Gregor Mendel · Science
How Mendel’s second great law - that different traits are inherited independently of one another - explained the endless reshuffling of characters across generations, and laid bare the combinatorial engine that makes every offspring a unique new deal of the hereditary cards.
Having cracked the inheritance of single traits, Mendel asked a deeper question: what happens when you track two traits at the same time? Suppose you cross a pea that is true-breeding for round and yellow seeds with one that is true-breeding for wrinkled and green seeds. The first generation, as expected, are all round and yellow (since round and yellow are dominant). But the real test comes in the next generation. Do the two traits travel together as a package - so that round always comes with yellow, and wrinkled always with green, just as they started? Or do they separate and recombine freely, throwing up new pairings that neither parent had - round-and-green, wrinkled-and-yellow?
This was a crucial question, because the answer would reveal whether hereditary factors are bound to one another or independent. If they travel as fixed bundles, heredity would be conservative, locking traits into the combinations they came in. If they shuffle freely, heredity would be combinatorial and creative, endlessly remixing traits into new arrangements. Mendel did the experiment, counted the offspring, and found his answer in the ratios - an answer that revealed heredity to be a vast shuffling machine, dealing each new organism a fresh hand from the same deck of ancestral cards.
The law of independent assortment has a consequence that is easy to state and staggering to contemplate: it makes the number of possible offspring combinations explode. With one trait having two forms, there are a few possible offspring types. With two independently-assorting traits, more. But heredity does not deal in two traits - it deals in thousands of genes, each able to come in different versions, each assorting more or less independently. When many independent factors shuffle and recombine, the number of possible combinations is not added but multiplied, and multiplication of many small numbers produces astronomically large ones.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned Mendel’s law of independent assortment - that the factors for different traits separate and recombine independently of one another, producing new combinations and the 9:3:3:1 ratio in a two-trait cross. Explain why this independent shuffling, multiplied across many traits, generates the vast variety among…
Leads to Aristotle.
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