Identical And Fraternal Twins example essay topic

Today most interest in twins serves to determine whether traits come from ones environment or from genetics. However, throughout time, twins have been a constant source of entertainment and amazement for society. Twins differ very little, and according to an e-mail interview with Dr. Nancy Segal, the American attraction to them is due to our high value and appreciation of independency and personal differences. Segal believes the similarity of twins contrasts greatly with Americans values and this contrast sparks interest, almost to the nature of a circus sideshow.

Because most singletons find the eerie alikeness of twins mystifying, myths and urban legends about twin behavior are not uncommon. For instance, the belief twins share more than just a special bond, but a type of telepathic connection, is commonly held. To explain this seeming telepathy, Piontelli writes: It would not be unreasonable to hypothesize that an uncommonly early and constant exposure to each other's rhythms, bodily substances, behavioral patterns, and body language may foster a heightened reciprocal sensitivity in some twins. Some kind of familiarity with each others rhythms may already start before birth. This would not imply esoteric 'telepathic understanding'. It may only suggest that twins, being constantly exposed to the manifestations of their co-twins, could sense from an unusually early age, and hence later perceive in an uncommon way, the signals of the other twin.

(120) So twins do not have extrasensory perception, but then, why are they so similar? Is it because they are raised in the same environment or is it simply because they share the same genes? Twins may be mystifying to most the general population, but they are very interesting to scientists studying genetics. Today twin study is the major source of information fueling the scientific debate of nature verses nurture. Nature refers to traits genetic or inherited from parents and nurture refers to all traits non-genetic or stemming from the environment. The answers received from this debate help to define human differences.

Understanding these differences can eventually prevent and help in the early treatment of disease. It may even be able to predict diseases in unborn children that would not show signs or symptoms for decades. Scientists cite these and other interesting reasons as impetus for studying twins. However, the majority of twin studies serve to determine whether genetics or environment decide human traits.

Studies of twins place them in two differing groups. Identical twins or monozygotic twins, according to Clegg and Wool lett, 'are formed by the accidental splitting of a single fertilized egg, so that the babies develop from one egg and one sperm. ' They also define fraternal or dizygotic twins as non-identical twins that 'develop from the fertilization of two eggs by two sperm' (16). It is common knowledge ones genes are inherited, half from ones mother, and half from ones father.

Since monozygotic, or MZ, twins are a split of the same egg and sperm, they contain identical genes. However, the genes of dizygotic, or DZ, twins are only as similar as typical brothers and sisters. Scientists are interested in fraternal twins reared together because, being the same age and in the same home, they share practically identical environments. They are also interested in studying identical twins that have been reared apart from one another, because they share identical genetics but have differing backgrounds. These groups of twins provide researchers with automatic control groups. By studying these two groups, scientists can see if DZ twins with the same environment are more similar or if MZ twins with the same genes are more similar.

Rowe explains the importance of these studies: The study of separated twins is the most direct method for estimating a trait's heritability. Separated MZ twins are reared by different parents, and hence have different family environments, whereas they possess identical heredity. Thus the effects of heredity are distributed against a background of different family environments, and we can infer genetic influence on a trait from resemblance in separated twins (37). Many scientists who study twins reared apart believe traits are mostly genetic and they seek to find a genetic predetermination for human characteristics, traits, diseases, and psychology. Segal was apart of one of largest studies of twins reared apart.

She also performs many different experiments, surveys and polls of twins. Expressing her opinion in the e-mail interview, she felt both nature and nurture play very important roles in determining human traits. The arrangement of twins into groups, MZ or DZ, presents a few problems and criticisms. In the e-mail interview Segal said there would be serious problems with research if twins were not properly assigned as identical or fraternal. In older studies this had been a problem, however today, scientists are able to do a complete DNA analysis on twins to determine their zygocity. Today, for a fee of less than two hundred dollars, many companies provide zygosity testing merely for the personal knowledge of the families of twins.

These are bloodless tests that simply compare the twin's cheek swabs (Proactive Genetics). Recently new information leads some scientists to believe that a third grouping of twins exists. This group is referred to as polar body twins or half-identical twins. It is hypothesized that these types of twins, more similar than fraternal and less similar than identical, occur when a mother's egg splits before fertilization.

Then the two eggs with identical DNA are fertilized by two different sperm. Currently, there are no tests to determine if twins are polar body twins (Proactive Genetics) Imitation arises as another complication associated with twin studies. Many critics claim twins learn there are benefits to imitating one another, which can skew results. As most children learn by imitating their parents, twins obviously learn by imitating each other as well. This behavior begins at a very early age (Piontelli 121). Many times a parent praises one twin, while the co-twin watches.

The co-twin proceeds to imitate their twin in order to be praised as well (Piontelli 122). However, according to the interview with Segal, imitation is an 'overblown criticism' of twin studies. Another criticism of twin research stems from a premise called the equal environments assumption or EEA. In Entwined Lives, Segal says, 'when researchers compare identical and fraternal twins' mental or medical similarities they assume that environmental influences affecting traits are the same for identical and fraternal twins' (318). In other words, scientists must assume that people in the lives of MZ and DZ twins treat them exactly the same. However, many researchers found MZ twins are "treated more similarly than their DZ counterparts" (Evans 77).