It is indeed possible for siblings to have different ancestry percentages, even when they share the same biological parents. Ancestry refers to the proportion of one’s DNA that originates from various ancestral populations or geographical regions. While siblings inherit genetic material from the same parents, the specific combination of DNA they receive is unique, much like drawing different hands from the same deck of cards. This genetic variability explains their differing ancestral breakdowns.
The Genetic Basis of Ancestry
Every individual inherits a complete set of genetic instructions, or DNA, from their parents. This DNA is organized into structures called chromosomes, with humans typically having 23 pairs. One chromosome from each pair comes from the mother, and the other from the father.
Each parent contributes roughly half of a child’s genetic material. However, the specific segments of DNA inherited by one sibling will not be exactly the same as those inherited by another, unless they are identical twins. This means that although siblings share the same parental gene pool, the unique combination of genes they receive creates distinct genetic profiles.
How Genetic Recombination Creates Sibling Differences
The primary reason for differing ancestral profiles among siblings lies in genetic recombination, a biological process occurring during the formation of sperm and egg cells. Before genetic material is passed from parent to child, the parent’s two copies of each chromosome undergo a shuffling process.
This involves two main mechanisms: crossing over and independent assortment.
Crossing over is the exchange of segments between homologous chromosomes, which are pairs of chromosomes containing the same genes but inherited from different parents. During this process, sections of DNA from the maternal chromosome can swap places with corresponding sections on the paternal chromosome. This creates new combinations of genetic information on each chromosome.
Independent assortment refers to the random distribution of recombined chromosomes into the reproductive cells. Each pair of homologous chromosomes aligns independently, meaning the inheritance of one chromosome pair does not influence the inheritance of another. This random segregation of chromosomes further contributes to the vast number of unique genetic combinations possible for each sperm or egg cell. As a result of these processes, each sibling receives a unique blend of their parents’ DNA, leading to variations in ancestral markers.
Interpreting Ancestry Test Results
When siblings take commercial ancestry tests, the results often show different percentage breakdowns for various ancestral regions, reflecting genetic recombination and independent assortment. These tests analyze specific genetic markers to estimate ancestral origins by comparing DNA to reference populations from different geographical areas.
Because each sibling inherits a unique, shuffled combination of their parents’ DNA, they may receive different “chunks” of ancestral genetic material. For example, one sibling might inherit more DNA segments linked to a specific European region, while another might receive a larger proportion from an Asian or African lineage, even though both parents carry DNA from all these regions. These differences are typically in percentages rather than entirely distinct regions, meaning siblings generally share many ancestral regions but in varying proportions. The estimated percentages from these tests are approximations based on complex algorithms and reference databases.