How to play Punnett Squares
Punnett Squares is a free GCSE biology game about genetic crosses. A Punnett square is a grid that shows every way two parents can pass on their alleles. Each question shows a cross between two parents and a square with one box to fill, marked with a question mark. You pick the pair of alleles that meets in that box. Each answer names the allele from each parent and the result, such as Aa. Tap an answer, or press its number key on a keyboard.
A game has ten questions. The first four use a cross where one parent is AA and the other is Aa. The next four use a cross between Aa and aa. The last two show a full square and ask for the ratio of offspring with the dominant trait to offspring with the recessive trait.
Tick the harder start for a tougher game. It starts with the Aa and aa cross and moves on to two Aa parents. In both starts the other boxes stay hidden, so you work out each one from the parents.
You score 10 points for each right answer. A 35-second clock runs on every question, and running out of time counts as a miss. Practice mode turns the clock off. The game saves your best score on this device, with its own best for the harder start.
The science behind Punnett squares
Some characteristics are controlled by a single gene. AQA gives fur colour in mice and red-green colour blindness in humans as examples. Each gene can come in different forms, called alleles. During sexual reproduction, each offspring gets two alleles of each gene, one from each parent's gamete. Gametes are cells with half the usual number of chromosomes, and two of them join to make the offspring.
The alleles an organism has are its genotype. The characteristic you can see is its phenotype. A dominant allele always shows, even when there is only one copy. A recessive allele shows only when there are two copies, with no dominant allele. If both alleles are the same, the organism is homozygous. If they are different, it is heterozygous. In the game, a capital A stands for the dominant allele and a small a for the recessive one.
A Punnett square puts one parent's alleles down the side and the other parent's along the top. Each box joins one allele from each parent. The four boxes show the chances for one offspring. So a 3 : 1 ratio means each offspring has a 3 in 4 chance of the dominant trait. AQA reminds you that most characteristics come from many genes working together.
AQA covers genetic inheritance in section 4.6.1.6 of Biology 8461 and 4.6.1.4 of Combined Science 8464. Every student should be able to complete a Punnett square. In AQA, building a whole cross from scratch is Higher Tier only. Edexcel 1BI0 and 1SC0 cover the key terms in 3.13, and Punnett squares and family pedigrees in 3.14. OCR J247 has single gene crosses in B5.1i and B5.1j. OCR J257 covers them in B1.2, points 2 and 3.
Tips for Punnett square questions
- Read across and down. The box takes the allele at the start of its row and the allele at the top of its column.
- One allele from each parent, every time. An answer with two alleles from the same parent is always wrong.
- Write the capital letter first, so a box reads Aa and never aA.
- To find the ratio, count the boxes with at least one capital A. Those show the dominant trait. The rest show the recessive trait.
- Two Aa parents give 3 : 1. An Aa parent and an aa parent give 1 : 1.
With the harder start, the square begins empty. Each answer fills in a box, so the full cross builds up as you play.
Use this in the classroom
Punnett Squares suits Years 9 to 11 during the inheritance topic. A game runs for about five minutes, so it fits at the start or end of a lesson.
Show the first cross on the board and ask students to draw the full square in their books before anyone answers. Then play the four box questions and check each box as a class. For the ratio questions, ask students to explain the ratio as a chance for one offspring. OCR suggests a coin toss or a dice roll to explore probability, and the game makes a good follow-up to that.
The harder start suits AQA Higher Tier groups, since those students must build a cross from scratch. After the game, give them a family tree and ask them to work out the parents' genotypes.
No account or sign-up is needed. The practice switch gives slower students all the time they need. See our teachers page for more ways to use the games.
Similar games
A gene is a section of DNA. Cell Builder shows that the nucleus holds the genetic material in animal and plant cells. In a bacterium, the DNA is a single loop outside any nucleus, as our bacterial cell page explains. For more biology, label the chambers of the heart in Heart and Circulation, or browse the biology games page.
Questions about Punnett squares
Common questions from students and teachers about Punnett squares, with short answers.
How do you fill in a Punnett square?
Write one parent's two alleles down the side and the other parent's two alleles along the top. Each box then takes the allele from its row and the allele from its column. The four boxes show the possible genotypes of the offspring.
What ratio do two heterozygous parents give?
Two Aa parents give AA, Aa, Aa and aa. Three of the four boxes have a dominant allele, so the ratio of dominant to recessive offspring is 3 : 1.
Is the Punnett square Higher Tier in AQA GCSE biology?
Completing a Punnett square is for every student. Building a genetic cross from scratch and using it to predict results is Higher Tier only in AQA Biology and Combined Science.
What is the difference between genotype and phenotype?
The genotype is the pair of alleles an organism has, such as Aa. The phenotype is the characteristic those alleles produce, such as fur colour in mice.