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Punnett Squares

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Punnett Squares

What's the chance of the coin landing on heads?

There is always a 50-50 chance that a coin will land on heads. Half the time it will land on heads and half the time it will land on tails. These rules of probability also apply to genetics. If a parent has one dominant and one recessive factor for a trait, then half the time the dominant factor will be passed on, and half the time the recessive factor will be passed on.

Probability and Punnett Squares

A Punnett square is a special tool derived from the laws of probability. It is used to predict the offspring from a cross, or mating between two parents.

An example of a Punnett square ( Figure below ) shows the results of a cross between two purple flowers that each have one dominant factor and one recessive factor ( Bb ).

The Punnett square of a cross between two purple flowers (Bb)

The Punnett square of a cross between two purple flowers ( Bb ).

To create a Punnett square, perform the following steps:

  1. Take the factors from the first parent and place them at the top of the square ( B and b ).
  2. Take the factors from the second parent and line them up on the left side of the square ( B and b ).
  3. Pull the factors from the top into the boxes below.
  4. Pull the factors from the side into the boxes next to them.

The possible offspring are represented by the letters in the boxes, with one factor coming from each parent.

Results:

  • Top left box: BB , or Purple flowers
  • Top right box: Bb , or Purple flowers
  • Lower left box: Bb , or Purple flowers
  • Lower right box: bb , or White flowers

Only one of the plants out of the four, or 25% of the plants, has white flowers ( bb ). The other 75% have purple flowers ( BB , Bb ), because the purple factor ( B ) is the dominant factor. This shows that the color purple is the dominant trait in pea plants.

Now imagine you cross one of the white flowers ( bb ) with a purple flower that has both a dominant and recessive factor ( Bb ). The only possible gamete in the white flower is recessive ( b ), while the purple flower can have gametes with either dominant ( B ) or recessive ( b ).

Practice using a Punnett square with this cross (see Table below ).

The Punnett square of a white flower (bb) crossed with a purple flower (Bb)
b b
B Bb Bb
b bb bb

Did you find that 50% of the offspring will be purple, and 50% of the offspring will be white?

Vocabulary

  • dominant trait : Trait that will appear in the offspring if at least one of the parents contributes it.
  • Punnett square : Chart that is used to predict the outcome of a particular cross.

Summary

  • A Punnett square is a special tool used to predict the offspring from a cross, or mating between two parents.
  • In a Punnett square, the possible offspring are represented by the letters in the boxes, with one factor coming from each parent.

Practice

Use the resources below to answer the questions that follow.

  1. What is a hybrid?
  2. If you cross an Aa individual with another Aa individual, what will the genotype ratio be in the next generation? What will be the phenotype ratio?
  3. If you cross an AABb individual with an Aabb individul, what will the genotype ratio be in the next generation? What will be the phenotype ratio?
  4. If you cross an AAbb individual with an aabb individual, what will the genotype ratio be in the next generation? What will be the phenotype ratio?

Review

  1. In peas, yellow seeds ( Y ) are dominant over green seeds ( y ). If a yy plant is crossed with a YY plant, what ratio of plants in the offspring would you predict?
  2. In guinea pigs, smooth coat ( S ) is dominant over rough coat ( s ). If an SS guinea pig is crossed with an ss guinea pig, what ratio of guinea pigs in the offspring would you predict?

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