Enter the parents' genotypes (for example, AaBb × AaBb). The calculator lists the gametes, draws a colour-coded Punnett square and counts the genotype and phenotype ratios as fractions and percentages (9:3:3:1 and others). The solution is laid out the school way: Given, P, G, F₁, answer.
How to build a Punnett square
A Punnett square shows all possible combinations of the parents' gametes and the probability of each offspring genotype and phenotype. The steps are the same as in a school genetics problem.
- State which trait is dominant and which is recessive, and assign a letter to each gene.
- Write the parents' genotypes (P).
- Find the gametes (G): each gamete carries one allele of each gene. A double heterozygote AaBb makes four gamete types, AB, Ab, aB and ab, in equal shares.
- Put one parent's gametes along the rows and the other's along the columns, then fill in the cells (F₁).
- Count identical genotypes and phenotypes: this is the ratio.
Typical ratios
- Monohybrid Aa × Aa: genotype 1 AA : 2 Aa : 1 aa, phenotype 3 : 1.
- Dihybrid AaBb × AaBb: 16 combinations, 9 genotypes, 4 phenotypes in the ratio 9 : 3 : 3 : 1.
- Trihybrid AaBbCc × AaBbCc: 64 combinations, 27 genotypes, 8 phenotypes in the ratio 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.
- Test cross AaBb × aabb: 1 : 1 : 1 : 1, one class per gamete type of the double heterozygote.
- Incomplete dominance: the heterozygote is intermediate, phenotype ratio 1 : 2 : 1.
Sex-linked inheritance
If a gene is on the X chromosome and Y has no allele, the trait shows in males from a single allele. Colour blindness and haemophilia are inherited this way: a son of an affected mother is always affected, while a daughter is affected only with two recessive alleles.
Limitations
The calculator handles genes on different chromosome pairs that assort independently. Linked autosomal genes and crossing over, lethal alleles, epistasis and complementary genes are not modelled; such problems give ratios that differ from the classical ones.
FAQ
What is a Punnett square and how do I use it?
It is a table with one parent's gametes along the rows, the other's along the columns, and the offspring genotypes in the cells. Write the parents' genotypes, find the gamete types, fill in the cells and count how many cells fall into each genotype and phenotype.
Where does the 9:3:3:1 ratio come from?
A cross of two double heterozygotes AaBb × AaBb gives 16 gamete combinations. Of them, 9 cells show both dominant traits, 3 + 3 show one dominant and one recessive trait, and 1 shows both recessive traits. It follows from independent assortment (Mendel's third law).
What is a test cross?
A cross of an individual of unknown genotype with a homozygous recessive one (aabb). The recessive parent only makes ab gametes, so the offspring ratio directly shows the gamete types of the tested individual. Uniform offspring mean a homozygous dominant parent; a 1:1 (or 1:1:1:1) ratio means a heterozygote.
How does incomplete dominance differ from complete dominance?
With complete dominance the heterozygote Aa looks like AA. With incomplete dominance it shows an intermediate trait (red × white gives pink), and the phenotype ratio equals the genotype ratio, 1:2:1.
How do I solve sex-linked inheritance problems?
Switch to the Sex-linked mode: the gene is on the X chromosome and absent from Y. For example, a carrier mother X^D X^d and a normal father X^D Y have only unaffected daughters, and half of the sons are colour blind (1/4 of all children).