Answer:
Observations on the outcomes of breeding experiments. The underlying processes were unknown at the time
Explanation:
Mendel gave his principles of inheritance way before concept of chromosomes, genes and alleles came. He crossed pea plants and observed seven different traits. These traits were: plant height, flower colour, flower position, seed shape, seed colour, pod shape and pod colour. According to the results of his experiments, he gave three principles of inheritance: law of dominance, law of segregation and law of independent assortment. He also coined the term "factors" which are known as alleles today. The three laws were also later proved using modern day concepts of genes, chromosomes and chromosomal crossover.