Siblings, in the context of genetics, are individuals who share one or both biological parents. Their genetic relationship is determined by the inheritance patterns of DNA during reproduction. Grasping the intricacies of genetic relationships among siblings is crucial for accurately determining the genetic connection between two individuals.
What is an inheritance pattern?
Humans inherit their DNA from their biological parents. Each parent possesses a complete genome, half of which[1] is contributed to each offspring through specialized sex cells: sperm (father) and egg (mother).
Types of siblings
Identical twins
Identical twins are a unique case. They arise from a single fertilized egg that splits early in embryonic development, resulting in two genetically identical individuals. Identical twins are usually of the same sex.
Full siblings
Full siblings share both biological parents (the mother and the father).
Fraternal twins
Fraternal twins develop when two separate eggs are fertilized by two separate sperm. Fraternal twins are no more genetically similar than any other siblings. The twins can be either the same sex or different sexes
Half siblings
Half-siblings share only one biological parent (only a father or only a mother).
3/4 siblings
3/4 siblings are genetically half way between full siblings and half siblings. Siblings will be considered 3/4 when one of the following combinations occur:
- Children are related as half-siblings AND first cousins: This will occur in one of the following cases: A. A woman has children by each of two brothers. B. A man has children by each of two sisters.
- Children are related as half-siblings and half-aunt or half-uncle and half-niece or half-nephew: This will occur in one of the following cases: A. A woman has children with both a man and his father. B. A man has children with both a woman and her daughter.
Full siblings
Since siblings inherit half their DNA from each parent, on average, full siblings will share approximately 50% of their DNA.
This 50% represents a combination of inherited genes from both parents.
The 50% shared DNA of full siblings is an average. Due to the random shuffling during meiosis, siblings can inherit slightly more or less than 50% of their DNA from each parent. This variation can be as high as 60% shared DNA with one full sibling and 40% with another full sibling.
Identical twins
Since Identical twins are a result of a split of a single fertilized egg, they share 100% of their DNA.
Fraternal twins
Fraternal twins, develop from separate fertilized eggs and share, on average, 50% of their DNA, just like full siblings.
Half siblings
Half siblings share around 25% of their DNA with each other. They inherit 50% from the parent they have in common.
Each half-sibling receives their other 50% of DNA from a different biological parent. These unique genetic contributions are not shared between the half-siblings. Therefore, the shared DNA between half-siblings comes solely from the parent they have in common, resulting in an average of 25% shared DNA.
3/4 siblings
Three-quarter siblings share on average 37.5% of their DNA.
All DNA testing companies measure the amount of DNA shared between two people in Centimorgans.
A centimorgan (cM) isn't a direct measure of physical distance on a chromosome, rather it is a specific unit of measurement that tells you the likelihood of genetic material being swapped between chromosomes during reproduction.
Each amount of DNA is reported in ranges and all calculations are probabilities. Before forming a definitive conclusion about the relationship at hand, one should consider the examining the various ranges as well as further information about the people at hand.
You can find various calculators to determine the relationship between two people:
- cM Explainer (MyHeritage Blog)
- Shared cM Project (by DNAPainter).
- ISOGG -Autosomal Statistics.
See also
Explore more about siblings' DNA
- Can Siblings Have Different Ethnicity Estimates? on the MyHeritage Knowledge Base
- The Power of Siblings – the DNA Tests of Five Brothers webinar at Legacy Family Tree Webinars
- Why are My Siblings Ethnicity Results Different? webinar at Legacy Family Tree Webinars
References
- ↑ Why siblings can be so different, according to science. Washington Post