The first genetics, Mendelian genetics, began at 1900, yet inheritance of Mendelian traits was localized to chromosomes by 1903, thus chromosomal genetics.
No homozygous mutant embryos were identified during gestation, and in a separate study only 1% survived until weaning (significantly less than the Mendelian ratio).
This paper showed that the inheritance of traits measurable by real values (i.e., continuous or dimensional traits) is consistent with Mendelian principles.