From ISOGG Wiki
Three-quarter siblings are siblings who are genetically half way between full siblings and half siblings. This can happen, for example, if they share the same father but different mothers but their mothers are sisters which makes them effectively both half-siblings and first cousins. An alternative scenario is if a woman has a child with both a man and his father which makes the children related both as half-siblings and half-aunt/uncle vs half-nephew/niece.
Three-quarter siblings can occur as a result of the following scenarios:
- a man has children with each of two sisters (the children are related as half-siblings and first cousins)
- a woman has children by each of two brothers (the children are related as half-siblings and first cousins)
- a woman has children with both a man and his father (the children are related as half-siblings and half-aunt or half-uncle and half-niece or half-nephew)
- a man has children with both a woman and her daughter (the children are related as half-siblings and half-aunt or half-uncle and half-niece or half-nephew)
Full siblings share on average 50% of their DNA when accounting for both fully identical and half-identical regions. Three-quarter siblings share on average 37.5% of their DNA with both full and half-identical regions. However, the range of sharing for both full siblings and three-quarter siblings is variable. Full siblings can theoretically share between 38% and 61% of their DNA. This means that there can be an overlap in the ranges at the low end of sharing for full siblings and at the high end of sharing for three-quarter siblings.
Andrew Millard has performed computer simulations to estimate the range of sharing for both full and three-quarter siblings to show the overlap.
Andrew Millard has produced additional simulations showing the distribution of fully identical regions (FIR) in full siblings and three-quarter siblings. While it only makes a small difference whether it is the mother or father that is shared the distribution of FIRs should in theory distinguish most relationships.
23andMe is the only company which provides a breakdown of the number of fully identical and half identical regions and also provides a visualisation in a chromosome browser. If the testing has been done elsewhere you can upload your results to the third-party website GEDmatch which also has a chromosome browser feature and will report both full and half-identical regions.
Galván-Femenía et al have developed a likelihood ratio approach for identifying three-quarter siblings in genetic databases which they tested on 5400 genomes from Catalonia in Spain. They were able to find eight pairs in the dataset. See:
Galván-Femenía G, Barceló-Vidal C, Sumoy L, Moreno V, de Cid R and Graffelman J (2021). A likelihood ratio approach for identifying three-quarter siblings in genetic databases. Heredity, published online 15 January 2021.
- If two kids have same dad but their moms are sisters and the sisters are from same mom and dad, are the kids full siblings or half?. Ask A Geneticist, The Tech Interactive, 9 September 2014.
- I have four children with two different fathers that were brothers... Ask A Geneticist, The Tech Interactive, 29 September 2011.
- Autosomal DNA statistics
- Coefficient of relationship
- Fully identical region
- Half-identical region
- The 37% figure is derived by adding together the average amount of sharing for half-siblings (25%) and first cousins (12.5%) or half-niece/nephew versus half-aunt/uncle (12.5%).
- See also the simulations from Visscher et al (2006) who estimated that full siblings share between 37% and 62% of their DNA. From Figure 1 in: Visscher PM, Medland SE, Ferreira MAR et al (2006). Assumption-free estimation of heritability from genome-wide identity-by-descent sharing between full siblings. PLOS Genetics 24 March 2006.