Friday, January 22, 2010

The Genetic Milkshake

One of the things that I find interesting is the amount of genetic material each of us share with our relatives. In most cases the numbers are averages; however, they are absolute in the case of three relationships. We only share 50% of our genetic material with each parent. No more and no less. This is because half of our genetic material comes from our mother and half comes from our father. Our children will share half of our DNA – no more, no less. Lastly, identical twins share 100% of their DNA, as they are from the same fertilized egg that split into two.

Outside of these relationships, the amount of DNA we share with our other relatives will vary. This is because our DNA is a unique code that is partially from our mother and partially from our father. Since each sperm and each egg is genetically unique carrying half of the genetic code from each parent, the ultimate combination is like a genetic milkshake – unique at the moment to the ingredients that are available.
Likewise, our siblings will have a unique genetic code. This is why in one family you will have differing heights, eye color, hair color, blood types, and other physical characteristics. Let’s consider one of these – eye color. Geneticists tell us that there are two genes that control eye color. It is the combination of these genes that produce the actual hue of our eyes.

The first gene (the “bey2” gene) has two forms – one that produces either a great deal of melanin (B) or very little melanin (b). The second gene (the “gey” gene) can produce some melanin (G) or very little melanin (b). If you notice here, it becomes apparent that there are three basic colors that emerge – Brown, Green, and blue. Every other color is determined by the combination of the genes. At the present, this has not fully been explored. One real mystery is how hazel eyes are produced.

So if a person has the following genetic parameters, his or her eye color will be determined as follows:

bey2 Genegey GeneEye Color
BBGG    Brown Eyes
BBGb    Brown Eyes
BBbb    Brown Eyes
BbGG    Brown Eyes
BbGb    Brown Eyes
Bbbb    Brown Eyes
bbGG    Green Eyes
bbGb    Green Eyes
bbbb    blue eyes

For example, gray and violet eyes are just hues of blue eye color. Geneticists really are not sure how eye hue is determined, as the great variance in eye color needs to be studied further.

By using the basics, we can come up with the following rules: Brown trumps everything; green trumps blue; and blue is recessive – it trumps nothing. A brown eyed person (because dominant genes win) may also carry green and/or blue eye genetic material. A green eyed person can carry blue eyed genetic markers. This is how my mother and dad, who both had brown eyes, were able to produce three children with differing eye color: one brown eyed child, one gray eyed child, and one blue eyed child.

If I look back to my grandparents, the eye color mystery begins to make sense. I had one brown eyed grandfather, a hazel eyed grandfather, a blue eyed grandmother, and a gray eyed grandmother. My brother with the gray eyes really has a form of blue eyes and there is something passed from my gray eyed grandmother that gives him that particular hue. While I have blue eyes, they are not as blue as others I know and this is probably influenced by my maternal grandmother’s gray eyes as well.

Just as three siblings can carry different characteristics, the degree of genetic material we share with our siblings can vary greatly. When we consider the possibilities because all humans have 23 pairs of chromosomes, it is estimated that over 10 trillion different combinations could occur.

While siblings are generally considered sharing 50% of our genetic material – it is possible that they could share 100% of the same DNA as us (highly unlikely – unless they are identical twins) and it is possible that two siblings could share 0% of the same genetic material (again, highly unlikely). The average is 50% - though it may be 65% shared with one sibling, only 30% shared with another, and 42% with a third. Just like our mileage, it will vary.

More on degrees of relationship and how this relates to genetics next time.


  1. Very interesting post. I find eye color very interesting...I actually made a custom section for it in my Family Tree Maker, there is now a spot for "Eye color" after Birth, Death, and Marriage. Both of my grandmothers had brown eyes while both of my grandfathers had blue eyes. My mom had hazel eyes while my dad had brown sister and I both have brown eyes, but neither of us as dark as my dad's eyes. It is interesting how it all works out.

  2. Thanks Andrea - it is interesting that from so few markers we have such a wide variety of hues and shades.