The puzzle Mendel inherited
In the 1860s most people pictured heredity as blending: a tall parent and a short parent should give medium children, the way mixing red and white paint gives pink. But blending had a problem nobody could solve — if traits truly blended, variation would shrink every generation until everyone looked the same. Gregor Mendel, working in a monastery garden in Brno, suspected the blending picture was simply wrong.
Mendel chose the garden pea for good reasons. Peas grow fast, are easy to cross by hand, and come in clean either/or characters — seeds are round or wrinkled, flowers purple or white, with almost nothing in between. He also started with true-breeding lines: plants that, left to self-pollinate, produced the same trait generation after generation. That gave him a clean starting point most naturalists never bothered to establish.
What the disappearing trait revealed
When Mendel crossed a true-breeding purple-flowered plant with a true-breeding white one, the children were not pale lavender. They were all purple. White had vanished. Under blending this made no sense. But then Mendel let those purple children self-pollinate — and white reappeared, in about one out of every four grandchildren. A trait that disappeared and then came back unchanged could not have blended. It must have been hiding, carried intact and passed on.
Mendel's leap was to propose that each plant carries two copies of a hereditary factor for each character, one from each parent. Those factors come in alternative versions — what we now call an allele. One version can mask the other without erasing it. The masked version sits silently, ready to show up again in a later generation. This is the seed of all Mendelian inheritance.
Why discreteness changed everything
Treating heredity as discrete, countable units turned biology into something you could predict with arithmetic. Because factors stay whole, you can ask: what fraction of offspring will show each trait? Mendel counted thousands of peas and found stable ratios — not vague tendencies but numbers like three-to-one that held up across experiments. Discreteness is what makes the rest of this track possible.
It is worth being honest about the limits, too. Not every trait obeys Mendel's clean either/or pattern — human height, for instance, really does vary smoothly, because many genes contribute at once. Mendel's genius was partly in choosing characters that show the rules in their simplest form. Those simple cases are the right place to begin, and we will meet the messier ones later.