Support/Connecting and Sharing/Comparison Features

Family Traits: Genome View

23andMe
posted this on April 10, 2012 08:45 AM

Family Traits: Genome View shows which parts of two related people's genomes appear to be inherited from a common ancestor. In reality, the genome is organized into 22 pairs of chromosomes, plus the sex chromosomes (two X, or one X and one Y). We show only one chromosome out of each pair, but we color them to show whether one copy is identical (i.e. half-identical), or both copies are identical (i.e. completely identical) between two people.

If you select a trait from the list, you can see which genes associated with the trait lay in the stretches of common inheritance.

Calculating identical segments

If two people are closely related, we expect them to have the same sequence across relatively large stretches of DNA - that is, more than would be expected by chance. This is because recombination mixes chromosomes so thoroughly that two people separated by enough generations no longer share long, intact stretches of DNA inherited from the same ancestor.

We can detect stretches of shared DNA by comparing SNP genotypes and looking for mismatches. If a continuous region of matching SNPs is longer than a minimum threshold which is set to avoid chance matches then it is likely to have been inherited from the same ancestor.

Note that our algorithms produce estimates of identity, and can occasionally miss identical regions or mistakenly call a non-identical region identical. It is particularly difficult to spot short regions of identity. As scientific knowledge improves and our database grows our estimates will be ever more precise.

Comparing parents to children

Children inherit 50% of their genomes from each parent, so if you compare a mother and daughter you will see that they are half-identical across the entire genome (as designated by the light color). In this case, the common ancestor is the mother herself.

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Comparing grandparents to grandchildren

Because a parent’s chromosomes recombine before being passed on to children, a child receives a mixture of his or her grandparents’ genomes. For example, the DNA a father passes on to his son is a mixture of DNA from the paternal grandmother and the paternal grandfather. Only patches of a grandchild’s genome—amounting to about 50%—will be half-identical to each grandparent’s genome.

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In our example, if you compared the grandson’s DNA to his paternal grandfather’s, you would see that about half of their chromosomal regions would be half-identical. If you then switched to compare the grandson to his paternal grandmother, you would see that all the stretches that were non-identical with the grandfather are now half-identical to the grandmother.

If you compared the same grandson to his mother’s parents, you would find a different set of patches. But again, the patches that were half-identical with the grandfather would be non-identical with the grandmother, and vice versa.

Comparing siblings and half siblings

The most informative comparison is between two siblings. It is possible to tell where the siblings share regions of complete identity—that is, short stretches of the genome where the two siblings effectively look like identical twins.

Two things must be true in order for this to happen. First, these stretches of the maternally-inherited chromosomes must come from the same maternal grandparent. Second, the same segments of the paternally-inherited chromosomes must come from the same paternal grandparent.

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Identical twins inherit entire chromosomes from the same two grandparents (one on each side). So in the Family Traits: Genome View tool, they would look completely identical across the entire genome.

Comparing unrelated individuals

Generally, two unrelated individuals will have no completely identical or half-identical stretches in their genomes. If they do, however, it is an indication that the two individuals are in fact related. The degree of relatedness is proportional to the total length of shaded genome; for example, one small chunk of half-identity on a single chromosome would indicate that two people are distant cousins. If a half-identical segment is shared on any of the 22 autosomes, this could have been inherited from an ancestor or either the maternal or paternal side.

 
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