Dihybrid Cross Calculator
Enter the two parent genotypes (for two genes, e.g. AaBb) to find the offspring phenotype ratio.
A dihybrid cross tracks two genes at once. Because the genes assort independently, you combine two separate monohybrid crosses by multiplying their probabilities. Crossing AaBb × AaBb gives the classic 9:3:3:1 phenotype ratio — 9/16 show both dominant traits, 3/16 and 3/16 show one of each, and 1/16 show both recessive traits.
How to Use This Calculator
Enter each parent's genotype for two genes as four letters — two for the first gene and two for the second, such as AaBb. Use a capital letter for a dominant allele and the same letter in lowercase for the recessive one (A and a for gene 1, B and b for gene 2). The calculator returns the offspring phenotype ratio, expressed as fractions of 16, with each class labeled.
The two letters for each gene must match (Aa, not Ab), because they are two alleles of the same gene. You can cross any combination — AaBb × aabb, AABb × AaBb, and so on — not just the textbook double heterozygote.
How the Calculation Works
By Mendel's law of independent assortment, two genes on different chromosomes are inherited independently, so a dihybrid cross is just two monohybrid crosses happening at once. For each gene, the tool works out the probability that offspring show the dominant phenotype versus the recessive one, exactly as a single Punnett square would. It then multiplies the per-gene probabilities together for each combination — dominant/dominant, dominant/recessive, recessive/dominant and recessive/recessive — to get the four combined phenotype probabilities.
Worked Example
Cross AaBb × AaBb. For gene 1 (Aa × Aa), 3/4 of offspring show the dominant trait (A_) and 1/4 show the recessive (aa). Gene 2 (Bb × Bb) is the same: 3/4 B_, 1/4 bb. Multiplying: both dominant = 3/4 × 3/4 = 9/16 (A_B_); dominant + recessive = 3/4 × 1/4 = 3/16 (A_bb); recessive + dominant = 1/4 × 3/4 = 3/16 (aaB_); both recessive = 1/4 × 1/4 = 1/16 (aabb). That is the famous 9:3:3:1 ratio.
What the Result Means
The four fractions predict the proportions of offspring phenotypes over many crosses. The 9:3:3:1 ratio is the signature of two independently assorting genes each showing simple dominance — seeing it in real data supports independent assortment, while a distorted ratio can signal linkage (the genes are near each other on the same chromosome) or gene interactions like epistasis. For crosses other than the double heterozygote, the ratio changes: AaBb × aabb, for instance, gives a 1:1:1:1 test-cross ratio.
Assumptions and Limitations
This tool assumes the two genes assort independently (are unlinked), that each shows complete dominance, and that the two alleles per gene are a dominant/recessive pair written as the same letter in upper and lower case. It does not model linked genes, incomplete dominance or codominance, multiple alleles, epistasis, or sex linkage, all of which alter the ratios. Enter exactly four letters per parent. The probabilities are exact expectations; real offspring counts vary around them by chance.
Who Uses a Dihybrid Cross Calculator
Genetics students use this to get offspring phenotype ratios for a two-gene cross, including the classic 9:3:3:1. Run it for homework on independent assortment, to check a two-trait cross, or to explore test crosses. Enter both parents' genotypes to see the ratio and the working.
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Open calculator →Frequently Asked Questions
What is the phenotype ratio of a dihybrid cross?
For two double heterozygotes (AaBb × AaBb) with independent genes and simple dominance, the offspring phenotype ratio is 9:3:3:1 — nine both-dominant, three and three with one dominant trait each, and one both-recessive.
Why is a dihybrid cross 9:3:3:1?
Because the two genes assort independently, so you multiply two 3:1 monohybrid ratios. (3/4 × 3/4), (3/4 × 1/4), (1/4 × 3/4) and (1/4 × 1/4) give 9/16, 3/16, 3/16 and 1/16.
What is the law of independent assortment?
Mendel's principle that alleles of different genes are distributed to gametes independently of one another, provided the genes are on different chromosomes. It lets you treat a dihybrid cross as two separate crosses.
Can I cross genotypes other than AaBb?
Yes. Enter any two four-letter genotypes, such as AABb × Aabb. The calculator computes each gene's phenotype probabilities and multiplies them, so it handles test crosses and other combinations too.
What if the 9:3:3:1 ratio is not observed?
A distorted ratio can indicate that the genes are linked (near each other on the same chromosome) or interact through epistasis. Independent assortment, and thus 9:3:3:1, assumes unlinked genes.
Does this handle incomplete or codominance?
No. It assumes complete dominance with a dominant/recessive allele pair per gene. Incomplete dominance and codominance produce different ratios that this tool does not model.
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