Genetics of Squash Shape (2/2)

In my first blog post, I examined a small biology puzzle: the classical model of squash shape genetics (at left) didn't seem to match what I was seeing in my own garden.

The classical model involves two genes ('A' and 'B', respectively).   The dominant allele of both genes must be present (A_B_) to produce a disk-shaped squash.   If one gene is present in the dominant form and the other is in the recessive form (A_bb or aaB_), spherical-shaped squash will result.   If only the recessive alleles of both genes are present (aabb), then elongated squash will result.   After some digging, I found this model comes from a 1927 paper by Edmund W. Sinnott in the research journal, The American Naturalist.

This model predicts that crossing a (AABB) PattyPan to a (aabb) Zucchini should result in (AaBb) progeny plants with disc-shaped squash.   The progeny plants I grew, instead each produced intermediate/elongated-fat squash.   I assumed this meant my original PattyPan squash was a hybrid that contained both recessive alleles (AaBb) and so I then calculated the probabilities of producing (aabb) progeny from crossing the potential male parents (PattyPan and Zucchini) to the female parent (PattyPan).   The best probability I calculated was \( p = \frac{1}{16} \).   I wasn't pleased with this result and decided I needed further data.

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The magic of the internet then made its presence known: Ottawa Gardener was forwarded to my original post in a discussion of their recent post.   They had grown Zucchini, PattyPan, and some Pumpkins, then tossed some of the PattyPan squash to their chickens.   The next year when they moved the chicken run, up came a batch of hybrid squash.   Most appeared to be intermediate between the Zucchini and the PattyPan, with a few looking intermediate between the PattyPan and the Pumpkin.   (I've rearranged their photo to make the diagram at left.)

Because there are three potential male parents (and two offspring types), the calculations of probability get somewhat more intricate and I won't go into them here.   The detail I found most interesting was the recreation of the intermediate/elongated-fat shaped squash at the bottom-left.

Upon digging into the research literature further, I found a 1910 paper by R.A. Emerson, again in The American Naturalist.   In this paper, the author describes the result of crossing "White Scallop" (disc) and "Yellow Crookneck" (elongated) squash as being intermediate in shape.   This is contrary to the Sinnott model of shape genetics and is the result that both I and Ottawa Gardener observed in our gardens.

It seems like the Emerson result was never followed up on and the Sinnott result erroneously became the standard model of squash shape genetics that has been used in textbooks ever since.

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I'm really happy to know that my garden results are consistent with results from others, but I still need more data.

The simplest model I've come up with requires there to be a third gene (C) that allows the first two genes (A & B) to produce disc-shaped squash.   Sorting out the genetics of a cross involving three genes is much harder than a cross involving one or two genes.   Fortunately, I've got several hundred F2 seeds from the F1 plants I grew.

I plan to grow several F2s this coming year and I've managed to find homes for a few more in others' gardens.   It may take several years of this to collect sufficient data to get an idea of what is going on.

Would you be willing to grow some of my experimental squash seeds, then send me photos/measurements of the fruit that grow?   Get a message to me and we'll work out how to get seeds to you.

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Citations and notes :

1. Emerson RA.   The Inheritance of Sizes and Shapes in Plants.   1910, The American Naturalist 44: 739-746.   (https://archive.org/details/jstor-2455667)
• "Yellow Crookneck" x "White Scallop" -> F1 "intermediate".
• This matches the results I found in my garden


2. Sinnott EW.   Inheritance of Fruit Shape in Curcurbita pepo.   1922, Botanical Gazette 74: 95-103.   (https://archive.org/details/jstor-2470204)
• Sphere x Scallop -> F1 disc -> F2 (3 disc):(1 sphere).


3. Sinnott EW.   A Factorial Analysis of Certain Shape Characters in Squash Fruits.   1927, The American Naturalist 61: 333-344. (http://www.jstor.org/discover/10.2307/2456386)
• Sphere(#103) x disc -> F1 disc -> F2 (3 disc):(1 sphere).
• Sphere(#22) x disc -> F1 disc -> F2 (3 disc):(1 spheroid).
• Sphere(#103) x sphere(#22) -> F1 disc -> F2 (9 disc):(6 sphere):(1 elongate).

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Part 1