Mendel Wins Again

Warning: Excessively geeky science content ahead.

I harvested all of this year’s corn crop a couple of weeks back, and it’s been drying down ever since. I chose Carol Deppe’s “Cascade Ruby-Gold” again, because I need me some more this. It’s a lovely corn that the casual observer would look at and say, “Nice Indian corn,” and shellac a few ears for hanging on the front door. Thankfully, I know better.

The ears tend strongly to one solid color, unlike some corns where the individual kernels on any ear might show color diversity. In Cascade Ruby-Gold, you get either dark red, yellow, or orange ears. It’s really pretty corn.

Taste the rainbow...
Taste the rainbow…

But as I’ve been putting out the corn to dry, I noticed some interesting (to me) things. The first was how many more red ears there are than the other colors. That got me thinking about genetics (naturally), and then I started noticing that there aren’t really three colors in the crop, but four – dark red, full yellow, and two varieties of orange, light and dark. Time for science!

I grabbed the kids – it’s a homeschool thing – and we sorted the corn by colors. The different shades of orange are a little subtle, but in the picture above they really show up well. We may be off a few ears in either pile due to the subjective nature of judging color, but it looks like we did a pretty good job. Next, we did some accounting:

  • Dark red – 83 ears
  • Dark orange – 46 ears
  • Yellow ears – 44 ears
  • Light orange – 22 ears

Hmm – interesting ratios. Dark red has almost exactly twice as many ears as yellow and dark orange, and four times as many ears as light orange. In other words, the ratio is 4:2:2:1. That got us to discussing how that could be. Here’s where the science comes in.

Brother Harold, meet Brother Gregor
Brother Harold, meet Brother Gregor

Everyone knows that genes control everything about an organism. At least we do now. Back in the 19th century, that wasn’t at all clear. An Austrian monk named Gregor Mendel was a curious sort, and did really cool experiments with his pea plants in the abbey where he lived. These experiments are classics of biology and are still taught today. Before anyone knew about genes or the structure of DNA, Mendel was able to propose a model of inheritance of traits just by observing his pea plants. That model fit in perfectly when we later learned how DNA actually works. Sure, there are those who say he fudged his data, because it’s just too perfect, but if he did that, he must have had some inside information on how DNA works. Maybe his boss tipped him off.

Back from Greg’s pea patch to my corn, I started wondering what factors were responsible for the ratios we saw. Clearly red color is a dominant trait, and must be able to mask the underlying yellow, which is probably actually the lack of red pigments. The oranges are really just incomplete expression of the red pigment over the yellow background. So that 4:2:2:1 ratio we saw must relate to two genes – a gene for red, and a gene for yellow.

But here’s the rub: that ratio is “off” for the classic dihybrid, or two-factor, cross. Mendel showed that he could always count on a ratio of 9:3:3:1 when he crossed pea plants and looked at two different traits, like seed shape and color. Crossing plants would yield peas with sixteen genetic combinations, or geneotypes, but only four traits, or phenotypes: Round and yellow, round and green, wrinkled and yellow, and wrinkled and green. Whenever he counted seeds from a cross, he saw those traits in a 9:3:3:1 ratio.

But in biology, one of the first things you learn (after “There’s no such thing as a free lunch”) is that every rule is outnumbered by its exceptions. It turns out that the classic dihybrid ratios only work out if the two genes in question are completely independent of each other. If they’re linked in any way, the ratios get complicated. In this case, a 4:2:2:1 ratio points to lethal recessive combinations – recessive genes that mix and cause the organism to die before ever getting to express a phenotype. Notice that 4+2+2+1 = 9, not 16, which is the number of possible combinations of crossing two genes between two organisms. That means that in Cascade Ruby-Gold, seven of the possible 16 genotypes are croaking before I ever see them.

All this is of absolutely no use to me as a homesteader. It’s just a interesting aside, and a chance to groove to a little science with the kids. Maybe it got them interested in biology enough to read up on genetics a bit, or maybe it’ll come back to them someday when they’re helping their kids study. Maybe they’ll even relate the story to their kids about their crazy old coot grandfather and the day he made them count corn cobs on the front porch for no apparent reason.

And now we get to eat our experiment. So Mendel wins, but we win bigger.

1 Comment

  1. APB

    re: Eating your experiments – this is pretty common behavior in the life sciences. Scientists (at least in academia) gravitate toward model organisms that they either like to eat, or have fun collecting. My best friend from high school went on to be an entomologist studying wasps in the tropics because he loved being in the rain forest. I had two professors in college that loved to eat their experiments – one studied lobsters, the other was a microbiologist who loved to make his own sauerkraut. Each of them would almost drool in class talking about their experiments. One guy I knew needed shark rectal glands to study sodium transport, and while he didn’t eat them (that I know of), he loved to spend every summer in Maine collecting sharks and liberating them of their private parts. And in grad school I took a rotation through a yeast lab run by a guy who couldn’t stop talking about the bread he baked.

    Science: it’s what’s for dinner. Except for the wasps and shark butts.

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