Of course nothing was known specifically about genes, or anything smaller than what people could see (except for some philosophical theories...anatomists etc.).
But mankind made a lot of conscious usage of heredity, especially in selective plant breeding and animal husbandry. Note that most of the grains (wheat, rice...but originally not much more than grass) were hardly digestible before they were bred to be so. Many fruits, like all the different rosaceae (apples, pears...) weren't very tasty, nutritious, big or resistant before mankind saw potential in them; in the latter case we're even talking about the time after the last ice age, so quite recently (but there are theories that neanderthal had some early forms of agriculture).
Heredity in animals and humans however, is intuitively known since "forever". That mating with healthy individuals is more likely to produce healthy offspring is at the very center of evolution - and knowing about genetic diseases, or at least the concept thereof, immediately follows.
A good indicator for this is how much importance pre-modern societies placed on the concept of "blood". Blood was just their way of saying genetics. They knew traits were inherited by the offspring, they just didn't know the mechanics. Also they didn't know exactly which traits were inherited and which not - but then again, neither Mendel did. Farmers and Herders had good eyes for that though.
I'm aware I'm not citing any sources and I'm not saying there aren't, but sometimes it is difficult to say how much people know once, especially in times where the only means of knowledge inheritance was through oral tradition. One might speculate, that in certain areas of botanics they might have even known things we don't know, especially about healing properties of plants and such.