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.


What is heredity?

All the time, people say that I look more like my Dad and my sister looks more like my Mom.  My sister inherited by mom’s blue eyes and light hair, and I inherited the green eyes and brown hair from my Dad’s side of the family.  When we talk about heredity, we often discuss it in terms of traits – things you can see like eye color to whether or not you can roll your tongue.  But really anything that you inherit, like personality traits or risk of disease, all are part of heredity.

Before we go any further, let’s get some confusing vocab out of the way:

  • Hereditary: Cause is genetic and has the possibility of being passed down to children
  • Familial: Multiple people within a family have the disease but the cause may be due to shared environmental factors. For example, everyone in the family may be obese, but it’s because they all have the same die.  Or multiple people in the family develop lung cancer, but it is because they all smoke
  • Congenital: Caused by something in utero (while pregnant).  This may or may not be inherited
  • Sporadic: Occurs by chance


For now we’ll focus on heredity, but we can come back to these other terms later.

So HOW do people inherit  traits? Through their genes! Quick quiz (and this is related to the question, I promise).  How many pairs of chromosomes humans have? 23 pairs.  Now why in the world am I highlighting the word “pairs”? If you have two copies of each chromosome (which you do – hence, the word “pairs”) then you have two copies of each gene (because genes are located on the chromosomes).  Where do these two chromosomes come from?  One of the sets of chromosomes comes from your mother (from the egg) and the other set comes from your father (from the sperm). So when you inherit traits, it’s very literally because the you get half of your DNA from one parent and the other half from the other parent.

You may be wondering how even though the chromosomes are the “same” that the genes on each chromosome are different that they can inherit such a huge variety of traits.  Even though 99.9% of our DNA is the same compared to any other person you’re sitting next, there is 0.1% that is different.  This may not seem like a lot, but with 3 billion bases in the human genome, that means that 3 million of those may be different (how this happens is the topic of another blog post).  These small differences change individual genes slightly, and as we know, that changes the protein the gene makes as well.  So the gene from your mom is slightly different from the gene from your dad.  The technical word for these two genes that are the same but slightly different, are called alleles. A great example of this is blood groups.  If you are blood group AB, you have one A allele and one B allele.  Gene alleles are responsible for our amazing differences and our ability to inherit traits from our parents!


Moxie with his corgi and lab traits

Moxie with his corgi and lab traits

Keep in mind, alleles and heredity isn’t just in humans.  Plants inherit traits because of gene alleles.  Dogs inherit traits based on different alleles – my dog Moxie is a corgi black lab mix and has traits of both (black body and short legs).  The majority of organisms (those with two copies of each chromosome) have heredity based on gene alleles from the parents.