We can think of different levels of cause.

Normal variations in the human genome, some of which provide advantage and some disadvantage, such as height, eye colour and proportion. Taller people are more successful in life, they are more likely to get better jobs, earn more money and so are advantaged. Their offspring are more likely to get a good start and hence survive, ensuring that trait is maintained. The opposite will also be the case, namely that there are normal variants which are disadvantageous in the present environment of C21st Urban humankind. If there was a sudden change, such as the earth being struck by a large lump of space debris, these disadvantageous genes might come in very useful, ensuring that, while different individuals lived or died, the species as a whole would continue. This variation is the vital genetic flexibility of our species.

This includes inherited genes that confer strong disadvantage, such as that for Huntington’s chorea. These tend to be very rare, but because they are only expressed after reproduction, carriers are not selected against by failure of reproduction.

Germ line mutations. New changes or polymorphisms in the germ line, (which are heritable). The ones drawn to the attention of the medical profession are usually those that disadvantage the child or adult. Few parents take their child to a doctor because the child is too intelligent for school, or taller than their peers, or better at sports, or can play the piano at 3 years of age.