Genetic Question for Dan

What is the genetic mechanism that controls continuously variable traits (height, bone thickness, pigmentation, etc.) ? I am aware the very simple case in flowers where the presence of normal petal color is determined by two copies of a gene, each of which produce a certain enzyme. A flower can have either zero, one or two copies of this gene, which results in a net production of the pigment enzyme in corresponding amounts of zero, about half-normal, or full-normal. The resulting flower color is then either pure white when there is no enzyme, whiteish-purple for medium enzyme levels, or deep purple when enzyme levels are fully normal. In principle, this is really a three-state set of trait values {0, 1/2, 1}. Although it is possible that environmental factors might cause the ½ level of enzyme to vary between individual flowers, resulting in a smearing out of the medium state in color space, such that the distribution of trait values appears to be a bit more continuous in the population

Is multi-gene encoding of enzyme/hormone levels the only way that continuously variable traits are controlled? What about genes being turned on and off? A single gene turns on and starts producing a certain growth hormone. The creature keeps growing until a certain condition is met, at which point the gene turns off, the hormone levels dwindle, and the creature’s shoe size has reached its maximum value. However, although only one gene encoded the recipe for the growth hormone, there must have been something else that prescribed the “turn off” condition, and that would be what is really responsible for the creature’s terminal value of shoe size. Different individuals with different shoe sizes would have identical growth hormone genes, but would have variation in the genes that encoded their shoe size via this “stop making hormone, my feet are big enough” mechanism. Unfortunately, we are right back to the fundamental question of how is a continuous numeric quantity is encoded by a set of genes. In this example, we would need to understand how the “hormone turn off” mechanism worked, and what knob in this mechanism could be continuously controlled via variation in some set of genes.

When you think about it, just about every little detail in an organism must have some sort of a control like this, since the length, curvature, and density of bones, muscles, organs, are all continuously variable during the growth of the organism. The final shape of each piece must be determined by possibly hundreds of specific dimensions and continuous quantities. How much is currently known about the mechanisms for encoding this information in our genome? If there are any serious flaws in my reasoning, or biological basics, please let me know, as I am very curious to understand this better.

Thanks

- Stephen