The relationship between the genes you inherit and the likelihood that you will develop a particular health problem is very complex. For example: it has long been known that some of the common diseases, which are responsible for a high proportion of deaths, such as cancer, heart disease and diabetes, are caused by a complicated interplay between genetic and environmental factors. Some people are more susceptible to the effects of factors such as over-eating, smoking or alcohol consumption it seems.
Susceptibility to infectious diseases, such as tuberculosis or malaria or mental illnesses such as schizophrenia or bipolar disease MAY also be determined by small individual variations on gene sequences, e.g. single nucleotide polymorphisms. This does not mean that individuals with these small genetic variations WILL suffer from these problems, but they are more likely to than someone who does not have them. Our understanding of the exact relationship between these small genetic variations and health are still emerging and there is much research still to be done.
Some conditions that are clearly genetically determined are much better understood however, and these include