Dihybrid Cross Punnett Square

What is a Dihybrid Cross?

A dihybrid cross is a breeding experiment between two organisms that are identically hybrid for two different traits. It allows geneticists to study how multiple physical features are inherited simultaneously from parents to offspring. This inheritance pattern was first popularized by Gregor Mendel in his historical pea plant experiments, leading to the formulation of the Law of Independent Assortment.

Understanding Mendel's Law of Independent Assortment

Mendel discovered that the inheritance of one trait does not influence the inheritance of another. For instance, whether a pea seed is yellow or green does not dictate whether it will be round or wrinkled. Under standard conditions, genes located on different chromosomes assort independently of one another during the formation of gametes.

When crossing two double heterozygous parents (such as $AaBb \times AaBb$), the alleles segregate. The expected phenotypic ratio for a dihybrid cross of two heterozygous individuals is $9:3:3:1$. This means:

• 9/16 of the offspring will display both dominant phenotypes.
• 3/16 will display the first dominant and second recessive phenotype.
• 3/16 will display the first recessive and second dominant phenotype.
• 1/16 will show both recessive phenotypes.

How to Create a Dihybrid Punnett Square

Determining the offspring probability for two traits is simple with a $4 \times 4$ Punnett square. Follow these steps:

1. Identify the Parent Genotypes: For example, let the maternal genotype be $AaBb$ and the paternal genotype be $Aabb$.
2. Determine the Gametes: Use the FOIL method (First, Outer, Inner, Last) to combine alleles of different traits for each parent. For an $AaBb$ parent, the possible gamete combinations are $AB$, $Ab$, $aB$, and $ab$. For an $Aabb$ parent, they are $Ab$ and $ab$.
3. Construct the Grid: Place the maternal gametes on the top columns and the paternal gametes along the left rows of a $4 \times 4$ grid.
4. Fill the Cells: Combine the alleles from row and column headers in each cell, keeping the dominant alleles first for each trait. E.g., crossing $AB$ with $ab$ results in $AaBb$.
5. Calculate Ratios: Count the occurrences of each genotype and phenotype to derive percentages and ratios.

Related Genetic Tools

If you are analyzing inheritance patterns, you might also find our other biology tools useful: Use the Blood Type Calculator to predict child blood type percentages using ABO and Rh factor genetics. You can also analyze gene distribution in populations using the Allele Frequency Calculator.