Summary
A grid used to predict the possible genotypes and probabilities of offspring from a cross.
In depth
List the possible gametes of each parent along the top and side, then fill the grid to show the combinations. Each box has equal probability, so ratios give probabilities.
For larger crosses, the product rule (multiply independent probabilities) and the sum rule (add alternatives) are faster.
Reginald Punnett introduced the grid around 1905. For a dihybrid cross it needs a 4 by 4 grid, and for three genes an 8 by 8, which becomes cumbersome; in such cases the product rule (multiply independent probabilities) and the sum rule (add alternatives) are quicker.
For example, the chance that two carriers of a recessive condition have an affected child is 1/2 times 1/2, which is 1/4, and the chance of two affected children in a row is 1/16. Each birth is independent: earlier children do not change the odds.
Common mistake: treating the grid ratios as guaranteed results for a small family, rather than probabilities for each child.
Frequently asked questions
What is Punnett Squares?
A grid used to predict the possible genotypes and probabilities of offspring from a cross.
Which area of biology covers Punnett Squares, and how advanced is it?
Punnett Squares is one of the 54 topics in Genetics. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Dihybrid Cross, Pedigree Analysis, Law of Independent Assortment.
Can you give an example of Punnett Squares?
For a 3:1 ratio, the chance of a Tt x Tt offspring showing the recessive trait is 1/4.
What should I learn before Punnett Squares?
Punnett Squares is a Beginner topic, so no earlier topic is required. Other Beginner topics in Genetics that pair well with it include Heredity, Gene, Allele.
What topics are related to Punnett Squares?
The closest topics to Punnett Squares are Dihybrid Cross, Pedigree Analysis, Law of Independent Assortment, Test Cross. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Punnett Squares?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Punnett Squares. To practise the whole of Genetics, use the Genetics quiz.
How is Punnett Squares different from Dihybrid Cross?
Punnett Squares: A grid used to predict the possible genotypes and probabilities of offspring from a cross. Dihybrid Cross: A cross following two genes at once, giving a 9:3:3:1 phenotype ratio when genes assort independently. Read both side by side and note what each one explains.
What is a common mistake about Punnett Squares?
Treating the grid ratios as guaranteed results for a small family, rather than probabilities for each child.
How should I study Punnett Squares step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Dihybrid Cross and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.