50 whys to look for genes: 40. Determine one’s ancestry

Examining a male’s Y chromosome can expose genetic variants (or a pattern of variants) common in certain regions of the world and thus tell you that the root of his ancestry going through father-paternal grandfather-etc. probably originated in that place.  Similarly, examining a female’s X mitochondrial DNA, which are transmitted from the egg and not the sperm, can tell her the root of her ancestry going through mother-maternal grandmother-etc. probably originated in a given place.

Each of us has, of course, many, many more ancestral roots than these two.  “Ancestry-informative markers”–variations at specific places on the genome that differ between regions, can be used to estimate proportions of one’s ancestry that originated in various different regions (wikipedia).

Complications

1. “Passed through” would be more accurate than “originated,” given that all Homo sapiens sapiens trace back to one woman, who lived in Africa between 100,000 and 200,000 years ago (wikipedia).  Similarly, all Homo sapiens sapiens males trace back to one man, who lived in Africa between 140,000 and 330,000 years ago (wikipedia).  (Many people also have a fraction of their genome from Homo neanderthalensis.)  So one needs to set some arbitrary point in the past for determining the region of one’s ancestors.

2. “Common in certain regions of the world” means common today.  The ancestors of people living in some region today need not have lived in that region at the particular point in the past chosen for determination ancestry.

3. Knowing the proportion of one’s ancestry that originated in a particular region does not imply that you possess a corresponding proportion of the genes that are common in that region.  The reason: At any time, in any region there is great variation among individuals in their genetic make-up (see previous post).  Indeed, it is difficult to “depict genetic relationships among humans in ways that allow simultaneously for similarity, diversity, and admixture at the same time as we depict ancestry” (previous series of posts).

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(Introduction to this series of posts)

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