If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA.
In contrast, identical twins (or monozygotic twins) result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two. As a result, identical twins share the same genomes and are always the same sex.
Is there a third type? Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called polar body or half-identical twins.
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
Just like having a boy or a girl in a typical pregnancy, the chance of having boy and girl twins is one in two.
Monoamniotic-monochorionic Twins
These types of twins share a chorion, placenta, and an amniotic sac. This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
One placenta and one amniotic sac.
This is the riskiest and rarest type of twin pregnancy. Fetal complications can arise due to tangling of the umbilical cords or an imbalance in nutrients, blood or other vital life supporting systems.
The boys are cousins, but because their parents are twins who married twins, their genetics are similar to those of siblings, Briana said. The Salyerses' unusual marriage is known as a quaternary marriage, and their sons are known as quaternary twins.
The term mirror twin is used to describe a type of identical, or monozygotic, twin pairing in which the twins are matched as if they're looking into a mirror — with defining characteristics like birthmarks, dominant hands, or other features on opposite sides.
For women born 1870-1899, moms of twins averaged reproductive spans of 14 years 11 months versus 14 years for singleton moms. Both results were statistically significant. Moms of twins also were older at the time of their last birth.
For a given pregnancy, the odds of conceiving fraternal twins are only determined by the mother's genetics, not the father's. Fraternal twins happen when two eggs are simultaneously fertilised instead of just one.
In fact, one British study tested ESP potential among identical twins and found that they were correct about 25% of the time, the same as any other siblings. There is one particular power though which twins possess, longevity. Fraternal twins live longer than singletons and identical twins longer still.
Twins share the same genes but their environments become more different as they age. This unique aspect of twins makes them an excellent model for understanding how genes and the environment contribute to certain traits, especially complex behaviors and diseases.
Multiples are about twice as likely as singleton babies to have birth defects, including neural tube defects (such as spina bifida), cerebral palsy, congenital heart defects and birth defects that affect the digestive system.
Many people believe twins skip a generation, but that's just a myth. The idea that twins skip generations likely comes from the fact that the genetic factors contributing to twins only come from the gestational parent's side.
As expected, first-born twins had greater birth weight than second-born twins. With respect to height, first-born twins were slightly taller than second-born twins in childhood.
Once the babies are large enough to stay in one position in the womb, the twin lowest in the uterus is known as Baby A and the one furthest from there is Baby B, according to the Stanford Medicine News Center. In the majority of vaginal births, Baby A is born first.
However, males are slightly more likely than females to die in the womb. And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
Twins and other multiple-birth individuals can suffer from much deeper and troubling loneliness than single-born individuals. Separation anxiety, which often begins at birth, is the underlying cause of loneliness for twins.
It's equally unsurprising that a Danish study found that twins are less likely to get married than non-twins. As twins have a partner from birth, the study suggests that they may not have the same desire for marriage as singletons.
Having two babies at a time is associated with a longer life, according to a new study. But that's not because doubling up on dirty diapers increases life span; instead, moms of twins are physically stronger in the first place.