Dihybrid Cross Calculator — Two-Trait Punnett Square & Inheritance Ratios

What Is a Dihybrid Cross?

A dihybrid cross is a genetic cross between two organisms that differ in two traits, each controlled by a separate gene. Unlike a simple monohybrid cross (which tracks one trait), the dihybrid cross tracks the simultaneous inheritance of two independent traits — producing a 4×4 Punnett square with 16 possible offspring combinations.

How to Use the Dihybrid Cross Calculator

1. Enter the genotypes for both parents using standard genetic notation:
   • Use 4 letters total — the first two for Trait A, the last two for Trait B
   • Uppercase = dominant allele (e.g., A, B)
   • Lowercase = recessive allele (e.g., a, b)
   • Example: AaBb = heterozygous for both traits
2. Optionally name your traits (e.g., "Hair type" for A/a, "Hair color" for B/b)
3. Click Build Dihybrid Cross to generate the full 4×4 Punnett square

Understanding the 4×4 Punnett Square

Each parent with genotype like AaBb produces 4 types of gametes: AB, Ab, aB, and ab. Crossing two such parents creates 16 possible offspring combinations. The calculator displays:

• Genotypic ratio — how many offspring carry each specific genotype combination (up to 9 different genotypes)
• Phenotypic ratio — how many offspring express each trait combination based on dominant/recessive rules

Classic Dihybrid Cross Results (AaBb × AaBb)

The most famous dihybrid cross — heterozygous for both traits × heterozygous for both traits — produces the 9:3:3:1 phenotypic ratio:

• 9/16 — dominant for both traits (A_ B_)
• 3/16 — dominant for Trait A, recessive for Trait B (A_ bb)
• 3/16 — recessive for Trait A, dominant for Trait B (aa B_)
• 1/16 — recessive for both traits (aa bb)

Genotypic Ratio Explained

The genotypic ratio describes the exact allele combinations in offspring. For AaBb × AaBb, there are 9 distinct genotypes:

AABB : AABb : AAbb : AaBB : AaBb : Aabb : aaBB : aaBb : aabb
   1   :    2   :    1  :    2   :    4   :    2  :    1   :    2   :    1

To simplify the ratio, divide all values by the smallest non-zero value (in this case, 1 — so the ratio stays 1:2:1:2:4:2:1:2:1).

Real-World Example: Hair Type and Color

Suppose we want to predict a child's hair characteristics:

• Trait A: Hair type — A = curly (dominant), a = straight (recessive)
• Trait B: Hair color — B = dark (dominant), b = light (recessive)

If the mother has genotype Aabb (curly, light hair) and the father has aabb (straight, light hair):

• Mother gametes: Ab, ab
• Father gametes: ab only
• Result: 50% Aabb (curly, light) and 50% aabb (straight, light)

Key Genetics Terms

Dominant allele

An allele that masks the effect of another allele when present (denoted by uppercase, e.g., A)

Recessive allele

An allele whose effect is masked when a dominant allele is present (denoted by lowercase, e.g., a)

Homozygous dominant

Two identical dominant alleles for a trait (e.g., AA or BB)

Homozygous recessive

Two identical recessive alleles for a trait (e.g., aa or bb)

Heterozygous

One dominant and one recessive allele for a trait (e.g., Aa or Bb)

Gamete

A reproductive cell (sperm or egg) carrying one allele from each gene. For AaBb, gametes are AB, Ab, aB, and ab

Phenotype

The observable traits of an organism (what it looks like)

Genotype

The genetic makeup of an organism (the alleles it carries)

Independent assortment

Mendel's law stating that genes for different traits sort independently during gamete formation (applies when genes are on different chromosomes)

Frequently Asked Questions

What does the 9:3:3:1 ratio mean?

When crossing two individuals heterozygous for two independent traits (AaBb × AaBb), statistically 9 out of 16 offspring will show both dominant traits, 3 out of 16 will show only the first dominant trait, 3 will show only the second dominant trait, and 1 will show both recessive traits.

Can I use any letters for my traits?

Yes! Standard practice uses A/a for the first trait and B/b for the second. You can use any letter — just make sure both parents use the same gene letters (same letter, different case). For example: CcDd × CcDd works perfectly.

What is a test cross in dihybrid genetics?

A test cross involves crossing an organism with unknown genotype with a homozygous recessive individual (aabb). The offspring ratios reveal the unknown parent's genotype. Use AaBb × aabb in our calculator to see a classic test cross result.

Does this apply to all organisms?

This calculator works for traits that follow Mendelian inheritance — autosomal traits with simple dominant/recessive relationships. It assumes genes are on different chromosomes (independent assortment). Linked genes, incomplete dominance, and codominance require different approaches.