Mendelian Genetics Calculator — Predict Inheritance Ratios

Mendel's Three Laws

How to Use the Mendelian Genetics Calculator

1. Open the Mendelian Genetics Calculator
2. Select Monohybrid (one trait) or Dihybrid (two traits)
3. Enter the parent genotypes (e.g. Aa × Aa or AaBb × AaBb)
4. Click Calculate to see the Punnett square and offspring ratios
5. Read genotype frequencies (AA : Aa : aa) and phenotype ratios (dominant : recessive)

Monohybrid Cross Reference

Beyond Simple Dominance

Incomplete dominance

In incomplete dominance, neither allele is fully dominant. A cross between red (RR) and white (WW) snapdragons produces pink (RW) offspring. The Aa × Aa cross gives a 1:2:1 phenotype ratio (red : pink : white) rather than the usual 3:1 — because the heterozygote has its own distinct appearance.

Codominance

In codominance, both alleles are fully expressed simultaneously. Human blood type AB is an example — both A and B antigens appear on red blood cells. The notation uses superscripts: I^A I^B for AB. The phenotype ratio from an I^A I^B × I^A I^B cross would be 1 I^A I^A : 2 I^A I^B : 1 I^B I^B.

Dihybrid cross — the 9:3:3:1 ratio

A dihybrid cross between two AaBb parents produces offspring in a 9:3:3:1 phenotype ratio (9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb), provided the two genes are on different chromosomes. This ratio is the cornerstone of Mendel's Law of Independent Assortment and only holds when genes are unlinked.

Genetic linkage and recombination

Genes on the same chromosome are linked — they do not assort independently. Crossing over during meiosis can recombine linked alleles, and the recombination frequency is used to construct genetic maps. Dihybrid ratios deviate from 9:3:3:1 when genes are linked; the closer two genes are on a chromosome, the less recombination occurs.

Common Questions

What is a test cross and when is it used?

A test cross breeds an organism of unknown genotype with a homozygous recessive (aa). If any offspring show the recessive phenotype, the unknown parent must be heterozygous (Aa). If all offspring show the dominant phenotype, the parent is most likely homozygous dominant (AA). Test crosses are the standard method for determining dominant genotypes.

What is the difference between genotype and phenotype?

Genotype is the actual allele combination (AA, Aa, aa). Phenotype is the observable trait (tall, short). Two organisms with different genotypes (AA and Aa) can share the same phenotype (both tall) if one allele is dominant. Environment can also modify phenotype independent of genotype.

Do Mendel's laws apply to all traits?

No. Mendel's laws apply to simple, single-gene traits with clear dominance. Many traits are polygenic (controlled by multiple genes), sex-linked, affected by epistasis (gene-gene interactions), or influenced by environment. Skin colour, height, and intelligence are all polygenic and do not follow simple Mendelian ratios.