Rules for Genetic Crosses: The Basic Cross

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Ok, so we’ve already done a cross, but in that situation, I wanted to focus more on the fact that we were dealing with a situation focused on only one gene. In this case, we’re still going to deal with only one gene, but now, we’re going to set out with the purpose of figuring out the parent’s genotype from using the offspring. If you’re just getting started with genetics, don’t worry. Soon, you’ll be able to do monohybrid crosses in your head. Scouts Honor.

So, here’s the problem:

Mrs. Payette breeds parrots and teaches them to talk. Unfortunately, learning to talk requires the ability to hear, and one of her male parrots, Dazzy, is deaf. He’s such a beautiful bird though, so she doesn’t want to keep him from breeding unless it’s there is absolutely no way that he’ll have hearing parrotlets. In this case, deafness is a dominant phenotype and the ability to hear is recessive. She wants us to figure out whether or not letting him breed is worth it.

Let’s identify the big problem. What is Dazzy’s genotype? We were told that dazzy shows a dominant phenotype, deafness, among a group of parrots who are all NOT DEAF, meaning that they are all recessive. So, we can assume that the rest of Mrs. Payette’s parrots are (dd) and that Dazzy is …. no wait, we don’t know what Dazzy is. His genotype is either (Dd) or (DD). Bingo.

There are no parrotlets yet, so there’s no way to figure this out. So let’s tell Mrs. Payette that we need behbehs.

(Elapsed time: 2 months)

SO, Dazzy and Cindy had parrotlets… 10 to be exact. Four of them can sleep through the squawking of all their comrades, so Mrs. Payette are pretty sure that they are deaf.

Because there are deaf parrotlets, we know that Dazzy’s genotype is (Dd). Did you figure out why? Well let’s make sure you know.

As you can see, if Dazzy were (DD), then ALL of his parrotlets would be deaf (Dd). Instead, 4/10 (almost half) are deaf, which makes sense if you look at the Punnett square. (We expected 50%. But hey nothing in life works out perfectly every time.)

Rules for Crosses:

1. FIND THE PROBLEM. Teachers are tricky; make sure to identify the actual question so that you don’t do a lot of work for nothing.
2. Identify the given information.
3. Try to identify all the genotypes of the individuals involved. If you can’t identify them all (which you probably won’t, since that’s the problem), then use the given information to find the not-given information.
4. Check your work with a Punnett Square.
5. Check to make sure that you answered the question completely – yet another way to easily lose points.

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As Always, Best of Luck,

Grey