of scientists assembled the complete genome sequences of two different woolly mammoths. Both specimens were discovered buried in the permafrost of Siberia, the coldest inhabited place on earth. Through radiocarbon dating, it was determined that one of the mammoths, found on Wrangel Island off the Siberian coast, died about 4000 years ago; the other mammoth, found in the town of Oimyakon, died about 45,000 years ago.

Analysis revealed that the genome sequences of these two animals differed significantly in the distribution of base pairs at which they are either homozygous or heterozygous. The Wrangel Island woolly mammoth had an extreme excess of runs of homozygosity (ROHs), regions in which the animal was homozygous for all of the base pairs. About 23.4% of the Wrangel Island animal’s genome was composed of ROHs that were greater than 500 kb in length; some of these ROHs were in excess of 5 Mb long. In contrast, only 0.83% of the Oimyakon animal’s genome consisted of ROHs longer than 500 kb.

a.	Explain how polymorphisms are detected when sequencing a genome. How would researchers know, for any particular base pair, whether a genome is homozygous or heterozygous?
b.	What does the extreme excess of ROHs in the Wrangel Island mammoth genome suggest about that animal’s parents?
c.	The Wrangel Island woolly mammoth is thought to have belonged to the last population on earth before the species went extinct about 4000 years ago. The answer to part (b) suggests one possible reason for the woolly mammoth’s extinction. Explain.