Written and maintained by CASRAI Editorial Board
Last updated
Short answer
A dihybrid cross tracks the inheritance of two traits at once, using parents that are heterozygous at two separate gene loci (for example RrYy × RrYy). When the two loci assort independently and each shows complete dominance, the offspring fall into four phenotype classes in a 9:3:3:1 ratio, shown by a 16-square Punnett square or by multiplying two 3:1 ratios.
What defines a dihybrid cross?
A dihybrid cross follows two traits simultaneously in the same individuals. Mendel used pea plants to follow the segregation of two traits at the same time in the same individual, and the defining feature is the parent: a dihybrid carries two alleles at each of two loci, written RrYy when R/r controls seed shape and Y/y controls seed colour. Mendel’s second law, independent assortment, is the idea this cross demonstrates. Genetics is the broader field; see what is genetics for scope and genetics vs genomics for how the two fields differ.
The classic pea example
The standard worked example crosses a pure-breeding yellow, round plant (RRYY) with a pure-breeding green, wrinkled plant (rryy). Every F1 plant is yellow and round, and every F1 plant is a dihybrid (RrYy). Crossing two F1 plants (RrYy × RrYy) produces the F2 generation, which shows four phenotypic classes in a 9:3:3:1 ratio.
What is a monohybrid cross?
A monohybrid cross follows one trait at one locus, using parents heterozygous at that locus (Rr × Rr). It gives the classical 3:1 phenotypic ratio among the progeny. A dihybrid cross is two monohybrid crosses run together, which is why the monohybrid ratio reappears inside the dihybrid result.
Monohybrid vs dihybrid cross
The two crosses differ in what the heterozygous parent carries and in how large the Punnett square becomes. The rows follow from counting gametes and squares, and the two ratios are the standard textbook results.
| Feature | Monohybrid cross (Rr × Rr) | Dihybrid cross (RrYy × RrYy) |
|---|---|---|
| Loci followed | 1 | 2 |
| Gamete types per parent | 2 (R, r) | 4 (RY, Ry, rY, ry) |
| Punnett square size | 2 × 2 = 4 squares | 4 × 4 = 16 squares |
| Phenotype classes | 2 | 4 |
| Genotype classes | 3 | 9 |
| Phenotypic ratio | 3:1 | 9:3:3:1 |
How to do a dihybrid cross: two equivalent methods
Method 1: the product rule
The probability of two or more independent events occurring together is the product of their individual probabilities. Each locus on its own gives a 3:1 split, so the four combined classes are products of those fractions.
| Phenotype | Shape fraction | Colour fraction | Product |
|---|---|---|---|
| Round, yellow | 3/4 | 3/4 | 9/16 |
| Round, green | 3/4 | 1/4 | 3/16 |
| Wrinkled, yellow | 1/4 | 3/4 | 3/16 |
| Wrinkled, green | 1/4 | 1/4 | 1/16 |
Method 2: the 16-square Punnett square
List the four gamete types of one parent (RY, Ry, rY, ry) down the side and the same four across the top. Each of the 16 squares combines one gamete from each parent. Counting the squares gives the same 9:3:3:1 result as the product rule.
How to determine genotype from a Punnett square
Read each square as the pair of alleles at each locus: the cell with gametes RY and ry is RrYy. Use an underscore to stand for “any allele” when only the phenotype is fixed. The four phenotype classes are then written R_Y_ (round yellow), R_yy (round green), rrY_ (wrinkled yellow) and rryy (wrinkled green, 1 of 16).
Counting squares by genotype shows that the 16 squares fall into nine genotype classes. In the order RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rrYY, rrYy, rryy, they occur in the ratio 1:2:1:2:4:2:1:2:1. Adding the classes inside each phenotype group (1+2+2+4 = 9 for R_Y_, 1+2 = 3 for R_yy, 1+2 = 3 for rrY_, and 1 for rryy) returns the 9:3:3:1 phenotypic ratio.
Are all dihybrid crosses 9:3:3:1?
No. The 9:3:3:1 ratio is the expected result only when specific conditions hold. Use this checklist before applying it.
When to expect 9:3:3:1
- The two loci segregate independently of each other.
- Each locus shows complete dominance.
- The phenotypes are distinguishable, with no interaction between the two genes.
Common reasons the ratio is distorted
- Linkage: loci close together on the same chromosome do not assort independently. It is described as one of the most important reasons for distortion of the ratios expected from independent assortment.
- Epistasis: one gene masks or modifies the expression of another.
- Duplicate gene action and complementary gene action: two genes act on the same trait and change the ratio.
Dihybrid crosses examples
- Pea plants, RrYy × RrYy: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
- Parental cross, RRYY × rryy: every offspring is RrYy, so the F1 is uniform and the 9:3:3:1 split appears only in the F2.
Where it matters for labs and teaching
In a two-trait cross, 9:3:3:1 is the expected benchmark. Genetics textbooks note that deviations from the 9:3:3:1 ratio may indicate that one or more of the conditions above has not been met, so an observed ratio that departs from expectation is a prompt to look at linkage or gene interaction. Whether observed counts differ from expected counts by more than chance is a statistical question; the chi-square test entry explains one standard way to compare observed with expected counts. The wider subject area is covered in what is biology.
Common misunderstandings
- “9:3:3:1 is the answer to every two-trait cross.” It applies to RrYy × RrYy under the conditions listed above. Other parental genotypes give other ratios.
- “Dihybrid means two plants.” It describes the parent’s two heterozygous loci, not the number of individuals.
- “Phenotype ratio and genotype ratio are the same.” The same 16 squares give 9:3:3:1 for phenotypes and 1:2:1:2:4:2:1:2:1 for genotypes.
- “The ratio is a prediction for each family.” It is an expected proportion across many offspring.
FAQ
How do I determine genotype from a Punnett square?
Combine the allele pair from the row gamete and the column gamete in each cell; for a dihybrid cross each cell holds two alleles at each locus. Group the 16 cells by genotype to get nine classes in the ratio 1:2:1:2:4:2:1:2:1.
What defines a dihybrid cross?
Parents that are heterozygous at two loci, with both traits followed at once. In the pea example the F1 plants are RrYy.
What is a monohybrid cross?
A cross that follows one trait at one locus and gives a 3:1 phenotypic ratio among the progeny of two heterozygotes.
Are all dihybrid crosses 9:3:3:1?
No. The ratio assumes independent segregation, complete dominance and distinguishable phenotypes with no gene interaction; linkage, epistasis, duplicate gene action and complementary gene action change it.
Sources
Checked 2026-10-29.
- Open Genetics, Thompson Rivers University, section 2.4 “A Dihybrid Cross Showing Mendel’s Second Law (Independent Assortment)”: https://opengenetics.pressbooks.tru.ca/chapter/a-dihybrid-cross-showing-mendels-second-law-independent-assortment/
- Leacock, Genetics (BIOL 3300), University of Arkansas at Little Rock, 4.2.02 “Dihybrid Crosses and Independent Assortment” (LibreTexts): https://bio.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Genetics_BIOL3300_(Leacock)/Genetics_Textbook/04%3A_Inheritance/4.02%3A__Mendelian_Genetics/4.2.02%3A_Dihybrid_Crosses_and_Independent_Assortment
