Identical twins are almost always the same sex, although there are some rare exceptions to this rule. Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes (usually, XX for girls or XY for boys).
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.
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!
Identical twins are completely random. It's not based on family history or genetics. Identical twins form when a single fertilized egg splits and forms two embryos. Now in this case, it's not because of mom or dad.
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.
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.
While men can carry the gene and pass it on to their daughters, a family history of twins doesn't make them any more likely to have twins themselves. 3 But, if a father passes on the "twin gene" to his daughter, then she may have a higher chance than normal of having fraternal twins.
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.
Fortunately telling twins apart is one fear that can be taken off the list. Most parents find that mixing up their babies is the least of their concerns. After only a few hours or days at most, they are distinguishable as individuals. Even if they look alike, parents just know the difference.
And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.
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.
While they may look alike, identical twins are unique individuals with their own personalities and interests. Identical twins, also known as monozygotic twins, share the same egg at conception and are always either both boys or both girls.
It is likely that twins' awareness of one another starts sooner than seven or eight months of age. An article by the late doctor, T. Berry Brazelton, observed that at age three to four months, an infant identical female twin seemed disoriented when her sister was removed from the room.
But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints. Like physical appearance and personality, fingerprints are largely shaped by a persons DNA and by a variety of environmental forces.
Several factors for causes of embryo splitting were suggested, including maternal age, prolonged embryo culture, ovarian stimulation, and zona pellucida (ZP) manipulation [6].
Best positions: deep penetrative sex is preferable. This helps to deposit the semen and sperm closest to the woman's cervix so they are given the best opportunity to get to the egg in the fallopian tube. Twelve hours before ovulation is thought to maximise the chances of conceiving with a boy.
Fraternal twins — the most common kind of twins — occur when two separate eggs are fertilized by two different sperm. Each twin has his or her own placenta and amniotic sac. The twins can be two girls, two boys, or a boy and a girl.
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.
According to The American Society for Reproductive Medicine, women who are fraternal twins have a 1 in 60 chance of having twins, and men who are fraternal twins have a 1 in 125 chance of fathering twins. It was previously believed that identical (monozygotic) twins were random — not genetic.
Identical twins form from the same egg and get the same genetic material from their parents — but that doesn't mean they're genetically identical by the time they're born.
Twins not only have a bestie from birth — they also live longer than singletons. And those two factors may be related, according to new University of Washington research.
Longer lives
When analyzing the data by gender, the researchers found that female identical twins lived, on average, about 63.4 years, whereas female fraternal twins lived about 61.4 years and the general Danish female population lived about 58.8 years, Sharrow said.
Identical twins will always have the same blood type because they were created from the same fertilized egg (fraternal twins can have different blood types — again, providing the parents do — because they are created by two fertilized eggs).
Analyses of data from the National Longitudinal Study of Adolescent to Adult Health show that MZ twins were significantly more physically attractive and healthier than dizygotic (DZ) twins and singletons.