Cheat Sheet: Dominance

The term “dominant” refers to the relationship between the two versions of a gene (more accurately, alleles) we inherit from each parent for the same trait.

For example, we all have two alleles that determine thumb-shape, one from mom, one from dad.  As it happens, we only need one of these alleles to code for straight thumbs in order to be born with straight thumbs.  Therefore this trait is said to be dominant, and the alternative, curvy thumbs, is said to be “recessive.”

It’s common to abbreviate dominant traits with a capital letter, and recessive ones in lower case.  For example, S=straight thumbs, and  c=curvy thumbs.  In this example, the combinations SS or Sc would give a person straight thumbs.  Only cc would result in “hitchhiker’s thumb,” the recessive trait where the thumbs curve backward in the upright, “thumbs up” position.

Ben Stiller has hitchhiker’s thumb, so he must have “curvy” thumb-shape alleles from both parents (cc).

Note that Anne Meara and Jerry Stiller could actually have straight thumbs and yet be carriers who were capable of passing along the “c” trait to Ben, if the alleles they got from their parents were, in both cases, S and c.  In this scenario, Ben Stiller would have had a 1 in 4 chance of inheriting the cc combination from his parents.

The “father of genetics” Gregor Mendel was the first to describe dominance when his experiments with pea plants showed pretty consistently that recessive traits like short, green or wrinkled appeared 1 in 4 times among the offspring of cross-bred plants.

Calculate the eye color of your child-to-be

Genetics is a field that tends to give you guesses rather than answers.

This online eye color calculator won’t tell the future, but it will show you more or less your odds of having a brown, green, or blue-eyed baby.

And that could be fun, whether you’re prognosticating about a baby in utero or fantasizing about babies you could have with whatever sexypants you have in mind.

Several genes contribute to eye color, and the factors that decide the trait are so complex that almost any parent / child combination of eye colors is possible.

Using this calculator, which uses a two-gene model, I learn that my partner and I would have a 1 in 4 chance of producing a green-peepered creature.  I also see some of the genetic factors at play.

For example, I learn that while I have brown eyes like my dad, I still possess genes for the green and blue eyes that my mom and her ancestors have had.

My eye color is my phenotype (i.e., brown eyes.)

My grandparents’ eye colors determine my genotype, or the combination of genes I carry.  My genotype BBGb, for example, describes me as a brown-eyed person (B) who is a genetic carrier of green (G) and blue (b) eye color traits.  When it comes to the expression of the trait, green trumps blue, and brown typically trumps all. (B for brown is written in upper case because it is “dominant” while b for blue is in lower case because it is “recessive.”)

The online calculator accounts for two of the known eye color genes: EYCL1, which is located on chromosome 19, and EYCL3, which is located on chromosome 15.  Using the Hardy-Weinberg principle, this tool computes your probability of having a brown, green, or blue-eyed baby.

The calculator was created by the San Jose, California-based Tech Museum of Innovation.