The claim

Allergy problems run in families.

The facts

In the family: Children may have the same allergies as their parents.

The environment may get most of the blame, but scientists have found that allergies such as asthma and hay fever have a powerful genetic component—just not in the classic Mendelian pattern. Unlike hair and eye colour, they stem from the interactions of a multitude of genes, some conferring protection and others contributing to the development of allergies. As a result, people may not inherit their parents’ specific allergies to ragweed or pollen, but will have an increased likelihood of developing an allergy in general, particularly when both parents have one.

One 1984 study of 344 families, for example, found that when neither parent had a history of asthma, only 6% of children went on to develop it. But in families where one parent had the condition, 20% of children had the diagnosis; in families where both parents had it, 60% of children had it too. More compelling evidence comes from dozens of studies on twins. Generally, when one identical twin suffers from hay fever, asthma or eczema, the other twin has it in 50-80% of cases. In fraternal twins, the percentage drops to 25-40%.

The bottom line

Environment and genetics both contribute to allergies, but studies suggest that genes play a critical role.


The Claim

Birth complications are more likely with boys.

The facts

Is it a boy?: Male babies are a little more likely to complicate childbirth.

A 2009 study conducted by researchers at Tel Aviv University examined 66,000 births and found that those involving boys were more often complicated, with a slightly greater chance of problems such as premature birth and a need for Caesarean delivery.

Other studies came up with similar results. One in 2002 looked at more than 90,000 births in 1988 and 1999. It found that male births were 1.5 times as likely as female ones to result in the arrest of descent, which occurs when the foetus essentially stops descending in the second, or “pushing", stage of labour.

Scientists point out that these findings do not indicate that male births are necessarily “high risk", just slightly more risky than female births. One reason, it seems, is the larger head size of male babies. But there is speculation that other factors, such as higher levels of androgens, may also play a role.

As one study concluded, “When we say ‘it must be a boy’ as a humorous explanation of complications of labour and delivery, we are scientifically more correct than previously supposed."

The bottom line

Studies have found that boys pose a slightly elevated risk of birth complications.

©2010/The New York Times

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