Things to Know About Genetic Variation

“Blah Blah Blah Natural Selection. Blah Blah Blah Evolution. Blah Blah Blah Mutation. Blah.”

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