How do genes affect traits? It’s just like “50 Shades of Grey”

In “50 Shades of Grey”, there isn’t any mention of molecular biology or genetics, but maybe there should be, because it’s a great example of how to explain how genes affect traits. And yes, I’m talking about dominance. Don’t get too excited – there won’t be any photos or explicit descriptions of sexy time (aka business time, if you’re  Flight of the Conchords fan).  We’re going to be talking about dominant and recessive genes.

You have two copies of each gene – one on each chromosome – and these are called alleles.  One allele is from your mother and one from your father, and these genes can be slightly different.  In some cases, the gene is dominant, which means that the variant of the trait that it is responsible for will take over – this would be represented by the dominatrix or Mr Grey.  Other genes are considered recessive – or the submissive, Anastasia Steele.  Christian Gray is the boss and what he says goes.  Only when the dominant person isn’t around does the submissive get to do what it wants.
dominant_and_recessiveAnother way to think of this is if you have a bully.  The dominant bully will take over whenever any other kids are around.  Only when the bully isn’t there, can the other kids do what they want.

What does this mean in genetics?  First, let’s get some terminology out of the way.  The dominant gene is usually represented in capital letters (for simplicity let’s call it gene “A”) and the recessive gene is in lowercase “a”.  Whenever the dominant trait, A, is present on one chromosome, that trait will be visible.  So if you have two As (AA) or one dominant and one recessive gene (Aa), you’ll only see the dominant trait.  Only when you have two recessive genes (aa) will you see the recessive trait.

punnetsquareLet’s give an example using eye color.  Let’s say that the dominant gene trait (A) is from brown eyes and the recessive trait (a) is for blue eyes.  With AA or Aa, you will have brown eyes, and with aa you will have blue eyes.  If you look at the image on the right, you may be having nightmares back to high school biology.  All this is representing are the possibilities of what trait a child might have depending on the parents genes.  If each of the parents has one copy of A and one copy of a, the child will randomly get either the A allele or the a allele from each parent. The boxes represent those possibilities.  From there you can ask and answer some really interesting questions – what’s the likelihood that with two Aa parents that the child will have blue eyes?  What do you think?  The answer is here.

What we have just discussed is what’s called “Mendelian Genetics” or “Mendelian Inheritance” named after the Monk Gregor Mendel who in the 1800s discovered these rules with dominant and recessive genes in pea plants.  What’s most interesting for us, besides being able to understand traits like eye color, is that over 4,000 diseases can be attributed to a single gene, either because it was inherited or by chance.

 

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