Some genetic disease genes are said to provide protection against
other disease if present in a single dos (only one gene)- but fatal if
present in a double dose (two genes).  For examples, having one sickle
cell gene imparts sime protection against t malaris-but gives the
bearer of two of the genes with a serious debilitating disease.
Q.Give a specific example of such a beneficial effect and discuss why
it might be so.