That’s actually written in my notes. Looking through some of my old notes to write this article, I found that quote and it was too good to pass up. Thankfully, I’ve grown a bit since then, and I can better explain this stuff to you (but of course, with the same teenaged angst).

As far as vocab terms go, “genetic variation” is one of the most straightforward ones that you’ll run across. For those of you who are a little slower than others, it means “VARIATION in a population based on GENETIC differences.” Are we all together here again?

Here’s something to keep in mind: you can’t tell differences in genetic variation by simply looking at an individual. Sure, you can look at hair color or eye color, and you’ll probably already know that these differences are based on genetics. BUT not every trait is genetic. Extreme “buff-ness,” for example is a trait that almost all members of the bodybuilder population have. Unfortunately, that’s not based on genetics. THEREFORE, we have to make a difference between GENOTYPE and PHENOTYPE. If you don’t see how that ties in right at this moment, that’s ok… it’s just something to keep in mind.

Discrete Characters vs. Quantitative Characters

Let’s talk turkey. Let’s say, for instance, that there exist only three genes in turkeys: one for feather color, one for gender, and one for everything else. For now, we’re gonna ignore the “everything else” gene and hone in on the other two.

The result of the gender gene is what we’d call a DISCRETE CHARACTER. It’s only got a certain number of results: male or female. There’s not really much of any middle ground there, so it’s a discrete characteristic.

The result of the feather color gene would be understood as a QUANTITATIVE CHARACTER. Although it’s possible that in some species color is either black or white (in which case, it would be discrete), but in most species, colors are the result of a few pigments blending together to give a certain final result. Like the range of browns (from almost white to almost black) in human skin, quantitative characteristics are usually part of a gradient.

If you’re ever having trouble assessing whether a characteristic is discrete or quantitative, ask yourself “are there only a few possibilities for this trait or are there only a few?”

Here’s an example: Is foot size a discrete or quantitative characteristic? (technically this is a trick question, but just follow me here) Because human adults show foot sizes that range from super small to super large, it’s pretty obvious that there are more possibilities than black vs. white. So foot size would be a quantitative character.

Average Heterozygosity

This is going to be the shortest paragraph I’ve written thus far. Average heterozygosity is the percentage of genes in a genome that are heterozygous. Boom. That’s all. If a population of Granny Smith apples had 500 genes, with heterozygosity in around 150 genes, then it could be said that that population has an average heterozygosity of 30%.

It’s actually that simple. But here’s the catch: it can’t measure variation any smaller than whole genes. If there was a mutation in a gene that changed the sequence but NOT the final result, using average heterozygosity would completely overlook that.

Nucleotide Variability

That’s why they invented nucleotide variability. Here’s how it works. Pick one individual from a population; compare his/her genome sequence to that of another individual from the same population. Do this again… A WHOLE LOT. Record results. What this does is tell scientists how much variation there is in the genomes of the population.

For instance, in a population of iPhones, it was found that there are approximately 122 million base pairs in the genome. After comparing the sequences of numerous iPhones, it was observed that any two iPhones differ on average by about 61 million pairs. Therefore, the average nucleotide variability among iPhones in that population was around 50%.

Other Sources of Variation

Those are the big boys that you should really be familiar with, however there are a few others that you should at least recognize.

Geographic Variation – brought about by changes in location that force populations to change in isolation from one another.

Cline – form of geographic variation. However, this is characterized by a gradient between numerous locations. If the percentage of people with a gene for black hair were to increase gradually as you move from Los Angeles to New York, that would be referred to as a cline.

Mutation – The ultimate source of all new alleles. The introduction of new characteristics due to random chance.

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I really hope this helps someone. Be sure to comment.

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Best of Luck,

Grey

Red Blood Cells

As much as I hate doing this, I’m going to stick with the traditional example of codominance for this article. But I have only decided to do so because it’s such a great way to learn this concept. The ABO blood group is widely known and used, so if you don’t get it here, then there exist 9872934598723405973984572034570923465 other sites or individuals who can explain it to you. So, moving right along…

Here in America, our laws state that we don’t discriminate. You can’t have an advantage over another person simply because of your race, gender, religion, or anything. (Our citizens disagree on some of these, but that’s an article for a different webzine.) HOWEVER, in America, you’re pretty much not allowed to do anything legally until you’re 18 years old. You can’t buy a house, start a business, or even call that 1-800 number on the tv commercials.

Codominance works in much the same way. Codominant genes don’t descriminate. If they are the dominant form of the gene, then they are all on the same playing field, and no one gene is any more powerful than another… unless you’re the under-18 form of the gene. Then, you’re recessive to ALL of the codominant forms.

