A Beginners Guide to Genetics: Eye Color
Genetics is a fascinating field of study, and one hub is by no means going to cover even a fraction of the knowledge, work and debate on the subject.
However, most of us will have some curiosity about how we got Grandpa's nose or Aunt Mary's temper, and this hub looks at the heredity of eye color in particular.
For the sake of simplicity, I have assumed there are only two shades of eye colour, Blue and Brown. There are, obviously, many different hues, but the principle is best illustrated using two differentials, with the assumption that any other eye color (green, hazel etc) is just a variant of either Blue or Brown eyes.
DNA and Heredity
On a simple level our DNA (Deoxyribonucleic Acid) is the recipe for how we are made. When we were simple fertilised embryos, this recipe was used to tell the cells how to grow and form and determines many factors, such as height, intelligence, sex and, or course, eye color.
Our genes are made up of two alleles that hold genetic information. We each receive one allele from each parent to make up our own individual set of two. The combined instructions in both alleles will determine the end result of whichever genetic trait is under examination.
For our illustration, the allele for Brown eyes will be BR and the allele for Blue Eyes will be bl. The capitalization is intended, and is explained below.
Dominant and Recessive Genes
So, each person has two alleles to determine eye color. Its is relatively simple to deduce that a person whose alleles are BR BR will have brown eyes, and that someone who has bl bl alleles will have blue eyes. But what happens with mixed alleles?
In many circumstances, one type of allele is dominant and another is recessive. For eye color, the brown eye allele is dominant BR (hence the capitals) and the blue eye allele is recessive bl. This means that someone who has alleles of BR bl or bl BR will necessarily have brown eyes, as the dominant trait takes precedence.
So what does this mean?
Well, consider the case of a test subject. We'll call him Ira. Ira has brown eyes. Ira's father also has brown eyes, but his mother has blue eyes. The potential genetic make up of Ira's family's alleles are as follows:
Because Ira's mother has blue eyes, we can be sure her alleles are both bl as this is the recessive gene.
Ira's father must have one BR allele, but the second could be either BR or bl.
Ira could potentially have the alleles bl BR or bl bl, depending on his father's second gene. However, as Ira has brown eyes, his alleles must be bl BR.
For eye color, whether Ira's father has two BR genes or BR bl genes is possibly interesting, but not necessarily important. However, for some genetic diseases or abnormalities, it can be important to determine exactly what the combination of genes is.
In most cases, DNA typing can easily solve the mystery. However, in our simplified example, some circumstancial evidence can help us solve the mystery of Ira's father's genes.
If Ira has a sibling with blue eyes, that sibling must have the alleles bl bl, meaning Ira's father must also have passed a bl gene to that child, making his genes BR bl.
So what about Ira's Children?
Because Ira has mixed alleles, BR bl, no matter what color eyes his wife has, his children have the potential to have either blue or brown eyes. If Ira's brown eyes were BR BR, no matter what color his wife's eyes are, his dominant BR gene would mean all his children would have brown eyes. Similarly, two blue eyed parents cannot produce a brown eyed child, as there would be no dominant BR gene to pass on.