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 percent of our DNA is the same compared to any other person you're sitting next to, there is 0.1 percent 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.

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.

Dr. Cathy Seiler is the Program Manager for the tissue biorepository at St. Joseph's Hospital and Barrow Neurological Institute. She has her BA in Biochemistry and Molecular Biology from Boston University and PhD in the Biological Sciences from Cold Spring Harbor Laboratory. Her research and teaching focuses on genetics, cancer, and personalized medicine. Find her on Facebook at 

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