I brought in the first snow peas to feed to the ravening boarders. The youngest loves snow peas though he insists on dissecting them first. He removes the tiny immature peas to eat then tears into the crunchy flesh. Watching him unsettles me a bit. He clearly approaches herbivory carnivorously.
Some of my peas are much taller than I expected and their flowers are purple not white. This is far from uncommon in peas, but any trait variation in peas brings to mind Gregor Mendel, the nineteenth century monk who conducted experiments to understand how inheritance works. Mendel focused on traits that could be easily identified. He also, through luck or careful selection, picked traits that were determined by a single gene.
A “gene” in this case (well in most cases) is a small section of an organism’s DNA that gets “translated” into a protein. The protein is what actually leads to a trait. In the case of my flowers, the purple comes from anthocyanin, a pigment that’s produced by the coordinated action of a bunch of proteins. This last is the issue that often gets skimmed over when people talk about genes, genetics and inheritance: the color comes from a whole bunch of proteins working together to make a molecule that looks purple. Each of these proteins has a gene; there is no single gene for “purple flowers.”
But I just said Mendel worked on traits that were determined by a single gene. He examined inheritance of either white or purple flowers. The white flowered plants he used are white because of a single mutation. One DNA “letter” (out of thousands in the gene) is changed from a “G” to an “A” that is from a guanine to an adenine (Hellens et al PLoS One 2010). Because of this the gene is translated as a bad protein. The protein’s job is to cause the translation of the other genes that make purple flowers. Since it’s mutated, no purple color making proteins get translated and so no purple flower. White flowers are really just lack of color flowers.
This sort of trait is actually a rarity. Mendel carefully selected his seven traits, the vast majority of traits in peas (or anything else) are determined by lots of different genes.
One of the most common misconceptions about genetics held by the public is that there is a single gene for most traits and diseases. More problematically coverage of science frequently supports this contention. BRCA is often described as “the breast cancer gene.” There are conditions caused by single gene defects (muscular dystrophy, cystic fibrosis and sickle cell anemia), but these are the exception.
My two indigent boarders look kind of alike but their genes were drawn from just what my wife and I have to offer. Very few of the traits you can see are determined by a single gene: precocious teeth, fussy eating, or the gift of gab. Most are a combination of effects of 10’s or 100’s of genes.
Of course inheritance is only part of the story. Did my boy develop his carnivorous approach to plant eating because of a genetic disposition, or did I train him to it with all this talk about eating your peas while they’re still screaming? We may never know.