A Punnett square is a square diagram that is used to predict the possible genotypes of a particular cross or breeding event. It allows you to easily determine the expected percentage of different genotypes in the offspring of two parents. In Punnett squares, the gametes produced by the male parent are at the top of the chart, and the gametes produced by the female parent are along the left side. The different possible combinations of alleles in their offspring are determined by filling in the boxes of the Punnett square with the correct letters (alleles). Each box in the table represents one fertilization event.
To correctly set up a Punnett square, you should follow these steps:
In the example below, two pea plants with purple flowers who are heterozygous (Bb) are being crossed in the Punnett square. In this cross, you can see that 25% of the offspring or 1 out of the four are homozygous recessive, giving them a white flower. Two out of the four offspring (50%) have the genotype Bb and one out of the four (25%) have the genotype BB, giving them purple flowers. This 3:1 ratio (3 with dominant and 1 with recessive phenotype) and the 2:1:1 ratio (2 heterozygous, 1 homozygous dominant, and 1 homozygous recessive) are what is expected when you cross two heterozygous genotypes. Of course, when the actual offspring are produced, the actual percentages will vary from the expected percentages.
To correctly set up a Punnett square, you should follow these steps:
- Take the factors from the first parent and place them at the top of the square .
- Take the factors from the second parent and line them up on the left side of the square .
- Pull the factors from the top into the boxes below.
- Pull the factors from the side into the boxes next to them.
In the example below, two pea plants with purple flowers who are heterozygous (Bb) are being crossed in the Punnett square. In this cross, you can see that 25% of the offspring or 1 out of the four are homozygous recessive, giving them a white flower. Two out of the four offspring (50%) have the genotype Bb and one out of the four (25%) have the genotype BB, giving them purple flowers. This 3:1 ratio (3 with dominant and 1 with recessive phenotype) and the 2:1:1 ratio (2 heterozygous, 1 homozygous dominant, and 1 homozygous recessive) are what is expected when you cross two heterozygous genotypes. Of course, when the actual offspring are produced, the actual percentages will vary from the expected percentages.