Why is it difficult to investigate human heredity?

Human heredity can be investigated, but it is very expensive and difficult. Genetics, which deals with the study of genes, the study of heredity, is a very precise science. But DNA has not yet been fully studied. The knowledge that we have can help us study heredity, find out the likelihood of genetic diseases in offspring, and so on. But there are not enough specialists and equipment for studying genes. Therefore, every person cannot afford it due to the high cost and lack of resources.