Twin women lock arms and become part of a large strand of DNA.

Asthma, Allergies, Twins: Exploring the Genetics

I go through phases in what I choose for reading material that can best be broken into themes. With some other random things thrown in, the 170 books I’ve read so far in 2020 (with just over a week to go) include themes on: pandemic disease (obviously), memoirs by foster parents, horrendous true crime stories, political memoirs, and a smattering of other fiction and memoirs.

The last theme I got into, though, has very few memoirs written about it, and that is the theme of identical twins separated at birth—because, yes, that’s a rather niche subject indeed. The most recent, Identical Strangers: A Memoir of Twins Separated and Reunited, mentioned the fascinating differences in asthma and allergy rates among identical twins.

The genetics of twin-dom and twin studies

I’m not going to go too deep into the genetics of twins, but here are the basics of it. Identical twins have what is generally considered identical DNA, whereas fraternal twins share the same amounts of DNA as any other siblings would. This means, of course, we’d expect diseases that have genetic linkage to affect both identical twins similarly. But, of course, we know that other factors may often “trigger” the development of certain diseases—or even other traits.1

This is where so-called "twin studies" come into play. You may have heard about these in a psychology class, which, in my case, was developmental psychology. (I dropped the class, but I remember this much: generally, twin studies are used to study the impact of nature, or genetics, vs. nurture, which I take to mean the environment in this case.) When twins are separated at birth, they carry their identical DNA and predispositions for traits or diseases into two different environments where their culture, exposure to different experiences, education, and potentially even socioeconomic status and opportunity can differ.1

While twin studies of separated twins and purposeful separation of twins are largely considered to be unethical now, they certainly contributed to a lot of understanding about a variety of health and social topics.1

Asthma and allergies in twins: What do we know?

20% of the way into Identical Strangers, my interest was piqued by the word asthma—of course.

"Surprisingly, identical twins don’t always have the same allergies. While one twin may be allergic to peanuts, the chance that the other will share the allergy is only 65 percent. With asthma, the correlation between twins is a startlingly low 20 percent. And though identical twins may be allergic to the same substances, the intensity of their reactions may differ.”1

Dr. Sicherer, a researcher at Mount Sinai’s Icahn School of Medicine, reports the same findings in a research study from 2000, which Sicherer quoted in 2009 when interviewed by Allergic Living, stating of identical twins who are allergic to peanuts, 64% both experience the allergy (though it may onset at different times), compared to 7% of non-identical twins—which we can expect, based on DNA, would be the same rate a non-twin sibling could be affected by the same allergy.2

When it comes to asthma, the factors surrounding who develops asthma are known to be complex, and a mix of genetic and environmental factors.3 With regard to twin studies (both twins raised together and apart), we also must remember twins may be at greater risk of developing asthma due to other environmental factors, including that twins are generally born smaller (low-birth weight—which is not conclusively linked to asthma development) and more likely to be born pre-term--which is a factor for developing asthma.3 Additionally, according to the analyses of twin studies with regard to asthma by Thomsen (2014), the intrauterine environment shared by twins before birth is more “hostile” than that of singletons (non-twins), and may contribute to the development of asthma.3

Among identical twins with asthma, it is said that asthma may be 52 to 75% inherited/genetically-linked, leaving a significant margin of room for the impacts of the environment on asthma development.4 A 2014 study suggests that, of families where one identical twin has asthma, only 50% of their “co-twins” also have asthma.5 This is, of course, despite their presumed identical genetics and living environment, underscoring the complexities that may lead to asthma development.

A multi-factor and complex genetic puzzle

We already knew asthma was complicated. But, of course, the fact that even identical twins don’t always both develop asthma or allergies makes our understanding of factors impacting the development of asthma perhaps even more complex than we’d have anticipated.

Do you have an identical or non-identical twin and have asthma, or know twins affected by asthma? I’d love to hear more about what you’ve encountered in the comments—sure, it’s not scientific, but it could be interesting!

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