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Biology 103
2003 Third Paper
On Serendip

Are Genetics Responsible for Allergies? A Study In Identical Twins

Melissa Teicher


Everyone has either suffered from some kind of allergy, or knows somebody who has suffered from allergies. Allergies are the source of irritating symptoms, ranging from a painless skin rash to life-threatening breathing problems. For years, researchers have been trying to find out the source of these allergies. Some have suggested that environmental factors or early exposure to certain foods can cause allergies later in life, while others say that allergies are caused by genetics. To test the latter theory, many researchers study identical twins to see if sets of twins share allergies. If both twins were to share a particular allergy, than this may prove that allergies are genetic.

To completely understand the remainder of this essay, one must understand the difference between identical twins and fraternal twins. Twin zygosity is the genetic relationship of twins. There are two types of twins: monozygotic twins, also known as identical twins, and dizygotic twins, also known as fraternal twins. Identical twins have exactly identical DNA strands; they are same sex and they have very similar physical traits. They come from one egg that is fertilized by one sperm. Some time after conception, the egg splits resulting in two babies. Fraternal twins only have half identical DNA; that is, only one strand of the double-stranded DNA is the same. They come from two individual eggs that are fertilized by two individual sperms. They are either same sex or different sex, and are just like siblings of same parents born at different times. There are other kinds of twins as well; for example, "mirror-image twins," "polar body twins," and "half-identical twins." These names refer to the time that the egg splits in identical twins. This essay, however, will deal with only identical and fraternal twins (5). The question now is, Are identical twins allergic to the same things? Since identical twins have exactly identical DNA, the sharing of allergies can shed some light on the role of genetics in allergies.

All sorts of food allergies affect eight percent of children and two percent of adults in the United States. Allergic reactions happen because one's immune system overreacts to regular foods that are ordinarily harmless to the general population (7). An allergy affecting many children and adults recently in the United States is an allergy to peanuts. In the last few years, tremendous amounts of people have developed this allergy, which seems, in most cases, to be very severe. To be exact, scientists estimate that three million Americans are at risk at having an allergic reaction to peanuts (7). In the past, some have said that the allergy is caused by early exposure to peanuts, either in early childhood or during prenatal stages. Now, many researchers say that a gene causes this allergy.

A study was done on 58 pairs of twins, consisting of 44 pairs of fraternal twins and 14 pairs of identical twins. In every set of twins, at least one of the two had a convincing history of a peanut allergy. The twins were observed for signs of allergic reaction, including hives, wheezing, repetitive coughing, vomiting and diarrhea, within sixty minutes of eating peanuts. The results were as such: 65 percent of the identical twins shared the allergy, while only seven percent of the fraternal twins shared it (3).

Something interesting found was that the allergic symptoms among individual sets of twins could vary. For example, one twin may have a skin rash, while the other twin may experience asthma in reaction to the peanuts. These variations may be attributed to early exposure, environmental factors, infections, medications, and so on (1).

Because of these results, Mt. Sinai School of Medicine claims that genetics accounts for 81.6 percent of the risk of peanut allergy (3). Similarly, a group of British researchers say that, considering genetic and environmental factors, the allergy is inherited 82 to 87 percent of the time. They also say that when ignoring the genetic factors, that percentage drops to 18.99 percent (7).

In response to this study, a professor of Biochemistry and Molecular Biology at the University of Arkansas Medical School is in the process of creating an allergy-free peanut by genetic engineering (3). If this allergy is, in fact, genetic, then this may be a wonderful alternative to those suffering from the allergy.

