Usually, it is easy to figure out the proper kinship term that should be attached to the members of a particular family. Sometimes, though, things get a bit tricky. For example, if two babies are born at the same time to a mother who has two uteruses, are those babies twins, or just siblings?
Genealogy is the study of family. Most aspects of genealogy are pretty regulated. The same kinship terms are used to indicate a particular genetic relationship in every family. For example, your grandmother is either the mother of your mother, or the mother of your father. This definition is the same for everyone, which makes it much easier to understand family relationships.
If your birth parents had children other than yourself, then your siblings are either your brothers or your sisters. Obviously, the boys are brothers, and the girls are sisters. If one of your parents had a child with a person who is not your birth parent, then that child is your half-sibling.
It actually doesn’t matter if your siblings, or half-siblings, are twins or not. You still call them your brother, or your sister. On a typical family tree, all of the children that a set of parents have are connected to the parents by a line, and connected to each other by another line, (that would indicate that the kids are not in the same generation as their parents are).
Things get a bit more confusing when you start talking about twins. Technically, twins are listed just like typical siblings are on a family tree. What gets confusing is the differentiation between types of twins. This wouldn’t be listed on a family tree, but it is still something very note worthy about a particular family.
There are two types of twins. Identical twins occur when one egg, that has been fertilized by one sperm, splits into two masses of cells, and each mass develops into a baby. These babies have the exact same DNA as each other. Identical twins are always the same gender as each other.
Fraternal twins occur when two eggs are fertilized by two different sperm. This leads to two babies that are developing in one womb at the same time. These twins could be the same gender as each other, or opposite genders from each other. Fraternal twins share some DNA, but not any more than typical siblings would share.
What if the babies are developing at the same time, in the same mother, but in two different wombs? It is rare, but possible, for a woman to have a double uterus. This is called “uterus didelphys”.
A twenty-four year old woman in Florida named Andrea Barbosa recently gave birth to a set of twins. One baby was in each of her uteruses. Nathan and Natalie were born on September 15, 2011, and are being called fraternal twins.
But, are they really twins? Technically, we have two separate eggs that were fertilized by two completely different sperm. This means the two babies don’t share any more DNA than two typical siblings. They developed in separate wombs, but at the same time. Are they twins, or are they siblings of the same age? I’m not sure.
Image by Chris Waits on Flickr