Understanding genetic relationships, such as those between half-siblings, relies on the predictable process of DNA inheritance. This process helps uncover the precise nature of familial ties.
Understanding Half-Siblings and DNA Inheritance
A half-sibling is defined as an individual who shares one biological parent with another person. This means they have either the same mother but different fathers, or the same father but different mothers.
The process of DNA inheritance involves recombination, which occurs during the formation of reproductive cells. During recombination, segments of DNA from a parent’s two chromosome copies are shuffled and exchanged, creating new, unique combinations of genetic material. This random shuffling ensures that each child inherits a distinct set of DNA from their parents, making every sibling, except identical twins, genetically unique.
The Amount of Shared DNA Between Half-Siblings
Half-siblings typically share about 25% of their DNA, on average. This shared genetic material originates from their one common biological parent. The amount of shared DNA is commonly measured in centimorgans (cM), a unit that represents the likelihood of recombination between two points on a chromosome. DNA testing companies use cM values to quantify genetic relatedness.
The amount of DNA shared between half-siblings can range from approximately 1,160 to 2,436 cM, with an average around 1,750 cM. This variability exists because of the random nature of DNA inheritance and recombination. While half-siblings inherit half of their DNA from the shared parent, the specific segments of DNA they receive from that parent can differ significantly due to the random reshuffling of genetic material during cell division. Consequently, some half-siblings may share slightly more or less than the 25% average.
Comparing Shared DNA: Half-Siblings vs. Full Siblings
Full siblings, who share both biological parents, typically share a significantly higher percentage of their DNA compared to half-siblings. Full siblings share, on average, about 50% of their DNA. This translates to a shared centimorgan range of approximately 1,613 to 3,488 cM, with an average of around 2,600 cM.
The clear difference in the average percentage and centimorgan range of shared DNA helps distinguish between full and half-sibling relationships. While there can be some overlap in the lower end of the full sibling range and the higher end of the half-sibling range, other genetic markers usually clarify the relationship. For instance, first cousins, who share a set of grandparents, typically share about 12.5% of their DNA, or around 680 to 1,150 cM.
Confirming Half-Sibling Relationships Through DNA Testing
Consumer DNA testing services utilize the amount of shared DNA, particularly the cM value, to identify and confirm half-sibling relationships. These services compare DNA segments between two individuals and calculate the total centimorgans they share.
Beyond total shared cM, DNA tests also analyze specific patterns of shared segments. For example, full siblings often share many “fully identical regions” (FIRs) across their chromosomes, while half-siblings typically do not share significant FIRs. The absence of matching segments from the non-common parent also provides strong evidence for a half-sibling relationship. Including a known parent’s DNA sample in the testing process can further increase the accuracy of relationship confirmation.