Food allergies are not the only types of allergies affecting most people. Asthma is, at times, a severe allergy that causes problems in the airways of the lungs. When one has an asthma attack, the muscles surrounding the airways contract, the walls of the airways swell, and mucus is produced inside the airways, all making it very difficult for air to pass through while breathing. This attack can be caused by animal hair, colds or infections, dust, mold, pollen, cigarette smoke, or even weather, exercise and air pollution. Researchers used to think that asthma was solely caused by environmental factors, for example air quality, but now they are considering the effect of genetics as well. A study took place in Arizona involving 344 families. From families where neither parents had a history of asthma, only six percent of children suffered from asthma. In families where one parents suffered from the condition, only 20 percent of children suffered as well. And in families where both parents had asthma, 60 percent of the children had asthma as well. This shows that there is a strong link between asthma and genes (8).

In another study, the Department of Biological Psychology at Vrije Universiteit in Amsterdam studied 3600 pairs of twins from the Netherlands. Those observed consisted of pairs of identical and fraternal twins, all five years of age. The researchers had the parents fill out a questionnaire inquiring of the following: a) the presence of wheezing and coughing in the last twelve months, and b) if the twin children were ever diagnosed with asthma, allergies, hay fever, eczema, bronchitis, or pneumonia. They found that 50 to 80 percent of the time, identical twins shared allergies. However, fraternal twins shared allergies only 25 to 40 percent of the time, which is half of the results of the identical twins (2).

In Australia, the Australian National Health and Medical Research Council Twin Registry studied 3808 pairs of fraternal and identical twins. The twins were of all ages, from children to adults, and were being observed for asthma, wheezing and hay fever. The results were similar to the last study; 65 percent of the identical twins shared allergies whereas only 25 percent of the fraternal twins shared allergies (6).

Another study was done hypothesizing that genetics and environment were both very involved in suffering from asthma. Of 84 sets of twins, at least one of the two had a history of asthma. Among the 84 pairs, 39 pairs were identical twins and 55 pairs were fraternal twins. The study showed that among the identical twins, both twins suffered from asthma 59 percent of the time; and among the fraternal twins, both twins suffered from the condition only 24 percent of the time. When looking at the results of the identical twins, they show that 41 percent of the time only one of the two will get asthma. This neither proves that genetics are the main factor nor that the environment is the main factor, but instead that both genetics and the environment are large roles in developing the condition of asthma (8).

The National Jewish Medical and Research Center has been observing identical and fraternal twin children to see if there is a correlation between children with severe allergies and behavioral problems. The behaviors that they are most concerned about are aggressiveness, depression and irritability. The study wants to see if the behaviors are problems because allergies are a nuisance for the children, or if the behaviors and allergies are linked genetically. The Center observed 200 twin children between the years of three and eleven. They found more of a correlation between the allergies and the behaviors among the identical twins than among the fraternal twins. Their claim is that allergies and behavioral problems are both genetic and linked. In fact, they say that genetics accounts for over 70 percent of the relationship between allergies and aggressive behavior. If this is true, then children with allergies may have a higher chance of having behavior problems as well. More so, a genetic link may potentially lead to a treatment for both allergies and behavior problems (4).

All of these studies have tested for the same thing, whether or not allergies are genetic. The results are not exact, but one thing is in common: allergies among identical twins, who have identical DNA strands, are shared more often than among fraternal twins, who have only one strand of DNA in common. The percentages of the results for identical twins are clearly not 100 percent. Some may say that the results are close enough to 100 percent to say that genetics are the cause, while others may say that it is not close enough to make any definite conclusions on the matter. Perhaps a more involved study should take place, one that involves extensive research on each specific kind of allergy among twins. Maybe it is possible for some allergies to be genetic while others may be products of environment, diseases, and so on. In short, all of these cases suggest that genetics may have something to do with allergies, but at this point, that is all that it is: a suggestion. The important thing is to know that genetics and the environment could potentially be the main factors in developing allergies; so if one twin in a set has an allergy, it is a good idea to have the other twin evaluated just in case.


References

1)Twins and Allergies
2)A Study of Asthma and Allergies
3)Genes Cause Most Peanut Allergies
4)Genetic Link
5)Proactive Genetics Inc.
6)Entrez-PubMed
7)AAAAI Patients and Consumers Center
8)Asthma and Genetics


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