My question is this: how much was known about human heredity before Mendelian inheritance was discovered? I imagine human beings would have always noticed that children end up looking like their parents.
Heredity was well known term in the Scientific Community when Mendel published his work, but was not understood. Darwin presented a hypothetical model of heredity in Origin of the Species, his Theory of Pangenesis he used it to explain evolution but he didn't demonstrate it. He proposed a hypothetical because his theory of Evolution required such a model land his fossil record suggested it. Mendel was the first to use the scientific method and model it, although nobody realized this at the time.
When Gregor Mendel published his work "Experiments on Plant Hybridization" in 1866, it was seven years after Charles Darwin published "Origin of the Species" which predicted and depended on the inheritance of traits in animals.
Darwin's hypothesized on the existence of a mechanism of heredity. Darwin supported his theory / conclusions on an exhaustive catalog of skeletal and fossil records; but he did not have live experiments or a good Biological model for what he called gemmules.
Pangenesis was Charles Darwin's hypothetical mechanism for heredity, in which he proposed that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing heritable information to the gametes.
At the time nobody recognized Mendel's study on plant hybridization could be applied to support Darwin's work on inheritance of traits in animals. Mendel's work went largely ignored for the next 30 years after Mendel published. Charles Darwin who was called to defend his Theory of Evolution almost immediately was entirely unaware of Mendel's supporting work.
Gregor Mendel: Initial reception of Mendel's work
Notably, Charles Darwin was not aware of Mendel's paper, and it is envisaged that if he had been aware of it, genetics as it exists now might have taken hold much earlier. Mendel's scientific biography thus provides an example of the failure of obscure, highly original innovators to receive the attention they deserve.
Mendel paper made two conclusions:
The Law of Independent Assortment states the building blocks (allels) for separate traits are inherited independently from other traits.
The Law of Segregation states that two heterozygous parents(Yy) offspring inherit a given trait in a pattern supported by dominant-recessive expression 1:2:1. where 1 part expresses homozygous dominant (YY), 2 parts express heterozygous (Yy), and the last part is homozygous recessive (yy).
Y y <- Parent #1
Y | YY Yy
y | Yy yy
| Parent #2
Two Genetic Laws which while discovered observing plants also pertain to animals. Gregor Mendel is known as the father of Modern Genetics for his work.
Darwin's work in Origin of the Species was predicated on the inheritance of traits across generations but it lacked a biological model for that inheritance provided by Mendel. Thus Mendel's model of inheritance in plants supports and complements Darwin's work. As did Thomas Morgan who expanded Mendel's genetic studies but in animals (Drosophila melanogaster) fruit flies. Also James Watson and Francis Crick's which discovered DNA in the 1950s. The physical mechanism which Darwin hypothesized, and which Mendel first studied nearly 100 years earlier.