Ancestry

The information and genetic relationship with the individuals from whom one is biologically descended

Ancestry is most formally defined either genealogically or genetically, but in common usage, is often self-assessed by an individual based off groups they feel represent their pedigree; alternatively genetic data can be used to infer it

Although the forms of ancestry discussed below allow for a relatively objective categorisation, in practice (especially with self identification), many people are not certain of their ancestor’s ancestral groups, especially considering the number of ancestors exponentially increases.

Genealogical ancestry

The lineage of an individual traced through their family tree, connecting them to their individual ancestors over successive generations

Often determined via historical or genealogical records that allow us to find identifiable ancestors in our family tree/pedigree

Pedigree

The recorded ancestry/lineage of a person or family

• Can be conceptualised as a graph from which we can deduce facts about relatedness
• The pedigree is fixed and objective

Ancestral categories

There is no strict way of defining and categorising ancestry, thus the ancestral categories one identifies with can be fluid

Most commonly ancestry is defined:

• Geographically: Asian, Eastern European, sub-Saharan African
• Culturally: Apache, Brahmin
• Geopolitically: Norwegian, Zambian, Vietnamese

Use in spoken parlance

If an ancestor belonged to a particular group X, one might say they have some X ancestry

• For example, if one of your eight great-grandparents belonged to the group X, then one might describe themselves as one-eighth X

In practice, most people have little comprehensive knowledge of their ancestors beyond a handful of preceding generations

Genetic ancestry

Examines the subset of paths through an individual’s pedigree by which the material in their genome has been inherited, since everyone’s chromosomes are inherited in one of many possible ways

At every given position in one of an individual’s chromosomes, DNA is inherited via a unique path of inheritance, since parental chromosomes are shuffled together during meiotic recombination

Genealogical vs genetic ancestry

Full siblings have identical genealogical ancestry (same pedigree), but differ in genetic ancestry since the way the chromosomal segments were inherited from their parents differs (unless they’re identical twins)

Ancestral recombination graph

The ARG is a subset of the ancestral pedigree graph representing the nodes corresponding to ancestors where there was a recombination/coalescence event contributing to the individual’s genome, and the paths between them (representing line of descent) that also bypass through individuals on the pedigree not in the ARG

Practicality

Though the ARG seems to provide an essentialist and objective notion of ancestry, it is very impractical to infer the ARG due to its reliance on patterns of genetic variation

Even if we are able to reconstruct the true ARG, the problem of obtaining the ancestral category identification of each member (fluid construct) still remains

Most genetic ancestry analyses take an alternative approach

1. An approximate summary without ARG reconstruction relying on ‘population ancestry’ (see admixture graphs) - this is still uncommon and not what consumer genomics companies do - see Genetic similarity

Categorising genetic ancestry

Genetic ancestry in some category X means some fraction of an individual’s genome is inherited directly from an ancestor in X (i.e.: an edge in their ARG passes through an ancestor in X)

• Genetic ancestry in X implies genealogical ancestry in X but not vice-versa
• We can try computing the proportion of an individuals’s genome inherited from ancestors in group X (approach approximated by 23andMe etc.)

Examination of DNA variations can provide clues about where a person’s ancestors might have come from and about relationships between families. Certain patterns of genetic variation are often shared among people of particular backgrounds. The more closely related two individuals, families, or populations are, the more patterns of variation they typically share.

Genetic similarity

The similarity in genetics between populations and individuals

Genetic similarity is not a direct form of ancestry analysis

Consumer genomics

Most consumer genomic companies specialise in genetic similarity even when using the word ‘ancestry’

• Some methods allow individual genomes to be represented as combinations of reference populations
• Can range from approximations of the ARG to other models with no direct relationship to genetic ancestry

Relation to ancestry

Genetic similarity can be informative about genetic ancestry on broad temporal and geographical scales

Rationale

If segments of your genome are found to be similar to individuals from particular continental groups (“European”, “Native American”), it’s likely that you have genetic ancestry from those groups in the past few hundreds or thousands of years

Caveats

Implicitly uses non-genetic information though (historical population information and geographical continuity of populations), which means finer-scale analysis reveals a larger disconnect between ancestry and genetic similarity

• At sub-continental scales especially, many groups have experienced episodes of replacement

Thus some of your genetic similarity to present-day individuals in a particular location may derive from shared ancestry tens or hundreds of generations ago in a different part of the world

A more practical issue is that measures of similarity are always sensitive to the choice and labelling of reference populations - may give the false impression that inference from genetic data is unbiased