Rules for Genetic Crosses: Incomplete Dominance

Smoothie Picture by Eric Schmuttenmaer

I like smoothies. In fact, I was drinking one in a coffee shop and working on an English paper when I came up with this idea. Yumm.

So, when you think of incomplete dominance (also called semi-dominance by some), think of a strawberry (S) and banana (s) smoothie…mmmMMMMmmm. There are two things that have a big effect on the final product. (Kinda like the two alleles in genetics. Follow me here)

There are three possible combinations of the two ingredients: strawberries (S) and bananas (s). There is an all-strawberry smoothie, (SS). There is the all-banana smoothie (ss). And then, there’s the combination of the two into strawberry-bananaish goodness (Ss).

HOLD ON. This time, the (Ss) isn’t just a strawberry smoothie?!?!? Nope. And that’s what characterizes incomplete dominance. The heterozygote (Ss), in this case the strawberry-banana yummness, shows a middle ground between the two extremes.

Just for practice, let’s look at a problem that deals with incomplete dominance:

Your mom has two crepe myrtle trees, both with purple flowers. Over the next few years, more trees spring up – mostly purple, but some red and some blue. Your mom thinks the blue is the prettiest color ever, and she wants more. What cross would you perform to get the most blue-flowering trees?

First thing’s first: identify what we can. We know that mom and dad (the two beginning trees) are purple, and that the children are purple, red, and blue. We don’t know which allele is technically referred to as the dominant one, but since it’s common sense that red + blue = purple, we can deduce that this is a problem that deals with incomplete dominance… so that’s not really a big deal. We’ll call the blue allele (B) dominant because mom like it, so the red (b) would be the recessive version.

Although common sense would say that both of these alleles are on the same level of dominance to each other, we use the traditional means of writing this out. Just for the ease.

By using this method, we can call the blue flowers (BB), the red flowers (bb) and the purple one’s (Bb). Which mix will give us the most (BB) offspring?

As you can see, the most blue flowers will result if we cross a blue tree with another blue tree. We tested each instance in which there was any blue allele present. (BB) plants true breeding and can ONLY produce blue flowers, meaning 100% of offspring will show the wanted phenotype. You can’t get any better than that.

Best of Luck,

Grey