Mendelian Genetics Calculator

The probability that a child shows a trait, inherits each genotype or carries the allele without showing it, from the genotypes of both parents. It is Mendel's first law put into numbers.

Probability of showing the dominant trait

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Probability of showing the dominant trait—

Probability of showing the recessive trait—

Probability of being an unaffected carrier—

Possible genotypes—

Phenotype ratio—

Boxes in the square—

How this was worked out

The formula

P(phenotype) = boxes with that phenotype ÷ total boxes

What it means

Mendel called an allele dominant when it shows up whenever it is present, and recessive when it only shows if the other one is missing. A heterozygote "Aa" shows the dominant trait and at the same time passes the recessive one to half its children: that is the unaffected carrier, the figure that matters in genetic counselling. The probabilities come from counting boxes in the Punnett square rather than from a separate formula, which is why the square is drawn here too.

How to work it out by hand

Write down each parent's genotype, capital letter for the dominant allele
Build the Punnett square by crossing both sets of gametes
Count the boxes that show each trait
Divide by the total number of boxes for the probability per child

What is worth knowing

The probability applies to each pregnancy, not to the family as a whole: two affected children do not use up the third one's quota. And carrier status only means one thing when a single gene is in play; with two genes the word stops referring to one trait, so a dash appears instead. This describes a monogenic trait with clean dominance or recessiveness; X-linked, mitochondrial and polygenic conditions follow other rules.

Frequently asked questions

What is an unaffected carrier?

Someone with one recessive and one dominant allele: they do not show the recessive trait but pass it to half their children. In the square they are the heterozygous boxes.

If the probability is 25 %, will one in four children have it?

No. The 25 % applies to each pregnancy separately. All four could be unaffected or all four affected; the coin does not remember earlier tosses.

Does it work for sex-linked conditions?

No. Haemophilia and colour blindness sit on the X chromosome and the outcome depends on whether the child is a boy or a girl, which this square does not distinguish.

Why is the carrier figure a dash with two genes?

Because "carrier" refers to one specific trait. With two genes at once you would have to say a carrier of which, and answering anything else would be misleading.

Can two unaffected parents have an affected child?

Yes, if the condition is recessive and both are carriers: that gives 25 %. It is the commonest case in genetic counselling.