Ontogenesis is, in
biology, the path of development of a specific organism. Theoretically, it covers the effects of the environment on the organism from
conception on. But it is generally used as a term to describe the development of the organism in the
womb, and possibly through early childhood, if that environment has a lasting effect on the organism (cats visual systems, for example, develop after birth.)
Any lasting effects of an organism's ontogenesis is described as ontogenetic. This is general counterpointed by genetic influences, which are the effects of the organism's genes. Ontogenetic effects represent a very blurry line in the nature vs. nurture debate. On the one hand, they are effects of the environment on a person. On the other hand, they are in some sense intrinsic to the organism from birth, and the environment in the womb is usually affected by the mother's genes, which are normally also inherited. This means that many ontogenetic effects would show up as correlations in genetic studies, and be mostly indistinguishable from genetic effects.
There are some obvious forms of damage that could be described as ontogenetic. The developmental problems caused by alcohol or drug use while pregnant would qualify. We're usually concerned with the differences caused by normal development, though.
The first interesting ontogenetic effect that I know of is handed-ness. Supposedly, left-handedness is caused by elevated testosterone levels during brain development in the womb. This inhibits the growth of the left brain and the right brain compensates in strength, making it more likely to dominate in motor control. This explains why more males are left-handed, and may also explain claims that lefties are more creative.
One way to determine whether a trait is an ontogenetic effect is to compare identical twins with fraternal twins and normal siblings. When this is done for left-handedness, we find that identical twins and fraternal twins have the same correlation for handedness, and that it is significantly more than for non-twin siblings. This leads us to the evidence that handedness is an ontogenetic effect. Since fraternal twins share the same uterine environment, we expect their ontogenetic effects to be the same. If the effect were genetic, we wouldn't expect fraternal twins to be any closer than normal siblings.
It's still very unclear what sorts of traits are ontogenetic rather than genetic (or affected by other aspects of the environment.) This leaves the outcome of cloning very much up in the air. Ontogenetic factors would be completely different for the clone, and I suspect that many of the similarities we associate with identical twins may be ontogenetic. It's quite possible that height, body type, hair color, are all to some extent ontogenetically determined, much like handedness.
Brave New World contains a lot of biological engineering that would be classified as ontogenetic. Gammas, deltas and epsilons are exposed to various chemical, radiation, and trauma throughout their development which has a lasting effect on their biology. Since the discovery of DNA, our ideas of biological engineering have shifted to the genetic, but there is still a broad range of possibilities related to ontogenetic factors.