Is it possible to have identical boy and girl twins?
Identical (monozygotic) twins are always of the same sex because they form from a single zygote (fertilized egg) that contains either male (XY) or female (XX) sex chromosomes. A set of boy/girl twins can only be fraternal (dizygotic). Boy/girl twins cannot be identical (monozygotic).
There Are 7 Different Kinds Of Twins. There are 7 different kinds of twins, categorized as: Identical, Fraternal, Half-Identical, Mirror Image Twins, Mixed Chromosome Twins, Superfecundation and Superfetation. Normally, except the first two kinds, others are known to be quite rare to occur.
- Dichorionic/Diamniotic (di/di) twins. Di/di twins are the most common type of twins and the lowest risk. Di/di twins can be identical if the egg split very early, but fraternal twins are always di/di. In di/di twins, each twin has their own placenta and their own amniotic sac.
- Half-identical twins are also called polar body twins. At the end of the normal process of ovulation, the developing egg divides into two cells. Each cell has the same number of chromosomes in the nucleus, but different amounts of cytoplasm.
- It is possible for twins to have different fathers in a phenomenon called heteropaternal superfecundation, which occurs when two of a woman's eggs are fertilized by sperm from two different men. Identical twins occur if that fertilized egg divides into two separate eggs, early in the pregnancy.
Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). Another abnormality that can result in monozygotic twins of different genders is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated.
- Fraternal twins are formed when two eggs meet two sperm in the womb. Each is fertilized independently, and each becomes an embryo. With identical twins, one egg is fertilized by one sperm, and the embryo splits at some later stage to become two.
- 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. It sounds like fraternal twins do indeed run in your family!
- The notion that twins always skip a generation is also a myth. The illusion may have arisen because men who inherit the gene from their mothers are unaffected by it ( they do not ovulate), but can still pass it on to their daughters, who, in turn, will have an increased likelihood of conceiving twins.
Updated: 2nd October 2019