The document discusses Gregor Mendel's pioneering work in genetics using pea plants. It explains that Mendel used Punnett squares to predict the expected ratios of traits being passed on from parents to offspring. While Punnett squares can predict ratios, the actual observed results do not always match expectations if one parent is heterozygous for a trait. Mendel's careful experiments demonstrated that traits can be dominant or recessive and are passed on through discrete units (now known as genes), fundamentally shaping the field of genetics.