A Punnett square is a square diagram that shows all the possible combinations of alleles of a certain trait that can occur in an offspring provided that the genotype of the parents is known.
I am sure that definition raised more questions than it answered, but don’t worry, all will be explained shortly.
Before we can understand what is a Punnett square and what is it used for, we need to learn some biological jargon.
- Trait: A trait is a characteristic of an organism such as its color or height. Traits are affected by inherited genes and by the environment of the organism.
- Genotype: It is the genetic makeup of the trait. Consists of alleles.
- Phenotype: It is the physical manifestation of the trait i.e., how the trait expresses itself in reality.
- Genes: The memory banks of the cells which are inherited from the parents, have various information about traits.
- Alleles: Each gene has 2 alleles for a particular trait, each allele has a different property for that trait (but it can be the same allele twice too). Alleles are denoted by using letters. The letter is capital for a dominant allele (e.g., ‘A’) and lowercase for a recessive (e.g., ‘a’).
It is important to note that not all traits are purely affected by genes, some are affected by the environment. The Punnett square method only works for traits that are dependent on genes only and are independently inherited.
The trait must also only be determined by the alleles used in the Punnett square.
Blood type is a trait that can be calculated pretty well using Punnett square. Height is not a trait that can be calculated using Punnett square as it is also dependent on environmental factors.
Blood type Punnett Square Calculator
Blood type is a trait solely dependent on genes, It is a good trait to be calculated by the Punnett Square method. Let’s take a look at how the Punnett square works while calculating the blood type.
Here are the steps you need to take when trying to use a blood type Punnett square calculator to determine the probability of inherited blood types.
1. Determine the Blood Types from a Parent’s blood group
Blood types can be either A, B, AB, or O. In the case of A, the alleles can be ‘AA’ or ‘AO’, where O is recessive and will never express itself when paired with ‘A’ or ‘B’ which are both dominant.
Similarly, for ‘B’ the genotype is ‘BB’ or ‘BO’. ‘AB’ is ‘AB’ as both of the alleles are co-dominant. For ‘O’ it is always ‘OO’ because it is a recessive gene, recessive genes only show up in the phenotype when they are homozygous.
Blood type is usually determined by a blood test. For this example, let us assume the father has blood type ‘A’, while the mother has blood type ‘AB’.
2. Make a Punnett Square
In the Punnett square, the father’s genotype will be written on the left, where each allele corresponds to one row. Since blood type ‘A’ can be either ‘AA’ or ‘AO’, you will need to write all four alleles.
Since the mother’s type is ‘AB’ which has only one genotype, her alleles will be written on the top alongside the columns.
The Punnett square should look like this:
3. Writing Out Possible Genotypes of the Offspring
Each cell in the square corresponds to a possible blood type that the offspring might have. The coinciding alleles in the row and column of that cell are the combination of the alleles in that cell.
You might notice that some combinations are repeated a few times. That is normal, it only means that the chance of those combinations occurring is higher.
The Punnett Square should look like this now:
4. Calculating the Probability of Blood types
When the square is all filled up, count the number of the various blood types that occurred.
In our example, the blood types that occurred are: ‘AA’, ‘AB, ‘AO’, and ‘BO’. The total number of results was ‘eight’.
Now add all the duplicate types; so, we have three ‘AA’, three ‘AB’, one ‘AO’, and one ‘BO’.
The probability of each blood type occurring in the offspring is calculated by dividing the number of times that combination occurred by the total number of results and multiplying the result by 100.
So, the probability for type ‘AA’ to occur is 3/8 * 100 = 37.5%.
The probability for blood type ‘AB’ to occur is 3/8 * 100 = 37.5%.
The probability for type ‘AO’ to occur is 1/8 * 100 = 12.5%.
And the probability for type ‘BO’ to occur is 1/8 * 100 = 12.5%.
Benefits of Punnett Square Blood type calculator
There are online tools that can calculate the probability of the blood type using Punnett square automatically. You only need to provide them with the blood types of the parents.
Some tools show a graphical chart that highlights the percentages of each blood type that can occur in the offspring.
When you are unable to get a blood test, this can be a handy way to guess what the blood type could be.
This is how you can use a Punnett Square to calculate the probability of the blood types that can occur in a child whose parent’s blood types are known.
It is not as accurate as a real blood test, and it cannot tell whether the blood type will be positive or negative (the Rh system).