RESEARCHERS in Oxford have found that the gene responsible for asthma, hay fever and other allergy-type illnesses is active only when inherited from the mother. If inherited from the father, the offspring is less likely to suffer from these illnesses.
The finding explains previous observations that children with mothers who suffer from asthma are more at risk of developing the condition than those whose fathers are asthma sufferers.
More recent studies have also suggested that the risk of allergies in young children is much greater when the mother is allergic than when the father is.
According to a report in the Lancet, the explanation may lie in a mechanism known as genomic imprinting which leads to some genes being switched off before conception. The majority of cells in the body contain two copies of our genes, one inherited from the father and the other from the mother. Often both copies work together, but sometimes only one copy is required and the other may be switched off.
In the case of the allergies, it appears the male copy is suppressed for some reason, while the female copy determines the child's susceptibility to asthma and hay fever.
Dr Bill Cookson and Dr Julian Hopkin from the John Radcliffe Hospital, Oxford, studied 723 people; 459 of them were asthma and hay fever sufferers.
They found that 125 (62 per cent) of brothers and sisters affected by allergy-type illnesses had inherited the relevant gene, which is situated on chromosome 11, from their mother, while 78 (38 per cent) had not. 'This distribution differs significantly from the expected 50:50 distribution,' the researchers say.
The maternal inheritance of the allergies may be due to the mother's influence on the child's immunity as it is developing in the womb - via antibodies she produces, or substances in the food she eats.
After birth, breast milk contains a high proportion of the mother's antibodies, white blood cells and substances from her diet. These factors could cause subtle changes in the infant's immunity and alter the risk of developing allergies in adult life.
Alternately, the gene responsible may be switched off in the male. The genes which control the growth of the placenta, which nourishes the foetus and supplies it with oxygen, come mostly from the father, whereas genes which control the growth of the foetus come from the mother.
Dr Cookson and Dr Hopkin suggest that a gene causing allergy in the placenta would be 'disadvantageous', which is why the male allergy gene is switched off. They point out that allergies do sometimes come from the father's side, but say that these are due to genes on another chromosome.
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