When I was a kid, I always wanted to be a detective and solve cases like The Three? Then, impressed by Dr. Hannibal Lecter’s ability to read other people like open books, a psychiatrist. Later I dreamed of being a profiler and in the middle of it a Shaolin monk for kung fu.

From all this the fascination has remained to find out something about a person out of simple visible information. External characteristics that reveal something about her or her relationship to other people. One of these features crosses my path again and again, as in the recent episode 7 “Means and Paths” of the first season of “Elementary“: The Eye Color.

In this episode, Sherlock excludes physical fatherhood because the child has blue eyes and his parents are brown.

So the question is: Is it possible to exclude paternity on the basis of the child’s eye colour?

Some genetics with Mendel

Before I answer this question, a little genetics, which most of us should know as Mendel’s laws from school, because we inherit half of our genetic information from one parent at a time. 50% come from our mother and 50% from our father.

This means that we have duplicate information.

In the case of the eye colour characteristic, we inherit the eye colour from the mother and the father. This leads to the question which eye colour wins. In other words:

Which characteristic dominates and is therefore visible?

In order to do this, you need to know the following. There is a dominant sequence for the eye colors:

Brown
Green
Blue
Grey

From top to bottom, dominance decreases and recession increases. If a child inherits the eye color blue from one parent and brown from the other, then the child will have brown eyes because brown is more dominant than blue (brown and blue = brown eyes).

However, there is something important to keep in mind.

In the example, the child inherits the information brown and blue. The parents have parents as well as the child and therefore have double information.

Now I can start answering the actual question.

Parents brown eyes and child blue

So what if both parents have brown eyes like Elementary? Can the child have blue eyes?

Yes, one parent must also have blue eyes and the other parent must have blue or grey, but no green, because green and blue make green eyes. Example:

Mother: brown and blue = brown eyes
Father: brown and grey = brown eyes

These possible combinations result for the child:

Brown(mother) and brown(father) = brown eyes
Brown(m) and grey(f) = brown eyes
Blue(m) and brown(f) = brown eyes
Blue(m) and grey(f) = blue eyes

It is therefore not possible to infer the child’s eye colour blue from the dominant eye colours of the parents, because the recessive characteristic (blue or grey) is not visible.

Therefore, it cannot be said that parents with brown eyes cannot have children with blue (or green or grey) eyes.

Child brown eyes and parents blue

Nevertheless, is there not a possibility of very probably excluding paternity for a child on the basis of eye colour?

This can be found out by anyone with a little logic or combinatorics.

So I thought for a moment.

If I have a dominant trait, I cannot deduce the recessive trait. But what I can do is to exclude the dominant from a more recessive trait!

Or put differently: If the child shows a dominant trait, one of the parents must also possess this dominant trait.

If the child has brown eyes, then one of the parents would have to have brown eyes (I write “must” here on the basis of “must”, in addition immediately more) (as a reminder: One cannot hide a dominant characteristic and sees it immediately).

This color row can be continued with the dominance order: If the child has green eyes, one of the parents must have at least green eyes. At least means green or brown, because besides the brown eye color, one of the parents could also have the green eye color. Thus the green “hides” behind the brown.

Why do I always “have to” write?

Every rule has the famous exception, for example if it is a rare mutation or disease. In the same way, a step in the complex production of melanin* can take a different course than intended. Then it can happen that the dominant characteristic does not appear in one parent.

So even in this case you cannot say with certainty that a child with brown eyes must have at least one parent with blue eyes.

Conclusion

The colour of the eyes can be used to draw conclusions about certain information, but they are not valid enough to conclude about the physical parents (in particular about the father) or, conversely, to exclude them. Therefore, no one should jump to conclusions. Because what works on television or in the cinema cannot simply be transferred into real life.

I would say, Mr. Holmes, you are wrong!

Too bad, that’s it with my detective / profiler / psychiatrist career. Maybe it will work out with the Shaolin monk. My hair tends to go there.

(Here’s another episode of Elementary in which Sherlock uses a number of external traits to conclude a probable paternity.)

photo by Jenna Hamra on Unsplash

* The Davenport model has its weaknesses. If a child has brown eyes but not parents, this does not necessarily mean that paternity can be doubted, because the process of making melanin is complex. It can happen that something fails during an (intermediate) step. Then one would not see the eye color brown, although the characteristic is present.

Remark: This is a translation with DeepL of my german article on fxhakan.info.