Fraternal twins are siblings who are conceived at the same time of two separate fertilized eggs. They are also sometimes called "biovular twins" or "non-identical twins." In fact, it is quite common for fraternal twins to be defined in comparison with identical twins. Identical twins are two siblings who are born of a single fertilized egg that splits in order to create two fetuses instead. One of the key features of identical twins is that they look incredibly similar, or identical. Also, identical twins always share the same gender.
Unlike identical twins, fraternal twins can look quite dissimilar as siblings sometimes do. In fact, it is common for fraternals to be confused for siblings that simply happen to be similar in age. This is especially true in late childhood, adolescence, and later in life. Furthermore, while these twins can be of the same gender, it is also possible for a pair of this kind of twins to be made up of one boy and one girl. This is because, instead of sharing all of their genes as with identical twins, fraternal twins share an average of 50 percent of their genes.
Another term for twins of this type is "dizygotic twins" which is sometimes referred to in a shortened term as "dz twins." This simply means that the twins are the result of two zygotes instead of one, as in the case with identical twins. Zygote is the scientific term of a fertilized egg. In the instance of identical twins, or "monozygotic twins," only one zygote is present at the moment of conception.
Much later in life, it is possible for identical twins to be confused with fraternal twins. This is because general health, one's level of personal care, and one's lifestyle can alter the manner in which one ages. If the two members of a pair of identical twins lead very different lives, it is possible for them to look more and more dissimilar as they age. Cosmetic choices such as hair style, clothing, and makeup can also make identical twins look more like they're fraternal.
There are some groups that are more likely to produce fraternal twins than others. Dizygotic twins are more common among older mothers, especially women who conceive after the age of 35. They are also quite common in Japan and in some parts of Africa. There is some evidence that there is dz twins are often the result of a genetic predisposition.