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.
And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.
Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or 'dizygotic' (two-cell) twins. These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.
This is a very rare birth of twins—a boy and a girl—from a single fresh compaction-morula produced by ICSI. We had to examine why there were two babies of different sex, which meant that it was a dizygotic twin pregnancy.
Mixed-gender twins are the most common type of fraternals, some 50 percent are boy-girl. To understand this combination: Males have XY chromosomes, females have XX chromosomes.
The study of 381 pairs of identical twins and two sets of identical triplets found that only 38 were genetically identical, Tina Hesman Saey reports for Science News. Most had just a few points of genetic mismatch, but 39 had more than 100 differences in their DNA.
It is estimated that 99.9% of identical twins have the same gender. But, semi-identical twins are so rare that they make up only about 0.1% of the twin populations.
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.
The short answer is that while there are plenty of stories out there about twins who claim to have read each other's minds and predicted when the other was in danger, Nancy Segal, PhD, director of the Twin Studies Center, at California State University, Fullerton, says that scientific research doesn't support the idea.
Although the majority of monozygotic twin pairs achieve an identical height, a proportion of them are discordant. Because monozygotic twins completely share their genome, differences in their adult height are not due to heritable factors.
Things like your eye and hair color are controlled by your DNA. This is why identical twins look so much alike -- they share the same DNA. And yet, they aren't exactly alike. They can be hard to tell apart but if you look closely, you see differences.
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).
If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than the standard two - one from the mother and two from the father. And, according to researchers, three sets of chromosomes are "typically incompatible with life and embryos do not usually survive".
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.
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.
Multiple pregnancy usually happens when more than one egg is fertilized. It also can happen when one egg is fertilized and then splits into 2 or more embryos that grow into 2 or more babies. When one fertilized egg splits into 2, the babies are called identical twins.
Polar body twinning is thought to occur when an egg splits - and each half is fertilised by a different sperm. This results in twins who appear very much alike but share approximately 75% of their DNA.
The chances of having identical twins is relatively rare: 3 or 4 per every 1,000 births.
Not necessarily. Identical twins may differ on a host of diseases and conditions. According to Haseley, “If a genetic change occurs in a gene that affects its function, health concerns may arise.” Twins may differ in their risk of cancer, rheumatoid arthritis or other diseases.
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.
At age 7, the mean IQ score of twins was 5.3 points lower than that of singletons in the same family, and at age 9, the score was 6.0 points lower. The lower intelligence of twins in childhood may partly be a consequence of the reduced fetal growth and shorter gestations of twins, say the authors.
It was concluded, among many other things, that identical twins are about 85 percent similar for IQ, whereas fraternal twins are about 60 percent similar. This would seem to indicate that half of the variation in intelligence is due to genes.
Identical twins possess exactly the same set of genes. Yet as they grow older, they may begin to display subtle differences. They may start to look different, develop different diseases or slide into different personalities.
Identical twins live longer, possibly because they have a BFF from birth, according to researchers from the University of Washington, Seattle, WA. Furthermore, this effect is greatest in male identical twins, according to results from their recent study, published in the journal PLOS ONE.
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.