Academics · Study Skills · Middle and high school science

Cross two parents.
Count what grows.

Pick the parents and the page draws the square and the ratios. Then grow ten, a hundred or a thousand offspring at random and see how close chance comes to the prediction, next to the counts Mendel published.

Open the Punnett squareWhat you get
6kinds of cross, from one gene to two
150crosses, every one checked
7,324seeds Mendel counted for one cross

The Punnett square

Choose a kind of cross, then tap a genotype for each parent. Tick Fill it in myself to complete the boxes before the page shows them.

Loading the Punnett square. If nothing appears, turn on JavaScript.

One square, four patterns

The grid is always built the same way. What changes is how a genotype shows.

3 : 1

Complete dominance

Rr × Rr peas: three round for every wrinkled. The heterozygote looks like the dominant parent.

1 : 2 : 1

Incomplete dominance

Pink × pink snapdragons: one red, two pink, one white. The heterozygote is in between, so every genotype can be seen.

1 : 2 : 1

Codominance

Roan × roan cattle: one red, two roan, one white. The heterozygote shows both, red hairs and white hairs together.

by sex

X-linked

A carrier mother and a red-eyed father: every daughter red-eyed, half the sons white-eyed. Sons have only one X.

Dominant does not mean common, strong or better: it only describes which allele shows in a heterozygote. And most traits people ask about, such as height or eye color in humans, depend on many genes and on the environment, so a one-gene square does not predict them.

What you get

Academics · Study SkillsFree

What you finish with
A finished Punnett square for any of 150 crosses with its genotype and phenotype ratios, a square you completed yourself and had checked, a count of randomly grown offspring set against the prediction, a first-try score on eight practice questions and a record of up to twelve crosses. A printed lab sheet and a spreadsheet.
Who it is for
Middle school and high school students learning heredity; biology teachers who want a quick model for class or a lab on chance; homeschool families; anyone reviewing genetics before a test.
Format
Browser tool with three parts: The square, Practice and My record. Works on a phone, tablet or computer, by touch or keyboard. Prints the current square with the record; saves the record as a .csv.
Access
Free. No account and no sign-in. Crosses, counts and practice results are saved only in this browser; no names are asked for and nothing is sent to Caveat.
Where it lives
The Study Skills space in Caveat Academics.
Support
Questions, a correction or a request: team@caveatacademic.com.
Two genes at once. The Two genes cross draws all sixteen boxes and sorts them into the four kinds of seed.

How to use it

  1. Pick a kind of cross. For One gene, also pick the pea trait.
  2. Tap a genotype for each parent and read the square, the ratio and the chances.
  3. Tick Fill it in myself to complete the boxes first, then press Check my square.
  4. Grow 10, 100 and 1,000 offspring and compare each count with the prediction. Add the cross to your record and print it.

Open the Punnett square

How the simulator stays right

Every cross enumerated twice, and chance kept separate from prediction.

How the square is built and counted

  • Gametes first. Each parent passes on one allele of each gene, so a parent with two different alleles makes two kinds of gamete, and a two-gene parent makes up to four.
  • Every box joins one gamete from the top with one from the side. The page writes the dominant letter first, as textbooks do.
  • Each box is equally likely, so a phenotype that fills 3 of 4 boxes has a chance of 3 in 4 for each offspring.
  • Grow offspring picks a box at random for each one. Small samples wander; large ones settle near the expected share. That is the law of large numbers, and it is why Mendel counted thousands of seeds.
  • All 150 crosses the page can set up were enumerated by a separate reference before the page was built, and the page’s tests compare every one.

Mendel’s counts, for comparison

In the second generation Mendel counted 5,474 round and 1,850 wrinkled seeds (2.96 to 1), 787 tall and 277 short plants (2.84 to 1), and 705 purple-flowered and 224 white-flowered plants (3.15 to 1).

For seed shape and color together he counted 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green: 556 seeds, close to 9 : 3 : 3 : 1.

For a teacher. Have students fill in a square with Fill it in myself, predict the count for 100 offspring, then grow them and explain the difference. The record prints as a lab sheet.

Sources

  1. Gregor Mendel, “Versuche über Pflanzen-Hybriden,” Verhandlungen des naturforschenden Vereines in Brünn 4 (1866), 3–47: the pea crosses and the counts quoted on this page.
  2. Reginald C. Punnett, Mendelism (1905 and later editions), where the square diagram that carries his name was introduced.
  3. Thomas Hunt Morgan, “Sex limited inheritance in Drosophila,” Science 32 (1910), 120–122: white eyes in fruit flies and inheritance on the X chromosome. Felix Bernstein’s three-allele account of ABO blood groups (1924).
  4. Next Generation Science Standards, MS-LS3-2, which names Punnett squares among the models for how genes pass from parents to offspring, and HS-LS3-3 on applying probability to the distribution of traits.
  5. All crosses are enumerated by a reference written for this page (guide/suite/pun_ref.py) and compared with the page’s own results by its tests.

A teaching model. Real inheritance includes linked genes, many-gene traits, new mutations and environmental effects that a Punnett square leaves out. The blood type cross uses the simple three-allele model and cannot be used to draw conclusions about a real family. Practice with the ideas, not a full genetics course. Support: team@caveatacademic.com.