No! In about 21% of identical (monozygotic or MZ) twin pairs, one twin is right-handed and the other is either left-handed or ambidextrous. Since identical twins share identical genes, this is evidence that handedness is not a totally genetic trait.
Left-handedness is more likely to occur in twins than in single individuals. Only about 10% of people in the general population are left-handed. But about 17% of all twins are left-handed.
The cause of hand preference is not well understood. It is likely to be a combination of genetic and environmental factors. Environmental factors such as stress during birth, hormone levels during pregnancy, or position in the womb have been suggested. It has also been proposed that mirror-imaging in the formation of MZ twins could cause their differences in hand preference. Cultural influences also have an effect, as parents may encourage right-handedness.
Hand preference is a classic example of the controversy over the relative importance of nature (genetics) vs. nurture (environment) in causing a trait. A recent study examining the whole genome of nearly 4000 left and right handed twins was unable to find a strong genetic factor in determining handedness. Researchers concluded that it is likely that there are many weak genetic factors in handedness.
If your twin develops certain diseases, you may have an increased risk of developing that disease. This risk may be higher if you are identical twins than if you are fraternal twins.
If you would like additional information about how your twin’s health might impact your own health, contact your health care provider or a genetic counselor. Genetic counselors may be located through the National Society of Genetic Counselors.
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. The bigger cell (with most of the cytoplasm) is the egg, and the much smaller cell with very little cytoplasm is called the polar body. The polar body usually degenerates and dies.
Polar body twins are theoretically caused when the polar body is fertilized by one sperm at the same time the egg is fertilized by a different sperm. In theory, this results in twins who share half their genes in common (from the mother), but the other half of their genes are different because they come from two different sperm cells.
In theory, polar body twins would share 75% of their genes in common, so polar body twins are also called half- identical (since identical twins share 100% of their genes in common, and fraternal twins share an average of 50% of their genes in common).
However, there is no proof that polar body twinning has ever occurred. And there are some theoretical arguments that it doesn’t happen. For example, genetic recombination occurs during the formation of eggs, so the maternal DNA in the egg may not be identical to that in the polar body. Also, with very little cytoplasm, it is unlikely that a fertilized polar body would be able to survive and develop into a normal embryo and fetus. There are no definitive tests to confirm polar body twinning, so it remains a theory only.