Punnet Squares

Today I’m going to be teaching you about punnet squares! Punnet squares are a simple way to find the odds of a person having certain traits such as eye color, nail shape, or genetic disorders. Of course, genetics is a really complicated subject, but punnet squares are a simple way to understand how traits are passed down through generations. Before we begin, here are some things to know:

  • Capital letters stand for a dominant trait, while lower case letters stand for recessive traits. Dominant traits override the recessive ones.
  • Alleles are what hold the certain traits. For a parent to pass a trait to their child, they actually pass down that allele (which is on a gene, which are segments of DNA, which are on chromosomes). Generally, each trait has two alleles.

This is Sharon and Mike. Sharon and Mike are expecting a baby, and they want to know what their child might look like.

Sharon has the following traits:

Black hair (B), Blue eyes (b), Freckles (F), Curly hair (C)

These are Mike’s Traits:

Red hair (r), Green eyes (g), Straight hair (s), No freckles (f)

Sharon’s traits are all dominant except for b (blue eyes), while all of Mike’s traits are recessive.

First, let’s look at the punnet square for hair texture.

This baby would have curly hair. This is because curly hair is dominant, and in every situation the baby has one curly hair allele, meaning curly hair is pretty much a definite. This baby’s alleles are heterozygous Cs.

Let’s move on to freckles.

In this situation, the baby does have freckles. This is because in every situation, Sharon gives the baby one dominant allele for freckles, and the dominant allele overpowers the recessive allele given by Mike. This baby’s alleles are heterozygous Ff.

Next is hair color.

You probably know the drill by now: Black hair is dominant, red hair is recessive, there are no situations where the baby has two recessive traits, therefore baby has black hair. This baby’s alleles are heterozygous Br.

Now for eye color!

This is where it gets fun! What do you do when the baby inherits two recessive traits from their parents? In this situation, the baby has a 100% chance of having the alleles gb (green, blue). Technically, these are both recessive traits. But, green eyes are actually dominant over blue eyes! So, even though green eyes are a recessive trait, the baby will still have green eyes. This baby’s alleles are heterozygous gb.

Now, last but not least, boy or girl? A baby’s sex is determined pretty much the same way other traits are determined. But instead of parents giving the child alleles, they each give the child a chromosome (specifically a sex determining chromosome).

Sharon is going to have two X chromosomes because she is a woman, and Mike is going to have one X and one Y because he is a man. Sharon is always going to give an X chromosome, while Mike could give either X or Y. If he gives an X the child will be a girl, and if he gives Y the child will be a boy. As you can see in the image above, the child has a 50/50 chance of being a girl or a boy.

So what does the child look like?

The baby has curly black hair, freckles, and green eyes.

He’s also a boy!

Now, what happens when the baby grows up and has a child?

His traits are pink and his wife’s are green.

And this, my friends, is how traits are passed down through generations.

Please ignore the fact that these babies have no bodies.

If you have any questions, feel free to comment and ask!

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