Identifying genetic relatedness between human individuals is useful for many applications.
A common approach to this is based on detecting identical-by-descent (IBD) segments shared by two individuals. IBD segments are chromosomal segments that the two individuals inherited from a common ancestor, and hence are identical in sequence except for any mutations that happened since that common ancestor. IBD segment ends correspond to sites where recombination happened.
At a given position in the genome two individuals can share 0, 1 or 2 IBD segments (IBD0, IBD1 and IBD2 states respectively). Looking at the fraction of the genome in IBD1 and IBD2 states, for a given pair of individuals, is a common practice to infer genealogical relationships between individuals.
In questions 24.1, 24.2 and 24.3 assume that parents of any individual are completely unrelated and do not share any IBD segments.
Indicate the expected fraction of the genome (in percentage, with 100 being the maximum) in IBD1 and IBD2 states between pairs of individuals with the following family relationships: (1) Monozygotic twins, (2) Parent-offspring, (3) Full-siblings, (4) Half-siblings, (5) Grandparent-grandchild.
Q24.1.1. Expected fraction of the genome in IBD1 state between monozygotic twins (percentage).
Q24.1.2. Expected fraction of the genome in IBD1 state between parent and offspring (percentage).
Q24.1.3. Expected fraction of the genome in IBD1 state between full siblings (percentage).
Q24.1.4. Expected fraction of the genome in IBD1 state between half-siblings (percentage).
Q24.1.5. Expected fraction of the genome in IBD1 state between grandparent and grandchild (percentage).
Q24.2.1. Expected fraction of the genome in IBD2 state between monozygotic twins (percentage).
Q24.2.2. Expected fraction of the genome in IBD2 state between parent and offspring (percentage).
Q24.2.3. Expected fraction of the genome in IBD2 state between full siblings (percentage).
Q24.2.4. Expected fraction of the genome in IBD2 state between half-siblings (percentage).
Q24.2.5. Expected fraction of the genome in IBD2 state between grandparent and grandchild (percentage).
Q24.3.1. Notable biological variation in IBD sharing is expected for monozygotic twins.
Q24.3.2. Notable biological variation in IBD sharing is expected for parent-offspring.
Q24.3.3. Notable biological variation in IBD sharing is expected for full siblings.
Q24.3.4. Notable biological variation in IBD sharing is expected for half-siblings.
Q24.3.5. Notable biological variation in IBD sharing is expected for grandparent-grandchild.
Q24.4.1. On average, IBD segments shared between a parent and an offspring are longer compared to those shared between full siblings.
Q24.4.2. If we trace one's family tree deep into the past, in the 30th generation we will find 2^30 people this individual shares IBD with.
Q24.4.3. The fraction of IBD1 genome between two randomly picked individuals from a single population is higher in populations with smaller effective population size.
Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF