page thirty-six

The Cloister

Thirty-five rooms have measured, dated, named and forecast things, and none of them has asked how anything gets from a parent to a child. Today's number fills a priory seed drawer with two pure lines of garden pea and one stray, and hides the rules of inheritance inside them. You have eight beds this summer, and a pencil.

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The seed drawer

Every plant carries two copies of each character, one from each parent, and passes one of them, chosen at random, to each seed. What you see is what the two copies make together. Pick a seed parent and a pollen parent, then sow. Keep any plant you like from a bed and it goes back in the drawer for the next cross.

Beds left 8 of 8 —

The beds

Newest first. Every plant in the bed is counted and sorted by how it looks, and the line underneath splits the bed one character at a time.

The ratio bench

Pearson's chi-squared test on your own counts. One character at a time, it checks a bed against the classic ratios. Two at a time, it asks whether they travel independently or stick together.

The notebook

Which form wins in a plant that carries one of each? Or does neither, so it comes out in between? What does the well plant carry? Then the hard one: two of today's characters sit on the same chromosome. Name them, and say in what percentage of seeds a crossover splits them.

Drawer
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The arrow keys walk to other days. Every day is a different drawer, different rules and a different stray.

What is real here and what is not

The rules are the ones a friar worked out with garden peas in Brno in the 1850s and 60s, and the ones Thomas Hunt Morgan's fly room added fifty years later. The priory, the lines and the rules of the day are the day's number.

  1. 01

    Two copies, one passed on

    Gregor Mendel read his pea counts in 1865 as factors that come in pairs, separate when seeds are made, and meet again at random. Cross two pure lines and the children all look alike; let those self and the hidden form comes back in about a quarter of the grandchildren. That 3 : 1 is the signature.

  2. 02

    Not every form wins

    Sometimes a plant with one of each copy comes out in between, as Carl Correns found with four o'clock flowers: red crossed with white made pink. Then 3 : 1 turns into 1 : 2 : 1, and the look of a plant tells you exactly what it carries.

  3. 03

    Linkage and crossing over

    Characters on different chromosomes assort independently, giving 9 : 3 : 3 : 1. Two on the same chromosome stay together unless a crossover splits them, and the further apart they sit, the more often it happens. In 1913 Alfred Sturtevant turned those percentages into the first genetic map. The page's own estimate from your beds is the maximum- likelihood one, the standard way to read any cross at all.

  4. 04

    The rest is simplified

    Every character here is one gene with two forms and complete penetrance. No plant fails to germinate, no seed is miscounted, no bee gets into the bed. Real peas have seven pairs of chromosomes, and a few of Mendel's own characters turned out to share one. The counting is still what you would have to do.

Thirty-six rooms, and one number doing all the work. Today it writes the rules of inheritance into a drawer of peas and lets you find them out the slow way.