Dihybrid Cross Calculators

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A dihybrid cross examines the inheritance of two different traits simultaneously, using individuals that are heterozygous for both traits (AaBb × AaBb). Mendel's law of independent assortment predicts that the alleles for two traits assort into gametes independently of one another (when genes are on different chromosomes). This produces the classic 9:3:3:1 phenotype ratio in F2 offspring: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb (where A_ and B_ represent dominant phenotype). Dihybrid crosses are foundational to understanding Mendelian genetics and the law of segregation extended to multiple gene loci.

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Setting Up a Dihybrid Cross

Parents: AaBb × AaBb. Each parent produces 4 gamete types: AB, Ab, aB, ab (each with probability 1/4). A 4×4 Punnett square has 16 boxes. Phenotype ratios:

  • A_B_ (dominant both): 9/16
  • A_bb (dominant A, recessive b): 3/16
  • aaB_ (recessive a, dominant B): 3/16
  • aabb (recessive both): 1/16

Ratio: 9:3:3:1

Worked Example

Mendel's pea experiments: round/yellow (RRYY) × wrinkled/green (rryy) → all F1 RrYy. F2: dihybrid cross RrYy × RrYy. Predicts: 9 round/yellow : 3 round/green : 3 wrinkled/yellow : 1 wrinkled/green.

Genotype Frequencies

AABB1/16
AABb2/16
AaBB2/16
AaBb4/16
AAbb1/16
Aabb2/16
aaBB1/16
aaBb2/16
aabb1/16

Law of Independent Assortment

Mendel's second law: alleles for different genes assort independently into gametes. Holds true only when genes are on different chromosomes (or far apart on the same chromosome). When genes are linked (close together on same chromosome), recombination frequency < 50% and the 9:3:3:1 ratio is modified.

Glossary

Dihybrid Cross
A cross involving individuals heterozygous for two traits (AaBb × AaBb); produces a 9:3:3:1 phenotype ratio when genes are on separate chromosomes and show complete dominance.
9:3:3:1 Ratio
The classic dihybrid F2 phenotype ratio: 9 (dominant both) : 3 (dominant A, recessive b) : 3 (recessive a, dominant B) : 1 (recessive both); predicts by Mendel's law of independent assortment.
Law of Independent Assortment
Mendel's second law: alleles for different gene loci assort independently into gametes; holds for genes on different chromosomes; violated by linked genes producing modified ratios.

Frequently Asked Questions

A dihybrid cross involves two pairs of alleles simultaneously. Crossing two heterozygotes (AaBb × AaBb): each produces 4 gamete types (AB, Ab, aB, ab) → 16 possible offspring combinations. The phenotype ratio is 9 (both dominant) : 3 (dominant A, recessive b) : 3 (recessive a, dominant B) : 1 (both recessive) = 9:3:3:1. This assumes: both loci have complete dominance; genes are on separate chromosomes (independent assortment); no epistasis (one gene masking another).

Step 1: Determine gametes from each parent. For AaBb: gametes are AB, Ab, aB, ab (each 1/4). Step 2: Draw a 4×4 grid with parent 1 gametes labeling columns and parent 2 gametes labeling rows. Step 3: Fill each cell by combining row and column gametes. Step 4: Count phenotype classes by identifying genotypes. A_B_ (dominant both) = AABB + AABb + AaBB + AaBb = 1+2+2+4 = 9 cells. A_bb = AAbb + Aabb = 1+2 = 3. aaB_ = aaBB + aaBb = 1+2 = 3. aabb = 1. Total: 16 = 9+3+3+1.

The law of independent assortment states that alleles for different gene loci assort independently into gametes during meiosis — the allele inherited at one locus does not influence which allele is inherited at another locus. This is mechanistically explained by chromosome segregation: homologous chromosomes align randomly at the metaphase plate during meiosis I, so maternal and paternal chromosomes for different pairs orient independently. The law holds when genes are on different chromosomes or sufficiently far apart on the same chromosome. Genes located close together (linked) violate this law and produce non-9:3:3:1 ratios.

Epistasis occurs when one gene masks or modifies the expression of another. Modified F2 ratios from epistasis: Dominant epistasis (9:3:4): genotype A_ masks aa; aa shows third phenotype regardless of B locus. Recessive epistasis (9:3:4 or 9:7): bb masks A_ expression. Duplicate dominant epistasis (15:1): A_ or B_ (or both) produce same phenotype. Complete epistasis (12:3:1): A_ masks B entirely. These deviations from 9:3:3:1 suggest epistasis or gene interaction. Detecting epistasis requires observing non-Mendelian ratios in dihybrid crosses and ruling out linkage as an alternative explanation.