The Punnett square simulates the sexual reproduction of two organisms, examining how the passage of one of the many genes that will be passed down occurs. The full square shows all the possible ways the offspring can inherit a gene and what the probabilities are for each outcome. Making Punnett squares is a good way to start learning about the fundamentals of genetics.
Steps
Part 1 of 2: Creating a Punnett Square
Step 1. Draw a 2 x 2 square
Draw a square and divide it into four more smaller ones. Leave some space above and to the left of the figure so that you can add the labels.
Read the additional information presented below if you can't understand the next steps
Step 2. Name the alleles involved
Punnett squares describe the possible methods of transmitting variants of a gene (alleles) in the case of sexual reproduction of two organisms. Choose a letter that represents the alleles. Use uppercase for the dominant one and lowercase for the recessive one. You can choose the letter you prefer, it doesn't matter which one it is.
- For example, you can call the dominant gene for black fur "P" and the recessive gene for yellow fur "p".
- If you don't know which gene is dominant, use different letters for the two alleles.
Step 3. Check the genotypes of the parents
To continue, you need to know the genotype of each parent for the selected trait. Each sexually reproducing organism has two alleles (in some cases, the same one that repeats) for each trait, so it has two-letter genotypes. In some cases, you will know exactly what the genotypes are, while in others you will need to derive them from other information:
- A "Heterozygous" organism has two different alleles (Pp).
- A "Homozygous dominant" genotype consists of two copies of the dominant allele (PP).
- A "homozygous recessive" organism has two copies of the recessive allele (pp). All parents showing the recessive trait (yellow fur) belong to this category.
Step 4. Label the rows with the genotype of one of the parents
Usually this side of the square is reserved for the female (mother), but you can choose the configuration you prefer. Label the first row of the grid with one of the alleles and the second row with the other.
For example, the female bear is heterozygous for the color of the fur (Pp). Write P to the left of the first row and p to the left of the second row
Step 5. Label the columns with the other parent's genotype
Add the second parent genotype for the same trait as column labels. Usually, it is either the male or the father.
For example, the male bear is homozygous recessive (pp). Write a p above both columns
Step 6. Complete all the squares by entering the letters of the rows and columns
The rest of the Punnett square is easy. Start with the first box. Look at the letter on the left and the one above. Write them both in the empty square, then repeat for the other three cells. In case both types of alleles are present, it is customary to write the dominant one (Pp, not pP) first.
- In our example, the upper left cell inherits P from the mother and p from the father, becoming Pp.
- The upper right box inherits P from the mother and p from the father, resulting in Pp.
- The lower left square inherits the p allele from both parents, becoming pp.
- The lower right cell will be pp, inheriting the p allele from both parents.
Step 7. Interpret the Punnett square
This table shows the probability of creating a descendant with some alleles. There are four different ways in which the alleles of the parents can combine and all four have the same probabilities. This means that the combination present in each of the four squares has a 25% chance of occurring. If one of the squares has the same result, add these probabilities to get the total.
- In our example, we have two squares with Pp (heterozygotes). 25% + 25% = 50%, so each descendant has a 50% chance of inheriting the combination of Pp alleles.
- The other two cells are both pp (recessive homozygotes). Each descendant has a 50% chance of inheriting the pp genes.
Step 8. Describe the phenotype
Often, you will be more interested in the child's real traits, not just his genes. In simpler situations, those where Punnett squares are generally used, they are quite easy to find. Add the probability of all squares with one or more dominant alleles to get the percentage that the progeny has the dominant trait. Add the probabilities of all boxes with two recessive alleles to get the percentage that the descendant has the recessive trait.
- In this example, we have two squares with at least one P, so each descendant has a 50% chance of having black fur. Two boxes instead have pp, so all children have a 50% chance of having yellow fur.
- Read the problem carefully to get more information on phenotypes. Many genes are more complex than the example shown in this article. For example, a flower species may be red with RR alleles, white with rr, or pink with Rr. In cases like these, the dominant allele is defined incomplete dominant.
Part 2 of 2: General Information on the Topic
Step 1. Review the genes, alleles and traits
A gene is a piece of "genetic code" that determines a trait of a living organism, for example the color of the eyes. However, the eyes can be blue, brown, or many other shades. These variants of the same gene are defined alleles.
Step 2. Learn about the genotype and phenotype
The set of all your genes makes up the genotype: the whole chain of DNA describing how you are "built". Your body and your personality are the phenotype: your real appearance, determined in part by your genes, but also by your diet, possible injuries and other life experiences.
Step 3. Learn how genes are inherited
In organisms that reproduce sexually, including humans, each parent passes one gene per trait. The son keeps the genes of both parents. For each trait, it can have two copies of the same allele or two different alleles.
- An organism with two copies of the same allele is homozygous for that gene.
- An organism with two different alleles is heterozygous for that gene.
Step 4. Try to understand dominant and recessive genes
The simplest genes have two alleles: one dominant and one recessive. The dominant variant also occurs in combination with a recessive allele. A biologist would say that the dominant allele is "expressed in the phenotype".
- An organism with one dominant and one recessive allele is defined dominant heterozygote. They are also known as porters of the recessive allele, since the latter is present but does not manifest itself in the tract.
- An organism with two dominant alleles is dominant homozygous.
- An organism with two recessive alleles is recessive homozygous.
- Two alleles of the same gene that can combine to make three different colors are defined incomplete dominants. An example of this phenomenon is seen in horses with the cream dilution gene, where the cc specimens are red, the Cc specimens are golden and the CC specimens are light cream.
Step 5. Find out why Punnett squares are useful
The end result of this table is a probability. A 25% percentage of having red hair does not mean that exactly a quarter of children will have red hair; it is only an estimate. However, in some situations even a rough estimate can reveal enough information:
- Those involved in breeding projects (usually, to develop new strains of plants) want to know which pairs offer the best chance of getting the desired result or if it is worth trying to merge a certain pair.
- Anyone with a severe genetic disorder or a carrier wants to know the likelihood of passing that gene on to their children.
Advice
- You can use any letters you like, you don't have to choose P and p.
- There is no part of the genetic code that makes an allele dominant. We only look at which trait is visible with a single copy of the gene and we define the allele that caused that trait as "dominant".
- You can study the inheritance of two genes at the same time using a 4x4 grid and a code of 4 alleles for each parent. You can apply this method to any number of genes (or use it for genes with more than two alleles), but the squares quickly become very large.
