I get these kinds of questions all the time. And except for a few traits, I have to pretty much say I don’t know.
Part of the reason for this is that most traits have not been studied in very much detail. Which makes sense in a world of finite research dollars.
As is right, we spend most of the limited money on trying to understand and find cures for diseases. Very little goes toward figuring out the chances that Junior will have grandpa’s nose.
The other big reason we can’t make good predictions is that the genetics behind most traits are surprisingly complex. Except maybe red hair, there really aren’t any simple dominant-recessive traits. Which means we don’t even have the parents with recessive traits to fall back on!
As you probably already know, we have two copies of each of our genes – one from mom and one from dad. These genes can come in different versions and some versions are dominant and some recessive. The dominant ones trump the recessive ones.
What this means is that if you have a recessive trait, then both of your copies of the gene in question should be the recessive version. So if two parents each had a recessive trait, then their child should end up with a recessive trait too. Because that is all the parents have to share.
This is why we can predict that two red haired parents will pretty much always have red haired kids (J.K. Rowling got the Weasleys right!). None of the other traits out there are so simple though.
Numerous studies have shown that ear lobe attachment, cleft chin, tongue rolling and whatever else you learned in high school biology are way more complicated than advertised. None of them are a simple, single gene, dominant/recessive trait. Even eye color is complicated.
Pretty much everyone knows that blue eyes are recessive to brown eyes. And so blue-eyed parents can’t have a brown eyed-child. Heck, one group of scientists even used this as a basis for an evolutionary model. Too bad it is wrong.
Blue-eyed parents can have brown-eyed kids. It isn’t as common as the other way around but it can and does happen.
So it is hard to guess what a child will look like just by seeing the parents. But if we could look at their DNA we could predict what their child might look like, right? No. In fact, we couldn’t predict what they looked like from their DNA.
People are often surprised to learn that even if scientists are given a complete genetic readout of a person, they would not be able to predict what he or she looks like. We could guess ethnic background and have a good shot at predicting if they had red hair. And that is pretty much it. Oh, except for eye color.
Scientists are starting to make real progress on figuring out eye color from a person’s DNA. By looking at six different markers they can predict whether someone has blue or brown eyes around 90% of the time. They are only right about hazel, green and other colors about 75% of the time.
So you can see that even “simple” eye color takes six genes to make any good predictions. How can anyone make a good prediction just from looking at two brown eyed people? They can’t without a good family history and even then, they can be fooled.
And that is with a well-studied trait where we have the markers and understand what is going on. All bets are off for the other traits. For now anyway.