What's a 3 4 sibling?

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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.[1] 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.[2] 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.

Segment-based methods

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. A less mathematical approach is to just sum the FIRs and HIRs. The total for full siblings will be similar to the amount shared in a parent-child comparison while 3/4 siblings will usually share less than 3000 total FIR+HIR cM. This logic is explained by this blog post https://blog.kittycooper.com/2019/10/why-dont-we-share-more-dna-with-a-sibling/

What will happen if you have a baby with your cousin?

The vast majority of children of first cousins are healthy and do not have problems due to their parents' relatedness. It is important to keep in mind that even for an unrelated couple, there is an approximately 2-3% chance that their child is born with a birth defect, genetic syndrome, or disability.

gap.med.miami.edu - If cousins get married, are they at risk of having children with genetic ...

If cousins get married, are they at risk of having children with genetic conditions?

Any couple related or not, also is at risk of having a child with genetic conditions or health problems! Although taboo in some cultures, marriage between first cousins is accepted in many cultures around the world. The vast majority of children of first cousins are healthy and do not have problems due to their parents’ relatedness. It is important to keep in mind that even for an unrelated couple, there is an approximately 2-3% chance that their child is born with a birth defect, genetic syndrome, or disability. The risk for children of first cousins is increased over this background, by about 3%. The risk for genetic conditions and health problems does increase as the relatedness of parents increases. For example, marriages between brothers and sisters are not typically culturally accepted. This is likely due to the dramatic increase in birth defects, genetic conditions and health problems seen in children of these matings.

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