All pregnancies start when a sperm fertilises an egg. This forms twins. These twins are called fraternal twins, dizygotic twins meaning two zygotes or non-identical twins. The technical name for this is dichorionic. Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different — for example, they might have different hair or eye colour.
Semi-Identical Twins Identified For Only The Second Time Ever | IFLScience
Back to Pregnancy. It depends on the type of twins. There's no evidence that identical twins run in families. However, non-identical twins can run in families. A couple is more likely to have twins if there are twins in the woman's family.
'Extraordinarily Rare' Semi-Identical Twins Were Born in Australia
However, recent research on the sex-specific nature of breastmilk suggests that OS twins might be at a developmental disadvantage relative to SS twins. There may therefore be evolutionary selection for the tendency to have SS twins and against the tendency to have OS twins. We performed a one-sample t -test of the proportion of SS dizygotic twins against the theoretical null value of 0.
Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2. If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA.