DNA, or deoxyribonucleic acid, serves as the fundamental blueprint for all living organisms, carrying the genetic instructions that make each individual unique. This complex molecule is passed down through generations, determining a wide array of traits, from physical characteristics to predispositions for certain conditions. Family members inherit varying amounts of this genetic material, which forms the basis of their biological relationships.
What Defines Half-Siblings
Half-siblings are individuals who share one biological parent but not both. This means they might have the same mother but different fathers, or the same father but different mothers. This genetic distinction sets them apart from full siblings, who share both biological parents and therefore inherit DNA from the same two sources. Cousins, on the other hand, share more distant common ancestors, typically grandparents, resulting in less shared DNA compared to half-siblings.
Expected DNA Sharing
Half-siblings, on average, share approximately 25% of their DNA. This percentage reflects the genetic inheritance pattern where each child receives half of their DNA from each parent. Since half-siblings share only one parent, they inherit half of that parent’s genetic material. Genetic genealogists often use centimorgans (cM), a unit describing DNA segment length (the human genome is approximately 6,800 cM), to quantify shared DNA. Half-siblings typically share between 1,160 and 2,436 cM, with an average of about 1,700 to 1,800 cM.
Variability in Shared DNA
While the average shared DNA between half-siblings is around 25%, the actual amount can vary due to the random nature of genetic inheritance. During the formation of reproductive cells (sperm and egg) through a process called meiosis, DNA segments are randomly shuffled and recombined. This means that each child inherits a unique and random combination of DNA from their parents. Consequently, even full siblings do not share an exact 50% of their DNA; similarly, half-siblings will not share precisely 25%. The actual percentage of shared DNA between half-siblings can range from approximately 17% to 34%.
DNA Testing for Relatedness
Consumer DNA tests analyze genetic markers to identify and quantify shared DNA between individuals. These tests compare DNA segments to determine the total number of shared centimorgans, which helps estimate the degree of relatedness. The results typically provide an an estimated percentage of shared DNA and a breakdown of shared segments across chromosomes. This information can confirm half-sibling relationships by showing a shared DNA amount within the expected range for this familial connection. While shared centimorgans offer a strong indicator, some companies may also analyze the number and length of shared DNA segments, with longer, fewer segments generally suggesting a closer relationship and providing further evidence to support or clarify half-sibling connections.