Are Allergies Inherited?
The immune system is designed to protect people against infection by reacting against and destroying harmful outsiders such as viruses and bacteria. Sometimes, though, this all goes wrong and the body starts to react against harmless things like grass pollen or peanuts – these are known as allergens.
In the allergic response, the body produces high levels of a naturally occurring chemical called histamine and this leads to the symptoms of allergies, including itching skin, rashes, wheezing, coughing, a runny nose or itching eyes. People who have become sensitive to one allergen can sometimes become allergic to another as well – this is called ‘cross-sensitivity’ or ‘cross-reactivity’.
Inheriting AllergiesPeople do not necessarily inherit individual allergies, but will inherit the tendency to develop allergies (known as ‘atopy’). Therefore, while a child of parents with grass pollen allergies will not necessarily have a grass pollen allergy, he or she will be more likely to develop some kind of allergy than a child of non-allergic parents will. It seems that children are more likely to inherit allergies from their mother than their father.
Twin Studies and AllergiesA study of 344 families divided the families up into groups with neither parent, one parent or both parents having allergies. Only 6% of children with neither parent with an allergy had allergies, whereas 20% of children with one parent with allergies or 60% of children with both parents with allergies had allergies themselves.
Twin studies are a good way of looking at genetics, especially those that compare identical twins and non-identical twins, as pairs of twins generally have the same upbringing in the same environment, but identical twins share exactly the same genes, whereas non-identical twins will have similar but not identical genes.
In a study of 3808 pairs of twins in Australia, it was much more common for identical twins to have similar symptoms of asthma and hay fever than non-identical twins, suggesting that asthma and hay fever can be inherited. In a study of 11,750 pairs of twins in Denmark, identical twins were also much more likely to both have hay fever than non-identical twins were. This study suggested that environment has an effect as well, such as exposure to different allergens at different times, or exposure to cigarette smoke.
In a twin study of peanut allergies, 65% of twins shared the peanut allergy, whereas only 7% of the no-identical twins both were allergic to peanuts.
The Genetics of AllergyWhile it would only be absolutely conclusive if 100% of twins shared the allergy, it shows that genes do seem to have an impact. The genetics behind allergies are quite complicated, and it seems that there isn’t just one gene per allergy, but that it’s the interactions between a number of different genes that has the effect, and this is further complicated by environment.
Because of this, the allergies shared in a family may not be exactly the same – one allergic member of the family may have hay fever, whilst another has allergic asthma or atopic dermatitis. Even if the actual allergy is the same (e.g. peanut allergy) the responses may not be the same – one person may have a rash, and another a runny nose, or asthma. There may also be a link between the genes for allergies and the genes associated with certain behavioural problems.