bp's science: genetics with Punnett Squares (v.1)

Last week we discussed four leaf clovers and their possible causes. One theory suggested that a recessive gene causes the fourth leaf. Well today, I'm going to talk genes and I'm going to do it with a few Punnett Squares. A Punnett Square is a chart that shows genetic variations of possible offspring that a mother and a father could create based on their genetic make-up.

First, let's take the four leaf clover example in the simplest terms. Say for example the father clover has four leaves, and since it is believed that four leaves are caused by a recessive gene (or a gene that only exhibits itself in the absence of a dominant gene), it's genetic make-up is cc (two little c's to represent recessive genes). So why two cc's? Well, offspring get genes from two parents, the dad gives a leaf gene and the mom gives one, so the offspring has two leaf genes. Now the mother clover has three leaves, but she is a carrier of the recessive gene, so her genetic make-up is Cc (the big C is the dominant three-leaf gene and the little c is the recessive four-leaf gene). When this father and mother get together, their offspring have a 50/50 chance of having four-leaf offspring. How do I know that? Cue the Punnett Square.

There are four combinations that can come from the Cc x cc monohybrid cross. These are Cc, Cc, cc, cc. Out of the four combinations there are two distinct differenct combos (Cc and cc), so there is a 50/50 chance of creating either a three- or four-leaf clover. The Cc combination is going to create a three-leaf clover while the cc combination creates a four-leaf clover.

Now let's relate some clovers to us. The first example, the 50/50 chance, is the same as us humans having a boy or a girl. The father has XY chromosomes and can either give an X or a Y, while a mother has XX chromosomes and can give an X or an X. Place these in a Punnett Square and you see your 50/50 chance.

And that's the Punnett Square in the simplest of terms. Genetics is cool. This is just the beginning. Read on for more Punnett Square combinations if desired.

If we have a father and a mother with three leaves each and a genetic make-up of Cc each, we'd get three different combinations (CC, Cc, cc) and a 25% chance of a four-leaf clover.
If we did a father and a mother with three leaves each and the father with a CC make-up and a mother with a Cc make-up, there is a 0% chance of a four-leaf clover, because there's only one combination Cc, and the dominant gene will always repress the recessive gene.