Genetics
It has long been known that kids resemble their parents, and that this phenomenon remains true throughout species. Two tall people will likely have a taller kid. Two really fluffy dogs will probably have fluffy puppies. A bush that grows blueberries will produce another bush that has blueberries. Humans have known about the ability to selectively breed for desired traits thanks to this principle since the dawn of agriculture thousands of years ago.
You are probably familiar with the role of genes and DNA in this, but that wasn't always the case. For a while, it was simply believed that offspring were a blend of their parents' traits. Then, everything changed when a friar with a green thumb started fiddling around with some peas.
Gregor Mendel
Mendel was an Augustinian friar from the 1800s who is famously lauded as the "Father of Genetics". At the monastery in which he lived, Mendel studied pea plants and uncovered many of the rules behind how heredity works and why "kids resemble their parents" as was mentioned above.
Thanks to his meticulous counting of hundreds of thousands of peas over several years, Mendel established these basic rules of genetics that are now referred to as the laws of Mendelian Inheritance.
Some Basic Terminology
allele vs gene
homozygous vs heterozygous
genetic locus
The Law of Dominance
explain experiments
pic to the right should be of experiments
dominant vs recessive
The Law of Segregation
idea of diploid organisms and haploid gametes