A Punnett square shows every way two parents’ alleles can combine. For one gene, each box is a 1 in 4 (25%) chance, and crossing two heterozygous parents gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio under complete dominance. Build squares here and check your ratios as fractions and percents.
How to fill a Punnett square
Write each parent’s genotype, for example Rr and Rr.
Put one parent’s two alleles across the top and the other parent’s down the side.
Fill each box with one allele from its column and one from its row. Write the dominant (capital) letter first.
Count the boxes: each of the 4 boxes is a 1/4 (25%) chance for one offspring.
Group the boxes by genotype (RR, Rr, rr) and by phenotype (round or wrinkled).
RR × rr: every offspring is Rr and shows the dominant trait; all are carriers.
RR × Rr: 1 RR : 1 Rr; every offspring shows the dominant trait.
RrYy × RrYy (two genes inherited independently): phenotypes 9:3:3:1 out of 16.
A probability is not a guarantee for each child or seed: each offspring is a new chance.
Key terms
Gene: A section of DNA that affects a trait, such as seed shape.
Allele: One version of a gene. R (round) and r (wrinkled) are two alleles.
Genotype: The pair of alleles an organism has, such as RR, Rr or rr.
Phenotype: The trait you can observe, such as round or wrinkled seeds.
Homozygous: Two matching alleles: RR (homozygous dominant) or rr (homozygous recessive).
Heterozygous: Two different alleles: Rr.
Carrier: An organism with one recessive allele that does not show the recessive trait, such as Rr.
Complete dominance: One dominant allele is enough to show the dominant trait. This lab assumes it.
About the model
The lab uses traits Mendel studied in garden peas, each modeled as one gene with complete dominance. Many real traits, especially in people, involve several genes or other inheritance patterns such as incomplete dominance and codominance.
Official GED Science facts (checked 2026-10-26)
The GED Science test takes 90 minutes, including 2 minutes of instructions and final review, with no break (GED.com test subjects page).
Science uses multiple choice and other question types: fill in the blank, drag and drop, select an area and drop down (GED.com).
The official Science topic areas are Reading for Meaning in Science, Designing and Interpreting Science Experiments, and Using Numbers and Graphics in Science (GED.com).
About 40 percent of the test is life science, 40 percent physical science and 20 percent Earth and space science (GED Assessment Guide for Educators, Science).
GED.com says you do not need to memorize formulas for Science; the information you need is provided.
The passing score is 145 in each subject, on a 100 to 200 scale (GED.com scores page).
What to do next
Work through the lab in Test style until you get each part right on the first check.
Read GEDPilot lesson GP-S03, Living Systems, Heredity, and Ecosystems, in the book or online in the library (/library/GP-S03, included with the book or a plan).
Take the free GEDPilot diagnostic (/tests) to see which subject needs you most.
Before you book Science, take GED Ready, the official practice test, and aim for the green Likely to Pass band (145 to 200).
You can schedule one subject at a time, so Science can be its own test day when you are ready.
What is a Punnett square? A grid that shows every way the alleles from two parents can combine. One parent’s alleles go across the top, the other parent’s down the side, and each box gets one allele from each. For one gene there are 4 boxes, so each box is a 1 in 4 chance.
What is the difference between genotype and phenotype? Genotype is the pair of alleles, such as Rr. Phenotype is the trait you can see, such as round seeds. Under complete dominance, RR and Rr have the same phenotype, so you cannot always tell the genotype by looking.
What do homozygous and heterozygous mean? Homozygous means two matching alleles: RR is homozygous dominant and rr is homozygous recessive. Heterozygous means two different alleles, Rr.
Why does crossing two heterozygous parents give a 3:1 ratio? Rr × Rr gives four equally likely boxes: RR, Rr, Rr and rr. That is a 1:2:1 genotype ratio. Three boxes have at least one R and show the dominant trait; one box (rr) shows the recessive trait. So the phenotype ratio is 3:1: a 3/4 (75%) chance of dominant and 1/4 (25%) of recessive.
If the chance is 1/4, will exactly one in four offspring show the trait? No. A probability describes what to expect over many offspring, not a guarantee for each one. Each offspring is a new 1 in 4 chance, so a family of four could have none or several with the recessive trait.
What is a carrier? A carrier has one recessive allele but shows the dominant trait, like an Rr plant with round seeds. Two carriers can have offspring with the recessive trait, which is how a trait can skip a generation.
What does a 9:3:3:1 ratio mean? It is the expected phenotype ratio when two parents heterozygous for two genes (RrYy × RrYy) are crossed and the genes are inherited independently. Out of 16 boxes: 9 show both dominant traits, 3 and 3 show one dominant and one recessive, and 1 shows both recessive traits.
Are Punnett squares on the GED Science test? They can be. The GED Science educator guide lists "Determine offspring ratios using a Punnett square" among its skills, and heredity is part of the life science content. Life science is about 40 percent of the test.
Independent preparation materials. Not affiliated with or endorsed by ACE or GED Testing Service. GED is a registered trademark owned by ACE. Practice results do not predict an official GED score, and GEDPilot does not guarantee a passing score.