Half-siblings

Main contributor: MyHeritage team

Half-siblings are individuals who share one biological parent but not both, as opposed to full siblings, who share two biological parents. Half-sibling relationships can arise from various scenarios, including remarriages, blended families resulting from divorce and subsequent unions, and cases of donor-conceived children.

Half-siblings vs. full siblings

A fundamental distinction between half-siblings and full siblings lies in the shared genetic makeup. Half-siblings typically inherit fewer shared segments of DNA compared to full siblings, who inherit genetic material from both parents. This genetic variation arises from the random assortment of genetic material during meiosis, the process of cell division that leads to the formation of eggs and sperm.

Autosomal DNA, which comprises the majority of our genome, is inherited from both parents and carries valuable information about our heritage. In contrast, sex chromosomes, specifically the X and Y chromosomes, play a significant role in distinguishing between male and female siblings. Male half-siblings share half of their X and Y chromosomes, while female half-siblings share one X chromosome from their common parent.

To quantify genetic relatedness, centimorgans (cMs) are used as a unit of measurement. Full siblings typically share around 2600 cMs of DNA, while half-siblings share about half of that, around 1700 cMs, on average. This substantial difference in shared DNA makes it feasible to differentiate between full and half-sibling relationships through genetic testing.

How to tell if a DNA match is a half-sibling

Siblings present a unique challenge when it comes to DNA matching, because while 100% of the DNA they inherited comes from the same two people, the sections they inherit from each parent are random. This results in a situation where most siblings share around 50% of their DNA, but that percentage can vary from around 37.5% to around 61%.

Half-siblings will share half that amount of DNA, but again, it's a wider range than are generally with other close genetic relationships.

If a pair of DNA matches share around 25% of DNA, they might be half-siblings, but they also might be:

Uncle/aunt and nephew/niece
Grandparent and grandchild

In rare cases, they might even be a full siblings — so even closeness in age isn't definitive evidence that they are half-siblings. Accordingly, it's impossible to determine a half-sibling relationship based on the two DNA kits alone. The results should be interpreted alongside genealogical records and/or the testing of additional family members to verify the relationship.

Ethical and emotional considerations

The revelation of half-sibling relationships can bring about profound emotional responses and ethical considerations. The newfound knowledge of half-siblings might challenge existing family dynamics, especially when the information is unexpected or withheld intentionally. In cases involving donor-conceived children, the ethical responsibility of sharing genetic information and respecting the rights of all parties involved adds another layer of complexity. (See also: Non-paternity event)

Future implications and research

As technology continues to advance, the accuracy of distinguishing half-siblings from other genetic relationships is expected to improve. Research in the field of genealogy and DNA analysis is likely to focus on refining techniques for identifying half-sibling connections and enhancing the precision of genetic analysis. These developments hold the promise of uncovering even more intricate family stories and relationships.

Explore more about half-siblings

The MyHeritage DNA test
Full or Half Siblings? from DNAeXplained by Roberta Estes
Shared DNA: How Much DNA Do You Share with Your Relatives?: article from the MyHeritage Knowledge Base
Stories of half-siblings who found each other via MyHeritage DNA on the MyHeritage blog:

References