The Ancestry Composition feature provides three different ways for you to view your ancestry assignments:
Sharing with a genotyped biological parent will allow your Ancestry Composition results to be re-phased using your parent’s genetic information. Your new Ancestry Composition estimates will be visible in Split View, along with the ability to see each parent’s contribution to your overall Ancestry Composition.
Phasing is the process of separating the mixed genotype data for each location into the two haplotypes, one from each parent. Phasing against one or more parents produces a higher-quality phasing. Phasing against biological children will also yield higher-quality phasing in Ancestry Composition, but Split View requires the addition of a biological parent.
Once you establish sharing with a biological parent or child, or if your child or parent is in the same account, Ancestry Composition will automatically re-phase your data and your ancestry estimates will be recalculated. Children can be phased against one or both parents, but a parent can only be phased against one child.
Chromosome View in the Ancestry Composition feature allows you to view your ancestry along each of your chromosomes.
Unless you have access to Split View, the parental contributions are not necessarily divided accurately between the top and bottom chromosomes shown in this view. For example, the Italian ancestry highlighted here comes primarily from one parent, but appears on both the top and bottom chromosomes:
If you are sharing genomes with a biological parent and have Split View enabled, Chromosome View will reflect the parental contributions. One copy of each chromosome pair will reflect the ancestry you inherited from each parent. For the same individual as above, after linking a parent, the highlighted Italian ancestry appears consistently on the bottom of the two chromosomes:
When Split View is available, the bottom chromosome will show the paternal contribution while the top chromosome will show the maternal contribution.
The X chromosome
The X chromosome is included in the Ancestry Composition analysis. Females will see their two X chromosomes represented in Chromosome View, while males will see a single X chromosome. Contributions from X will be included in both the Map View and Split View totals as well.
The X chromosome is present in two copies in females, but just one in males. So while females receive equal amounts of DNA from their mothers and fathers, males receive slightly less from their fathers as the Y chromosome is significantly smaller in size than the X chromosome and is not included in the Ancestry Composition analysis.
The 22 autosomes are labeled by size, with chromosome 1 being the largest and 22 the smallest. We've arranged the chromosomes in Chromosome View of Ancestry Composition to reflect their relative size so that the results are easier to view.
Chromosomes 13, 14, 15, 21, and 22 are slightly indented because the 23andMe genotyping chip has no markers for the short arms of these chromosomes. The indention keeps the relative length of the chromosomes consistent.