Wednesday, November 27, 2013

Genetics of Squash Shape (1/2)

In 2012, I saved some seeds from a "Patty Pan" squash I grew in the garden.   This spring, I grew three of the saved seeds and had two survive to maturity.   I was expecting the plants to produce the flying-saucer shaped fruit that the "Patty Pan" squash is known for, so I was surprised to see what developed.   The fruit from both plants was elongated and grew in yellow (later maturing to orange).

I decided it was time to investigate the genetics of squash fruit shape.   I recalled seeing this figure in some ancient genetics text book explaining the common shapes of squash as being due to two interacting genes and eventually I found a version of it.   The "Patty Pan" squash shape is caused by having a dominant allele for each of the two genes, while the elongated "Zucchini" shape is caused by being homozygous recessive for each of the two genes.   A squash with a spherical/pumpkin shape has a dominant allele for only one of the two genes.

Since my "Patty Pan" squash produced elongated children, it had to carry a recessive allele for each gene, thus it had the genetic composition of (AaBb).   I didn't control the pollination which led two the seeds I grew, so the father is unknown.

The first model for this cross is that the pollen came from a male flower on the same plant.
(AaBb) x (AaBb) -> (aabb)

Both pollen and egg had to carry recessive versions of each gene.   A quarter of the egg and pollen cells would be double-recessive (or double-dominant), so the probability of this cross resulting in the observed progeny would be 1/16.   The probability that two progeny would have the double-recessive trait is (1/16)^2 = 1/256.   This is not exactly a likely probability.
The second model for this cross is that the pollen came from a male flower on a nearby "Zucchini" squash I was also growing.

(AaBb) x (aabb) -> (aabb)

A quarter of the egg cells and every pollen cell would be double-recessive, so the probability of this cross resulting in the observed progeny would be 1/4.   The probability that two progeny would have the double-recessive trait is (1/4)^2 = 1/16.   This is also not exactly a likely probability.

A third model is that the classical story of squash shape is wrong for the genetics I am playing with.   Generally, I would consider this also to not exactly be a likely probability.



To discriminate between the three models, I need more data.   Right now there are too few data points to be certain and unlikely events happen all the time.

I still have six viable-looking seeds from the parental "Patty Pan" squash and this season I saved almost every seed from a fruit each of the two progeny plants.

Next year, I plan to grow out the six remaining first generation seeds and as many of the second generation seeds as I can find homes for.   I expect I will be sharing lots of squash with everyone I know.

Part 2