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What is a 3 4 sibling?

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.

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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/

Further reading

See also

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Who has the rarest eye color in the world?

Albinism is when there is an absence of melanin which causes tiny red blood vessels to appear visible. Out of all the unusual eye colours, pink and red are the rarest in the world. According to the National Organization for Albinism and Hypopigmentation, only one in 20,000 people have albinism.

feelgoodcontacts.com - These are the most rare eye colours in the world - Feel Good Contacts

Where do rare eye colours originate from?

Rare eye colours originate from genetic mutations. Scientists believe that thousands of years ago, everybody in the world had brown eyes. They believe that the first person to have blue eyes had a genetic mutation that caused the body to reduce its melanin production. Blue eyes are most common in the UK, Ireland, Finland and Sweden. According to World Atlas, 8% - 10% of people in the world have blue eyes. Between 55% to 79% of people have brown eyes with dark brown eyes being most prominent in Africa, East Asia, and South East Asia. Light brown eyes are most common in West Asia, America and Europe. Green eyes affect around 2% of people worldwide; however, they make up 86% of the population in Ireland and Scotland. People are not born with green eyes; they usually develop at around 6 months old. Green eyes originate from areas around the Caucasus Mountains. Around 3% off people in the world have grey eyes. They are most common in Northern and eastern Europe. Only 1% of people have pink and pale red eyes; they are typically albinos. Albinism happens when there is a problem with one of the genes inherited from the mother. Like red eyes, heterochromia is rare and effects less than 1% of the population. Heterochromia is the result of a genetic mutation caused by variations in the distribution and concentration of melanin. However, it can also be caused by an eye injury or a medical condition.

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