There are two kinds of twins: identical and fraternal. Identical twins develop from a single egg that splits after fertilization and therefore have 100% the same genes. Fraternal twins occur when two separate eggs in the mother’s ovaries are fertilized by two separate sperm, and share roughly 50 percent of their genetic data.
Twin studies have shown that shyness, willingness to take risks and propensity toward holding religious beliefs are genetically inherited to some degree, as is the risk of diseases such as osteoporosis, asthma and heart disease.
But he still established that by studying twins, scientists could get at the root of one of the greatest debates in the study of human development and genetics, a term he coined that’s still relevant today: the competition between the forces of Nature vs Nurture. Below are a few facts which make twins so fascinating!
Identical twins do not have identical fingerprints
You might think that because identical twins supposedly share almost the same DNA, they must also have identical fingerprints. Well, that’s not true. Fingerprints are not solely generated based on DNA. When identical twins are conceived, they start out with the same fingerprints, but during weeks six through 13 of pregnancy, as the babies start to move, they each touch the amniotic sac, and unique ridges and lines are formed on each twin’s hand that result in different fingerprints.
Tall women are more likely to have twins
Gary Steinman, MD, PhD, an attending physician at Long Island Jewish (LIJ) Medical Center, discovered that taller women have more insulin-like growth factor (IGF), a protein that is released from the liver in response to a growth hormone that stimulates growth in the shaft of longer bones. Having higher levels of IGF results in increased sensitivity of the ovaries, thus increasing a woman’s chance of ovulating. According to Steinman, the more IGF a woman has, the greater chance she has of becoming pregnant with twins, because IGF “govern[s] the rate of spontaneous twinning.”
Mirror image identical twins have reverse asymmetric features
About 25 percent of identical twins develop directly facing each other, meaning they become exact reflections of one another. According to About.com, “they may be right- and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions.” This occurs when the twins split from one fertilized egg more than a week after conception.
Mothers of twins may live longer
A study published in the journal Proceedings of the Royal Society B focused on the population of women in 1800s Utah and found that women who gave birth to twins were very strong and healthy to begin with, so were likely to live long lives. As LiveScience writer Stephanie Pappas explains it, “Twins could be an evolutionary adaption in which healthy moms take the chance to pass on double their genes at once.” However, because the data only includes women who conceived twins naturally — IVF was obviously not used in the 1800s — the findings are not definitive.
