Review of Standard Consanguinity Relationships
The Legality of Polygamy. From Wikipedia.
There are two kinds of half-siblings, with varying degrees of consanguinuity (blood-sharing):
Rare Types Of TwinsThere are no records of how many sperm donations are made every year in the US, nor of how many children are conceived, but it is frequently said to be between 30,000 to 60,000 births per year. [Wendy] Kramer with the DSR [Donor Sibling Registry] argues that number is woefully out of date.Still, the scandals tend to find the media spotlight. There was the man whose samples were used to produce more than 150 children, the many parents who end up unintentionally having children of different races (in one instance because a clinic was said to have confused “Donor 380” with “Donor 330”), the woman whose dead husband’s samples were misplaced and allegedly used to impregnate other women, or the Georgia clinic that allegedly marketed a donor to hopeful parents as a neuroscience genius studying a PhD in engineering but who was in reality an ex-con who had never attended college and had a history of psychiatric hospitalization. His sample has been used to conceive at least 36 children. . .A 2018 survey of almost 500 donor-conceived people conducted by Kramer’s website found that almost a quarter of them had used DNA testing to track down their donor.As more donor-conceived children find their biological parents and half siblings, new and extended family units are forming. One 2016 study of 419 donor-conceived children found more than a third of them get together once a year with the half siblings they’ve discovered, and a fifth of them meet up together several times a year. Some 42% of them said they considered their half sibling to be part of their immediate nuclear family.
Here’s the run-down on various types of twinning. I threw in chimerism because this is essentially reverse-twinning and thereby relevant to discussing numbers of eggs and sperm and egg fusion events.
Monozygotic twins - one egg, one sperm, fertilized egg splits (also called “identical” twins)
Dizygotic twins - two eggs, each fertilized separately by its own sperm (also called “fraternal” twins and no more similar genetically than typical siblings born at different times)
Chimeric person* - two eggs, two sperm, the two fertilized eggs fuse early to form one person [*Not actually a twin, but a related concept.]
Sesquizygotic twins - one egg, fertilized by two sperm
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies from two separate biological fathers. The term superfecundation is derived from fecund, meaning the ability to produce offspring. Homopaternal superfecundation refers to the fertilization of two separate ova from the same father, leading to fraternal twins; while heteropaternal superfecundation is referred to as a form of atypical twinning where, genetically, the twins are half siblings. Superfecundation, while rare, can occur through either a complex single occurrence of sexual intercourse, separate occurrences of sexual intercourse, or through artificial insemination.
It is also possible for ordinary fraternal twins to appear to be heteroparental mixed race twins (a.k.a. "mixed twins") when, in fact, that were full siblings who simply inherit different genes from parents who have ancestry from more than one continent, most often in interracial marriages, but also in the case of children of people with mixed race ancestors even if both parents are identified as belonging to the same race, something that is true to some degree in the United States for almost all African-Americans, and for most Native American and Hispanics.
Cousin marriage rates. Image from Wikipedia.
Double cousins are when two siblings of one family have children with two siblings of another family (brother/brother, sister/sister or brother/sister, sister/brother).The children are related to each other through both of their parents and are considered double related.Double second cousins can happen in two ways.
* From the relationships of two first-cousins with two other first-cousins.* From the relationships two double-first-cousins with two other persons.Double first cousins share both sets of grandparents and have twice the degree of kinship of ordinary first cousins.How Does This Affect Genetics and DNA?Genetically, double cousins could test with numbers consistent with half-siblings.Normal first cousins (cousins with one parent of each as a sibling while the second parents are unrelated), can share roughly 12% of their inherited DNA. Double cousins, because both parents are siblings to each other, can share roughly 25% of inherited DNA.This is the same amount of DNA that you would share with a grandparent, a half-sibling or an aunt or uncle.
Real Life Examples
Double CousinsThe family tree below is an example of double cousins. Anna and Marie Clemens are sisters, twins as a matter of fact. Joseph and Theodore Schneble are brothers.Their children are considered double cousins.They will share both sets of grandparents through the Clemens and Schneble family.Because Anna and Marie are twins, they may even share more than 25% shared DNA. However, we would need to know if Anna and Marie are identical or fraternal twins. UPDATE: In speaking with Schneble descendants of Anna and Marie and seeing pictures of them, they were not identical twins.What if Joseph and Theodore were twins as well?According to the website FamilyPedia.wikia.com, when identical twins have children with identical twins, the children are genetically indistinguishable from full siblings. When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings.