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0:00
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.
0:17
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.
1:02
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.
1:37
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.
