Investigation 1: Inheritance of One Trait in Zea Mays (Corn), Monohybrid Cross Now that you have a basic understanding of the terminology used in studying genetics, it’s time to learn how to perform a monohybrid cross. Only one trait is studied in a monohybrid cross ("mono" means one, hybrid means heterozygous). Figure 9.1 uses corn color to illustrate how to outline a monohybrid cross using a Punnett square. This outline predicts the genotypes of three generations: P for the parent generation, F1 (also called monohybrids) for the first filial generation, and F2 for the second filial generation. In this example, the purple kernel is dominant, so a P will be used for this allele and a p for the recessive yellow allele. The two parents of the P generation are homozygous dominant (PP) and homozygous recessive (pp) (also known as true breeders). In a cross between these two parents (PP x pp), the homozygous dominant parent donates a P allele to its offspring and the homozygous recessive parent donates a p. Therefore, 100% of the F1 offspring will be heterozygous (Pp), or monohybrid. Now, to figure out the expected genotypes of the F2 generation, you begin by crossing two F1 offspring (also known as a monohybrid cross – Pp x Pp). Carefully follow along on the square in Figure 9.1. Since these parents are heterozygous, they can donate either a P or a p allele to their offspring. This is where a Punnett square comes in handy. Since each plant has two different possibilities for the allele it donates, you multiply 2x2 to determine that you need a square with 4 cells. The gametes for one parent go across the top of the Punnett square and the gametes for the other parent go down the other side of the Punnett square. Then you can match the two gametes to fill in each cell in the Punnett square. After you have determined the genotype, complete the cell by writing in the phenotype. Now you can calculate the genotype and phenotype ratios. Count the number of each genotype. In this example, the genotype ratio is 1 PP: 2 Pp: 1 pp. Then count how many offspring have each phenotype. In this example, the phenotype ratio is 3 purple: 1 yellow. In an actual population of corn, there will never be just 4 kernels. This is why you want the ratios. The interpretation of these results is that in a population of corn, it is expected that 25% will be homozygous dominant, 50% will be heterozygous, and 25% will be homozygous recessive. That is, 75% will be purple and 25% will be yellow.
Fill in the Punnett square for the monohybrid cross for kernel color, Pp x Pp.
9.1 Questions In a population of 215 kernels:
Question 9.1 How many would you expect to be purple?
Question 9.2 How many would you expect to be yellow?
Question 9.3 What is the genotype for homozygous dominant individual?
Question 9.4 What percentage of kernels are expected to be homozygous dominant?
Question 9.5 How many kernels are expected to be heterozygous?
Question 9.6 What is the phenotype of heterozygous corn?