5.11.2010

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.

3 comments:

Jess said...

Cool stuff, Ash.

jo said...

"Cue the punnett square." I love this. If I ever get to teaching ungrateful teenagers (just kidding) may I use this phrase?

You've got to love the punnett square. It's like your best friend at a party where you know no one else. It really is a relief to see it when nothing else makes sense.

Now on to multiple alleles! ha ha

Chap said...

Punnett Squares bring back such happy memories of dusty lecture halls at Berkeley (how clean BYU always looked in comparison!) Thank you for that walk down memory lane.