Hey Nerds,

I was talking to a family friend who owns a contracting business. It’s a family-owned company, and problems are always in abundance.

He was telling me about one particular instance in which half of the blueprints for a project that he had been commissioned to complete had gone missing. Of course, my sciency mind wandered (construction talk is frightfully boring) to genetics. Then, BOOM, epiphany.

Just for now, think about cells as a contracting company. The genetic code will act as a blueprint (I know that I contradict myself in a later post, but go with me here).

In this instance, a dominant allele will code for one copy of the blueprint. A recessive allele will code for a blank piece of blue paper.

So, lets look at the possibilities:

Homozygous dominant (XX) has two copies of the blueprint, and therefore has no problems building the house.

Heterozygous (Xx) has only one copy of the blueprint and one sheet of blank blue paper. The project takes longer, but the information is still at least there. And the house still gets built.

Homozygous recessive (xx) have no copy of the blueprints. This contracting company cannot build the house, so the lot that the house would have been built on remains in its default state: undeveloped.

Hopefully this will help some of you get a good grasp on the difference between dominant and recessive. When you get deeper into bio, you’ll find that it’s not actually that simple. But for all intensive purposes, this is all you need to know right now.

Best of Luck,
Grey

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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

In zebras, purple eyes are dominant to yellow ones. (I don’t know if this is true, but just go with it.)  A purple-eyed zebra-man has little zebralets with yellow-eyed zebra babe: 487 purple-eyed 520 yellow-eyed. What are the genotypes of the man and his kids? What are the probabilities of their possible children?

We’ll call the purple allele (P) because it’s dominant, and we’ll call the yellow allele (p) because it’s the recessive version of the gene. Now that we know how we’re gonna write the genotype of the parents, let’s figure those genotypes out.

We know that the female genotype is (pp) because we were told that, in this example, yellow is both recessive and the color of the female’s eyes.

Dad’s genotype is going to be a little harder to decide – but not much. Remember that with dominant alleles, both homozygotes and heterozygotes will show the dominant phenotype. If you’re not really good with the genetic jargan, that means that Dad will have purple eyes whether his genotype is (PP) or (Pp). That’s where we come in; we have to figure out which one he is.

This is usually where most people have problems. “Uhhhh… what now?” What a great question. Let’s figure out the children’s genotypes – since we already figured out everybody else’s. There are both purple and yellow eyed kids. That means that there are either (Pp) or (PP) AND there are (pp) represented.

REMEMBER that the children can only get alleles from mom and dad. SO if mom and dad don’t have them, the children can’t. Since Mom’s genotype is (pp), then she can only give (p) alleles to her kids. Since we know that there is going to be at least one (p) in all the kids, we can deduce that NONE of the kids will be (PP) and that the purple-eyed kids will have (Pp) as their genotype.

HEY! Wait a sec, we missed something. OH YEAH, we can also figure out Dad’s genotype now. Remember what I said earlier about kids only being able to get alleles from the parents? Let’s look at those yellow eyed kids again. They have a genotype of (pp) and we already know that one of those (p)’s came from Mom… so what about the OTHER one? It had to come from Dad, who has purple eyes. THEREFORE, his genotype must be (Pp).

FINALLY, we did ALL THAT WORK. Phew! But let’s check the question to make sure that we answered the question. I made that mistake a lot when I took genetics- so check to make sure that you’re done. There were two parts: genotype and probability. We know that Dad’s genotype is (Pp) and that his children are (Pp) and (pp). BUT we haven’t figured out the probabilities yet. Sorry.

It’s not that bad. Quit whining. Let’s just do this. It’s all down hill from here. Watch.

Mom: (pp)

Dad: (Pp)

If you didn’t get it that easily, check out my article on understanding Punnett Squares. But as you can see on the graph, 2/4 of the children are (Pp) and the other 2/4 are (pp). THEREFORE, we know that 0.5 of the children are (Pp) and the other 0.5 are (pp). And that makes sense seeing as 487+520 are 1007, and both numbers are just about half. NOW, we’re done.

Let’s recap the rules that we learned.

1. Name the alleles with the letter of the dominant. That way, it’s easier to remember which one is dominant.
2. The DOMINANT is capitalized and the recessive is not.
3. Use the genotypes of what’s given to find out the genotypes of what’s not.
4. If statistics are given, check the given data with the stats to make sure it makes sense.

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Best of Luck,

Grey