Dihybrid Cross Punnett Square Calculator
Calculate dihybrid cross punnett square with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Formula
Phenotypic Ratio (AaBb x AaBb) = 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb
In a dihybrid cross between two heterozygotes, each parent produces 4 gamete types. The 4x4 Punnett square yields 16 offspring combinations. With independent assortment and complete dominance, the phenotypic ratio is 9:3:3:1. Different parental genotypes produce different ratios.
Worked Examples
Example 1: Classic Mendel Dihybrid Cross (AaBb x AaBb)
Problem: Cross two organisms heterozygous for both traits. What are the expected phenotypic and genotypic ratios?
Solution: Parent gametes: AB, Ab, aB, ab (each parent)\n16 offspring combinations in 4x4 Punnett square\nPhenotypic ratio: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb\nGenotypic ratio: 1 AABB : 2 AABb : 2 AaBB : 4 AaBb : 1 AAbb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb
Result: Phenotypic ratio 9:3:3:1 with 4 phenotypes and 9 genotypes
Example 2: Test Cross (AaBb x aabb)
Problem: Cross a dihybrid individual with a homozygous recessive individual to determine the dihybrid's gamete production.
Solution: AaBb gametes: AB, Ab, aB, ab\naabb gametes: ab only\nOffspring: AaBb, Aabb, aaBb, aabb\nEach genotype appears in equal proportions (1:1:1:1)\nAll 4 phenotypes equally likely at 25% each
Result: Phenotypic ratio 1:1:1:1 confirming independent assortment
Frequently Asked Questions
What is a dihybrid cross?
A dihybrid cross is a genetic cross between two organisms that are heterozygous for two different genes. This means each parent carries two different alleles for each of the two traits being studied. The classic example is Mendel's cross of pea plants that were heterozygous for both seed shape (Rr) and seed color (Yy). A dihybrid cross produces a 4x4 Punnett square with 16 possible offspring combinations, which typically yields the famous 9:3:3:1 phenotypic ratio when both genes assort independently.
How do you set up a dihybrid Punnett square?
First, determine the genotypes of both parents for two genes. Next, figure out the possible gametes each parent can produce by combining one allele from each gene. For a dihybrid cross, each parent produces 4 types of gametes. Place one parent's gametes along the top of a 4x4 grid and the other parent's gametes along the side. Fill in each cell by combining the gamete from the column with the gamete from the row. This produces 16 offspring genotype combinations that you can then analyze for genotypic and phenotypic ratios.
What is the expected ratio from a dihybrid cross of two heterozygotes?
When both parents are heterozygous for both genes (AaBb x AaBb), the expected phenotypic ratio is 9:3:3:1 under Mendelian inheritance with independent assortment. This means 9 out of 16 offspring show both dominant traits, 3 show the first dominant and second recessive, 3 show the first recessive and second dominant, and 1 shows both recessive traits. The genotypic ratio is more complex with 9 distinct genotypes. This 9:3:3:1 ratio was one of Mendel's key discoveries and supports the law of independent assortment.
When does a dihybrid cross NOT produce a 9:3:3:1 ratio?
Several factors can alter the expected 9:3:3:1 ratio. Genetic linkage occurs when two genes are on the same chromosome and close together, causing them to be inherited together more often than expected. Epistasis is when one gene affects the expression of another, producing modified ratios like 9:7, 12:3:1, or 9:3:4. Incomplete dominance or codominance at either locus changes the number of distinguishable phenotypes. Lethal alleles can eliminate certain genotypes from the offspring pool, also modifying the expected ratios.
How do I use a Punnett square?
A Punnett square predicts offspring genotype ratios. Write one parent's alleles across the top and the other's down the side. Fill in each box by combining the row and column alleles. For a monohybrid cross of two heterozygotes (Aa x Aa), you get 1 AA : 2 Aa : 1 aa, or a 3:1 phenotype ratio.
How do I calculate genetic cross ratios for dihybrid crosses?
A dihybrid cross (AaBb x AaBb) follows independent assortment, producing a 9:3:3:1 phenotype ratio. Set up a 4x4 Punnett square with gametes AB, Ab, aB, ab. The 16 squares give 9 A_B_, 3 A_bb, 3 aaB_, and 1 aabb. Modified ratios indicate epistasis or linkage.