When you look at your fraternal twins, you might be amazed that two kids so different in looks and personality are members of the same family, let alone twins. Even if your twins are identical, you might marvel that two kids with the same genetic makeup, who look so much alike that their own grandma can't tell them apart, can have such different personalities. Both the genes you passed on to the them and the environment they grow up in -- which includes everyone who crosses their path, not just you -- can make even identical twins very different individuals.
Identical Twins' Appearances
Identical twins develop from the same embryo, which splits at a point in early development, before the embryo attaches to your womb. Because they have the exact same DNA, they often look so much alike at birth that you might have had to resort to tying different colored ribbons around their feet or leaving their hospital bracelets on for the the first month. Identical twins might look completely different if one hogged all the nutrition and the other didn't get enough; one could be fat and happy and the other skinny and cranky -- or the other way around. As twins grow, accidents and illnesses can take their toll on one but not the other, causing changes in one's appearance but not the other's.
Fraternal Twins' Appearances
Fraternal twins develop from different sperm and eggs, so they share only the same amount of DNA that any two siblings would, around 50 percent, according to an article published in the Spring 2001 issue of "The American Journal of Psychology." Fraternal twins can inherit a different eye color, hair color or height than their twin, and, of course, one might be a boy and one a girl, which certainly makes it easy to tell them apart. Fraternal twins born to racially-mixed African-American and Caucasian couples can even have completely different skin tones, with one appearing Caucasian and the other African-American.
Identical Twins' Personalities
Even though they start out with identical genes, identical twins won't have the exact same experiences in life, and life experiences can affect personality. While you might do your best to treat identical twins exactly alike, showing no favoritism, dressing them the same and trying to treat them the same, simple things can affect development. You might always feed one before the other simply because his crib is closer to the door or because he cries the loudest. Identical twins will usually have more similarities in personality than fraternal twins or singleton brothers and sisters. One twin might be the leader in everything physical but completely lost without his twin in social situations.
Fraternal Twins' Personalities
Fraternal twins can have personalities as different as day and night, both because they inherit different genes and also because life will treat them differently. Because personality traits, such as an extroverted personality, openness and agreeableness, are partly inherited, you might notice that both your identical twins have the same gregarious, easy-going personality, while one fraternal twin might have this personality type while the other doesn't. Fraternal twins differ more in personality than identical twins raised in different environments, who often have eerily similar personality traits, even if they've never met. Because they look different, you might be less likely to dress them alike or raise them as a unit; their individuality is easier to recognize than that of identical twins.