Free fantasy genetics / fan-fiction OC bloodline calculator. Define a trait, set each parent's genetic status (homozygous dominant, heterozygous, homozygous recessive) and visualize the Punnett-square odds.
Built and maintained by Paul Clark, Redmoon Software
Use when designing a character's family tree or a fan-fiction OC's offspring odds. Standard Mendelian inheritance — perfect for a simple visible trait.
A bare Punnett square calculator does the same math. This one wraps it in narrative trait names that suit world-builders.
| A | a | |
| A | AA | Aa |
| a | Aa | aa |
Children have a 75% chance of displaying the trait and a 50% chance of silently carrying it.
Two parents combine into a Punnett square: each cell is one of four equally likely offspring genotypes (AA, Aa, aA, aa).
Aa and aA are functionally identical — both heterozygous. AA shows the trait; aa hides it; Aa shows it but carries the recessive.
Percentages are simply the count of matching outcomes divided by four.
Not yet — one trait at a time. Run the tool again with the next trait.
Standard Mendelian: A is dominant over a. Aa shows the trait; aa does not.
They show the probability that offspring inherit each genotype based on the alleles each parent can pass on. For example, two heterozygous parents yield a classic 1:2:1 genotype ratio and a 3:1 dominant-to-recessive appearance ratio.
Homozygous means both copies of the gene are the same allele (either both dominant or both recessive), while heterozygous means one of each. A heterozygous parent can pass on either allele, which is why it produces variable offspring.
Input
Trait "Silver Hair", Parent A: Aa (carrier), Parent B: Aa (carrier).
Output
75% chance child shows trait. 25% no trait.
AA × AA = 25% homozygous dominant, 50% heterozygous (carriers, show trait), 25% homozygous recessive (no trait).
The tool crosses two parent genotypes and reports the offspring distribution — the standard Mendelian square, applied to whatever trait you name. Two heterozygous parents (Aa × Aa) give the classic 1:2:1 genotype ratio and a 3:1 phenotype ratio, which is where the familiar "one in four" comes from.
The probabilities are per offspring and independent. Three children showing the dominant trait does not make the fourth more likely to show the recessive one, and that misunderstanding is common enough to be worth stating.
Single-gene dominant/recessive inheritance is the exception in real biology, not the rule. Most visible traits are polygenic — eye colour and height involve many genes — and simple Mendelian models of them are wrong. Incomplete dominance, codominance, sex linkage and epistasis all break the square.
For worldbuilding that hardly matters; a clean model of an invented trait is usually what you want. It matters if you are using this to reason about a real trait, where the honest answer is that a single square almost certainly does not describe it.
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