Punnett squares

What is a Punnett square?  That is what we had to figure out in our last two labs.  In my last post I talked about alleles and how they affect genetic traits.  Well it turns out that alleles are represented or written as capital and lowercase letters.  For example, if a trait determined whether a plant was tall or short, we could call that trait H for height.  If the allele was dominant we would write it as H, if it was recessive we would write it as h.  

The two alleles put together determine a trait's genotype.  For example: If the trait for a plant being tall was dominant it would be represented as H,  that means that the short gene is recessive or h.  That means that if a plant was short it's genotype would be hh, remember that both alleles have to be recessive for the organism to have the recessive trait.  If the plant was tall it's genotype would be either HH or Hh.

  Remember that if only one of the alleles is dominant the organism has the dominant trait.

So what does this have to do with punnett squares?  Well, a punnett square is a way that we can use two organism's genotypes to determine the chances their offspring will have certain traits.  The way this works is by creating a punnett square.  This is what a blank punnett square looks like.

it looks like... a square.  but it is really very helpful.  So, in our example let's say a tall plant "Hh" breed's with a short plant "hh".  We put those genotypes on the sides of the square like so.

After that we just cross the alleles and fill in the blanks.

From this we can determine that 50% of the offspring would be tall "Hh" and 50% would be short "hh".  This does not mean that exactly half of the offspring will be tall and the other half will be short.  This only shows that there is a 50% chance of the offspring being tall.  Just like if you were to flip a quarter twice you would not necessarily get one head and one tail, it's the same thing with this.  Hopefully I explained punnett squares well and did not bore you to death.