Genetic factors do play the biggest role in the development of allergy so you are right to point that out. Despite this, be reassured that a baby breastfed exclusively for the first six months of his or her life will show
a significant difference in their health. The basis of formula milk which is cow’s milk protein (and, to a lesser extent, soya) are highly associated with the development of allergy so it makes sense to avoid this in the infant’s early diet for as long as possible. Unlike formula milk, breast milk contains antibodies designed to help an immature immune system making sure it does not become overwhelmed by whatever foreign proteins that challenge it. It is not surprising, then, that the research in this area consistently concludes that exclusive breastfeeding for six months (and longer as part of a weaning diet) is associated with a reduced incidence/severity of allergy in high risk infants, that extends well beyond childhood. This protection from breast milk is very significantly enhanced when the mother also makes dietary restrictions (avoiding cow’s milk, dairy, eggs, peanuts etc) throughout her pregnancy and lactation. Making any form of dietary restriction requires the advice of a dietician to ensure an adequate intake of calcium in the mother. Breast milk contains factors which are not found in formula milk, and as yet, cannot be added: growth factors, infection-fighting cells, antibodies, anti-bacterial and anti-viral cells, hormones, enzymes and prostaglandins – all of which play complex roles in the maintenance of the infant’s health and growth. A baby fed on formula milk may be affected by the presence of cow’s milk/soya protein, plus there is the possibility of errors/contamination 
during manufacture or in the home-preparation of the milk. By continued exclusive breastfeeding for six months, not only will you have reduced the risk of your baby developing allergies, you will have conferred on him/her a significantly reduced risk of gastro-intestinal illness, respiratory illness, ear infections, diabetes, obesity and respiratory wheeze. You may have raised his/her IQ and will have reduced your own risk of developing ovarian cancer, pre-menopausal breast cancer, and hip fractures. The longer the breastfeeding continues, the greater the health gains for both mother and baby in terms of incidence, severity and morbidity. Apart from the problems of sore nipples, painful breasts, frequent feeding and the social embarrassment associated with breastfeeding, which are to do with wrong public attitudes and poor support, the advantages of breastfeeding for both mother and baby far outstrips the discomforts.
Please note that one of the only times when medical practitioners advice against breastfeeding is when the mother has HIV or hepatitis B.