Before you study Mendel's second experiment, it is important for you to understand the mathematical tool that Mendel used to predict how many purple plants and how many white plants would emerge during his breeding experiments.
Long after Mendel's experiments, the British geneticist Reginald Punnett developed a simple method by which you can easily predict the likelihood of F1 plants inheriting particular traits from their parents. The slide show demonstrates how to form a Punnett square for two homozygous parent pea plants.
Step 1: Identify the genotypes of the parents.In Mendel's experiment, pollen from the purple-flowered plant is deposited on the carpel of the white-flowered plant. If you know that the pollen contains homozygous purple alleles (AA), and the carpel contains homozygous white alleles (aa), you can write the genotypes this way: Male - AA Step 2: Consider the law of segregation.According to the law of segregation, each offspring gets one allele from each parent. The sperm provides one gamete, and the egg provides the other. Male Gamete - A or A Step 3: Draw an empty Punnett square.The number of columns and rows of the Punnett square is determined by the number of male and female gametes. In this example, the male and female have two gametes each, which means you need a table with two rows and two columns. Place the male gametes across the top of the Punnett square and the female gametes down the side.
Step 4: Fill in the allele pairs.Next, combine the alleles of the gametes in the empty boxes, and fill in the possible genotypes as shown in the picture.
Step 5: Calculate the possible genotypes.The possible genotypes in this Punnett square are Aa, Aa, Aa, and Aa. All of the possible genotypes are dominant, which means all four offspring have heterozygous alleles that produce purple flowers.
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Question
A homozygous tall plant (with alleles TT) is crossed with a homozygous short plant (with alleles tt). Predict the outcome of this pairing using a Punnett square. (Create your own Punnett square, and then click Reveal Answer to check your work.)
