Consanguinity and genomic sharing in human evolutionary inference

Published on March 18, 2015   51 min

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Other Talks in the Series: Human Population Genetics II

Hello, I'm Trevor Pemberton, a Professor of Biochemistry and Medical Genetics at the University of Manitoba. Today I'm going to talk about the influence of cultural and population processes on patterns of identity by descenting human populations and their importance in understanding human evolutionary history and phenotypic variation.
I will start by defining what identity by descent is and the population and cultural processes that give rise to it. Next, I will introduce the inbreeding coefficient as a measure of identity by descent levels in individual genomes, the pedigree and genomic estimators used to calculate it, and give an overview of its patterns in worldwide human populations, and how these reflect their different cultures and histories. Then I will introduce runs of homozygosity as an approach to detect identity by descent regions in individual genomes, and how inbreeding coefficient estimates based upon this approach correlate with those obtained with the genomic estimator. Finally, I'll review some recent findings on genomic patterns in runs of homozygosity in worldwide human populations, their utility for understanding human evolutionary history, and briefly outline their importance in human phenotypic variation.
For a pair of individuals a genomic region is said to be identical by state if they have an identical nucleotide sequence in that region. An identical by state region is identical by descent if both individuals have inherited it from a common ancestor, that is the region has the same ancestral origin in these individuals. Genomic regions that are identical by descent are identical by state by definition. but regions that are not identical by descent can still be identical by state due to the same mutations arising in different individuals, or a combination of events that change the ancestral origin of the segment without altering its sequence.
The basic principles that underlie identity by descent sharing in individual genomes are shown in this diagram. Chromosomes with different ancestral origins in the two founders are represented by different colors. At each parent/ offspring transmission there exists an opportunity for chromosomal crossover to occur during the formation of the parents' gametes, where recombination exchanges genetic material between homologous chromosomes. These recombinant chromosomes are then passed stochastically on to the offspring, with this process repeated in subsequent generations. For convenience, if we consider the chromosomal complement of the two founders to derive from different ancestral origins, then they share no genomic segments identical by descent. Their offspring would, however, be expected to share many long identical by descent segments since recombination has only had a single opportunity to disrupt the ancestral haplotypes they inherit from the founders, while in subsequent generations fewer and shorter segments will be expected as additional crossover events further fragment the ancestral haplotypes segregating in their lineages.

Consanguinity and genomic sharing in human evolutionary inference

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