Punnett Squares are a simple tool for seeing how likely it is for a baby to inherit a specific trait from either a mother or father.
parents that are heterozygous for the purple/white color alleles.
From commons.wikimedia.org Punnett_square_mendel_flowers.svg, by Madeleine Price Ball.
It works not just for people, it works for all forms of life with two genders.
Punnett squares don’t work for most inherited traits – it only works for some traits.
Consider humans – do we have a food allergy? How do we build muscle? What is our skin color? How do our bodies process oxygen? None of these things are simple; none depend on just one gene inherited from either a mom or dad.
All of these parts of our biology are complex: they depend on many genes working together.
Punnett squares are only used to investigate traits when the genes for these traits are independent of each other.
What does that mean? Let’s use some simple visuals
Here is a (purple) cell. See the (pink) nucleus in the center? That’s where our genetic material is.
Our genetic material is our genes, which are wrapped up into chromosomes.
Let’s unfold these chromosome into a straight line. Here we see them, super simplified.
learn.genetics.utah.edu
When men and women combine a sperm and egg cell, they can create a baby – this baby inherits half of the genes from the mother, and half from the father.
During the process of making new eggs, or new sperm cells, the chromosomes duplicate, and then randomly swap pieces.
learn.genetics.utah.edu
You’ve probably done something like this – Ever shuffle a 52 deck of cards?
How many ways can you shuffle this one deck?
The math is wild, we won’t even attempt to touch that today
(but read here outside of class if you like)
Crazy Math: Every time you shuffle a deck of cards, chances are that you make history
But there are billions of billions of combos possible.
So what about eggs or sperm? There’s more than 52 genes, right?
So when our cells “cut the deck” and “shuffle” genes, how many combos can result? Trillions and trillions. That’s why no two sperm or egg cells are every exactly alike.
learn.genetics.utah.edu
THIS is what brings up back to “When can we use Punnett squares”
We can only use them for genes that are INDEPENDENT of each other.
That means that they aren’t physically stuck together.
This only happens when either (a) the genes are on the same chromosome, but far apart, or (b) they are on different chromosomes.
learn.genetics.utah.edu
Experiment in class – cut and shuffle a deck of cards.
Lay out the first 20 cards. Photograph. Now put those cards back in the deck.
Cut and shuffle a deck of cards, again. Lay out the first 20 cards. Photograph.
Repeat this, five times.
When you are done, compare the five results: In what ways are these similar? In what ways are these different? How does this result to the shuffling of the deck when our bodies produce new eggs or sperm?
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