Punnett Square Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Apr 06, 2022

How does the inheritance of traits work? The Punnett square calculator provides you with an answer to that and many other questions. It comes as handy if you want to calculate the genotypic ratio, the phenotypic ratio, or if you're looking for a simple, ready-to-go, dominant and recessive traits chart. Moreover, our Punnet square maker allows you to calculate the probability that a rare, recessive genetic diseases will be inherited.

Hey, perhaps you're looking for a more advanced Punnett square calculator, with 2 traits and 4 alleles, or an extreme, gigantic three trait Punnett square generator ?

This Punnett square generator will teach you the basics of genetics, and will guide you, step-by-step, on how to create your own genetic square. Read on!

How to do a Punnett square? - examples

Making a simple, 1 trait gene chart is extremely easy! You just need to remember that not all the genes can be used to create a Punnett square. Here's a shortlist of rules to follow:

  • Given traits must be inherited independently (their genes can not be located close to each other in the genetic material);
  • External factors cannot influence the inheritance of a gene; and
  • A given trait must be defined only by the alleles we're going to use in the genetic square.

✅ The blood type inheritance makes a good example of a trait that is perfect to use in the Punnett square calculator.

❌ The height of a child cannot be predicted using the Punnett square method - there are too many variables and genes affecting this trait.

Traits are inherited through genes, the memory banks of the cell. Every gene has two versions, called alleles. We use capital letters for dominant alleles (A), and lowercase for recessive alleles (a). Dominant alleles are superior in terms of strength - if a dominant allele is present, the trait it carries will always be visible. Recessive alleles' features will only be visible if there are no dominant alleles.

If you already know you blood type... why don't you check who you could possibly donate it to? 🩸

Punnett square calculator in practice

Let's say we need to know the probability that our patients' baby will inherit a genetic disorder called cystic fibrosis.

  1. Find out the manner of inheritance.

Autosomal recessive. (Autosomal inheritance means that described genes are located on regular chromosomes [1-22], and not sex chromosomes [X,Y])

  1. Study the parents' genetics.

There are children with cystic fibrosis in both of families. Both parents are healthy, but they still may be carries since the disorder is inherited in an autosomal recessive manner.

  1. Fill in the square! We need two Punnett squares for this particular case.
  • A - Healthy, dominant allele
  • a - Recessive allele of Cystic Fibrosis

First situation: both parents are carriers.

♂️\♀️ Aa

There is a 25% chance (1/4) of giving birth to a child with cystic fibrosis.
On average, 75% of children born to these parents will be healthy: out of those 2/3 will be carriers, and 1/3 will inherit no cystic fibrosis alleles.
(When the percentages get confusing - try the percentage tool.)

Second situation: only one parent is a carrier.

♂️\♀️ Aa
In this situation 100% of babies will be born healthy. 50% of them will inherit one improper allele, making them carriers.

Go ahead, play with our Punnett square calculator and try all of the possible options!

Genotypic ratio and phenotypic ratio

Phenotype describes the appearance, that is, what's visible. Genotype describes hidden genetic properties of a trait.
What's the difference? Why does it matter?
Let's look at the genetic table below.

♂️\♀️ Aa

Now, let's calculate the genotypic and phenotypic ratios:


Genotypic ratio

  • AA : Aa : aa = 1 : 2 : 1

Phenotypic ratio:

  • A : a = 3 : 1

Because allele a is recessive, when it appears with a dominant allele, the trait it carries is not visible, but the allele is still there, ready to potentially be inherited in the future.

Autosomal alles - homozygous or heterozygous?

Here are some basic definitions which may be crucial for the proper use of the genetic calculator:

  • Homozygous dominant - Where one set of alleles of one gene describes a particular trait. We can use this concept when both of those alleles are dominant (AA).

  • Homozygous recessive - We use it when both of described alleles are recessive (aa)

  • Heterozygous - We use it where one allele is recessive (a), and the other is dominant (A).

Mendelian inheritance

The basic rules of genetics were created by Gregor Mendel in 1865, thanks to his simple experiments conducted on garden peas. During that era, humanity had no microscopes, complex scientific technology, or the slightest concept of genes. With simple experiments and insightful observations, he was able to draw conclusions that are useful up to this day - it's no wonder he's called the Father of genetics.

  1. Traits are unitary (red color vs. yellow color);
  2. There are two versions of every gene (now we call them alleles);
  3. There are types of alleles which are superior to the other types (dominant alleles);
  4. Alleles are segregated in a random way;
  5. The chance either allele will be inherited is equal; and
  6. Genes are inherited independently.

A few centuries later, we can undoubtedly say that Mendel was not entirely right - some of the genes are inherited together, because of their close proximity on the chromosome. Moreover, some of the genes are codominant: two different dominant alleles can coexist and be visible in the phenotype at the same time. Blood types inheritance is an excellent example of that, since dominant alleles A and B cooperate in creating the AB blood type.

Types of Punnett squares

Our Punnett square maker works on autosomal alleles (chromosomes 1-22), but it can be used for other things.

Let's think about X-linked diseases - disorders that are inherited only via the female line of the family. Every woman has two different X chromosomes inherited from her parents. If one of them is faulty or sick, the second, healthy one may take its function. Every man, however, is equipped with only one X chromosome. This way, only one incorrect allele can cause diseases among men, but not among women.

Hemophilia is a rare genetic, X-linked disease. We want to know the chances that a male patient with hemophilia will have a baby with this disorder. His partner is healthy, and has no traces of the disease in their family.

  • XD - Healthy X chromosome;
  • Xd - X chromosome with Hemophilia gene; and
  • Y - Y chromosome.
♂️\♀️ XDXD

We can clearly see that all of the patient's children will be healthy. However, all of his daughters will be carriers, and may transfer the disease to the next generation. All of his sons will be completely free of the disease.

Łucja Zaborowska, MD, PhD candidate
You can also use the larger, dihybrid cross Punnett Square Calculator 🏷.

A - Dominant allele

a - Recessive allele
Mother's genotype
Father's genotype
Chance for AA
Chance for Aa
Chance for aa
Punnett Square
Phenotype and Genotype
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