I was writing an e-mail to my cousin this morning, who is the daughter of my mother’s fraternal twin. It got me to thinking about a book on twins I had read many years ago, that had discussed a possibility of there being another type of twin besides fraternal and identical, so I searched the archives and came up with the January column about five types of twins. The type of twin I was talking about wasn’t listed there, so I’ll ask you more about it if you don’t mind.
Identical twins are one egg fertilized by one sperm that then splits into two identical fetuses, and fraternal twins are two different eggs fertilized by two different sperm. The third kind I had read about was one egg, fertilized by two sperm, that then splits into two fetuses which would share half their genes. I often wondered if my mother and her twin sister were this kind of twin. Their facial features and coloring were pretty much identical, but one was tall and thin, and the other one was short and curvaceous. I suppose the kind of identical twins that shared nourishment could account for this difference also, but that third kind of twin seemed to be a possibility also.
I wondered if you could enlighten me about what you had heard about this third type of twin.
It’s helpful if the first person to open a topic provides the link to Cecil’s column, so that everyone can be on the same page (so to speak): Are there five kinds of twins?
I’m no obstetrician – and I don’t even play one on tv – but I never heard of this before.
Best of my knowledge (fueled by reading and PBS), it’s one egg + one sperm = one fertilized egg. If that egg splits, the resulting two people are identical twins. (Which makes sense, it’s the same fertilized egg.)
Your mother and aunt are obviously fraternal twins, two eggs, two different sperm.
That information actually came from a book I read well over twenty years ago on twins, and at that time they were just entertaining the theory that a third kind of twin might be possible.
That same book said that if identical twin sisters from one family married identical twin brothers from another family, legally their children would be cousins but genetically they would be siblings.
The book came from the local public library which, since then moved to a new building and sold or gave away many of the older books. But maybe I’ll try to find it again and look up the author, and that way I could find out where the original information came from.
A sub-catagory of fraternal twins would
be when two eggs are fertilized by two
different men. Twins who are half-siblings.
This has happened a few times. The most
bizarre case on record was an in vitro
fertilization where a Dutch white couple
had a set of twins one white, one black.
Sloppy lab practice was to blame.
Genetically, children of identical twins are half-siblings, yes. It has to do with percent genetic inheritance. You get half your genes from your mother and half from your father. Normal siblings (and fraternal twins) share half of their genes with one another (on average; the genetics and math for determining the bell curve of these percents would be beyond the scope of the discussion and likely beyond my scope :))
Your uncles/aunts by blood and grandparents carry one-fourth of your genes; cousins carry one-eighth of one another’s genes. (For cousins: you have half your mother’s genes. She has half her brother’s genes. His son has half of his genes. Thus your mother’s brother’s son–your cousin–has one-eighth of your genes). Half-siblings carry one-fourth of one another’s genes.
But identical twins are, genetically, identical (hence the name). Their children each get half of their genes, and half of those on average will be the same. Thus one-fourth, the same as half-siblings.
Children of two sets of identical twins are, genetically, children of the same parents, and therefore full siblings. They are also, genealogically, “double cousins.” THe decution of that term shall be left to the reader, 'cause it’s 0230 and I need sleep
The book probably did mention about the half-sibling thing, and it makes sense, but I don’t remember after so many years, and haven’t gotten to the library yet (spending too much time by this computer).
Are ya guys just funnin’ me about the term “decution,” or was that a typo? I love new words also but couldn’t find it in Websters.
