The Punnet square was invented by the English geneticist Reginald Punnett in the early 20th century. It represents a simple method for calculating the theoretical genotypic ratios, with which the expressions of a gene are manifested in the offspring generated by the crossing of two "parents". A monohybrid cross is defined as a cross from which the results of a single gene are taken into account.
Steps
Part 1 of 2: Prepare a Punnet Square
Step 1. Study genes and genotypes
The genotype is the genetic code of an individual that is passed on to the offspring. An individual's genotype derives from the alleles of two chromosomes inherited from their parents. For example, a gene codes for hair color, but one allele could be assigned to blonde hair and another to brown.
- Each individual has two chromosomes with two alleles that make up the genotype and are represented with two letters.
- The uppercase letters indicate the dominant alleles, while the lowercase ones are assigned to the recessive ones.
- It is not important which letter you choose to represent the gene you are studying, so choose one that makes sense to you; generally, the first letter of the dominant allele is used.
- For example, B can be used for the dominant gene which codes for brown hair and b for the recessive one which codes for blond hair.
Step 2. Draw a 2 x 2 table
Just as the name indicates, the Punnet square is a square divided into cells. Draw it and divide it into four smaller squares, drawing two lines (one vertical and the other horizontal) running through it in the center.
- Make sure that there is enough space in each cell to write the two letters.
- Also, remember to leave some space above and to the left of the table.
Step 3. Above the table write a parent's genotype
Suppose the mother has brown hair and a Bb genotype; accordingly, you have to write B above the upper left square and b above the upper right square.
- It doesn't matter where you write each parent's genotype.
- You must write only one letter above each box.
Step 4. Write the other parent's genotype on the left side of the square
Suppose the father also has brown hair, but with a BB genotype; accordingly, you should write a B to the left of the top left box and another B to the left of the bottom box on the same side.
Part 2 of 2: Performing the Intersection
Step 1. Match the alleles to each other using the boxes as guidelines
Each allele can be rewritten in the two cells below or to the right of it, depending on its position. For example, if the B allele is above the upper left corner, report the letter B in the two boxes immediately below. If the B allele is written to the left of the upper left box, you must enter it in the two cells immediately to the right. Keep filling in the boxes until they are all occupied by a pair of alleles from the parents.
- By convention, the uppercase letter corresponding to the dominant allele is always written first, followed by the lowercase letter of the recessive allele.
- As for the example of the two brown-haired parents, their genotype could be either BB or Bb; you must therefore know the specific genotype. However, if one of the parents was blond, you would know that his genotype is recessive bb.
Step 2. Count the number of each genotype
When performing a monohybrid cross, there are only three possible combinations: BB, Bb and bb. The BB (brown hair) and bb (blond hair) genotypes are homozygous for the gene, which means they have two identical alleles for a gene. The Bb genotype (brown hair) is heterozygous, meaning it has two different alleles for the gene. Some crosses lead to the formation of only one or two genotypes.
- In the example considered, crossing BB with Bb the Punnet square shows that there are two possibilities of obtaining a genotype BB and two of Bb.
- If you cross two homozygous parents with the same genotype (BB x BB or bb x bb), all the offspring will have a homozygous genotype (BB or bb).
- If you cross two homozygous parents with different BB x bb genotypes, all the offspring will have the Bb genotype.
- If you cross a heterozygous parent with a homozygote (BB x Bb or bb x Bb), you will get two homozygotes (BB or bb) and two heterozygotes (Bb).
- If you cross two heterozygous parents, Bb x Bb, you will get two homozygotes (one BB and one bb) and two heterozygotes (Bb).
Step 3. Calculate the phenotypic ratio
Using the calculations from the previous step, you can determine the relationships between the phenotypes. Phenotype is the physical characteristic encoded by the gene, such as hair or eye color. Assuming that the trait shows complete dominance, the heterozygous genotype (the cross that presents two genes different for hereditary characteristics) manifests the dominant phenotype.