As for the point of multiple alleles, think of them as all the different races of adults in America. They all have the same abilities (legally), but they come in so many different flavors! Codominant genes are the same way. One gene is no more powerful than the other, unless one is the under-18 form, and there can be many different types!

Let’s get right into the example of the ABO blood group. In this system, there are three main alleles that decide what type of blood cell clothing that is produced. Remember that genes code for proteins, so the genes of the ABO blood group code for proteins. The thing to know, though, is that these proteins are presented on the outside of the cell, much like clothing for us. These three types are “make an A sock” (A), “make a B sock” (B), and “go barefoot” (o).

{If you’ll notice, both the “A sock” (A) and “B sock” (B) alleles are capitalized, and the “barefoot” (o) allele is not. This is codominance. In this case, we would say that both (A) and (B) are dominant to (o). }

Do you remember how to figure out the possible combinations? There are three possible alleles, but we can only choose two because there are two copies of each gene in the final cell. If you don’t know how to figure this out, then check out my post on permutations. To save time, I’ll just tell you that there are six combinations: (AA), (AB), (BB), (Ao), (Bo), and (oo).

So, lets do some practice. Suppose a type (AB) woman had a son with a type B man – the kid is a type A. What is dad’s genotype?

Step 1: Gather given info.

So, here’s what we know:

• Mom = (AB)
• Dad = (B?)
• Kid = (A?)

Step 2: Figure out genotypes

The kid already has one (A) allele, and we need to know what that other one is in order to figure out dad’s genotype. Since we know that the kid is type A, he only has 2 possibilities: (AA) and (Ao). We could use this information in two ways – we could either run two punnett squares, or we could use common sense.

That other allele, either (o) or (A), had to come from Dad… who we already know is a type B. Which of those two alleles could he have given the boy in order to still be type B? Since the (A) allele is codominant to the (B), possession of this allele would mean that dad is a type AB. Since we already know that he’s a type B, and that the (o) allele is recessive to the (B), then dad’s other allele must be the (o).

Step 3: Check answer against question

Whoops, we never properly answered the question, which asked us for dad’s genotype… not his other allele. So the formal answer to this question is: Dad’s genotype is (Bo).

… The OTHER half of ABO blood typing…

Yes, there’s more. I can’t leave you here without covering the Rh group component of the ABO blood system. You see, just as (A) and (B) and (o) code for the red blood cell’s socks, the Rh gene codes for the shoes. You may have head people talking about their blood types as AB positive or O negative. This positive (R) and negative (r) refer to the Rh gene… and any crosses that you perform concerning blood types will cover the ABO gene and the Rh gene, meaning blood type crosses are actually dihybrid crosses.

Let’s solve a problem so that we can all go home.

Jenny Anderson, Janie Anderson, and Josie Anderson (all unrelated) each go to the same veterinarian at the same time on the same day so that their Yorkies can have pups. Each has only one pup. The nurses lose track of which pup belongs to whom, since the babies happened to each look alike by some awful coincidence. You are the head vet, and it’s your job to figure out which pup belongs to which dog. You remember reading about the ABO/Rh blood groups when you were in introductory bio; although you don’t remember whether or not its the same in dogs, you decide to give it a try anyway. Your blood test results are below. Which pup belongs to which dog? (Assume that each parent is a heterozygote.)

• Jenny’s Female = O+
• Jenny’s Male = B+
• Janie’s Female = B-
• Janie’s Male = A-
• Josie’s Female = A+
• Josie’s Male = O-
• Pup 1 = O+
• Pup 2 = AB-
• Pup 3 = B-

Since step one has already been done for you, I guess it’s time to move on to step 2.

Step 2: Figure out genotypes

In this case, we can’t really do anything until we figure out exactly what types of children each couple can have. For this, we’ll employ the use of three punnett squares.

Now, let’s examine the results and the parent types. (Since the problem said to, we assumed that everyone that COULD be a heterozygote was a heterozygote.) Let’s start with the first pup, type O+. Since both Jenny and Josie have dogs that COULD have had O+ pups, we can’t really decide who’s dog it is yet. So let’s move on to pup 2, AB-. Janie was the only one of the three dogs that COULD have had AB- pups, so pup 2 must be hers. In the case of pup 3, only Jenny’s dogs could have had a pup with B- blood. So pup 3 must be Jenny’s. This only leaves pup 1 and Josie, so they must go together.

Good job, Doc.

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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