Dihybrid Crosses

Introduction - In corn, the dominant gene R, determines the presence of colored aleurone. Individuals possessing one copy of the gene will exhibit purple kernels. Recessive phenotypes (r) result in yellow kernels. In addition, the dominant gene S produces the endosperm phenotype smooth. Smooth kernels appear hard and starchy. The recessive phenotype produces wrinkled kernels (s), which are shrunken in appearance.

In this lab, we will examine the mode of inheritance of these two genes by looking at the progeny of a fertilization event between a male corn flower and a female corn flower. Each kernel is the progeny of one of such fertilization events. By counting the different kernel types, we should be able to determine their pattern of inheritance.

The purple color is produced by a pigmented layer within the grains. If the layer is not pigmented (colorless), the yellow color of an inner tissue shows through. Sweet corn grains wrinkle upon drying while starchy grains remain smooth. Individual kernels may be (A) purple—starchy, (B) purple—sweet, (C) yellow—starchy, or (D) yellow—sweet. Because these traits are easy to see, a monohybrid or dihybrid cross provides a very easy way to see if these traits are inherited independently (Law of Independent Assortment, Mendelian), or are linked (Non-Mendelian).

Question - Are the genes for color and smoothness inherited independently (on different chromosomes) or are they linked (on the same chromosome)?

Collect and enter data - Examine an ear of corn was produced by a dihybrid cross involving two pairs of heterozygous genes Count the number of each type of kernel and enter your data here.

Analyze your data

• Independently Assorted - A ChiSquare value less than 11.3 [probability of .01 (1%) ] suggests that we can accept the null hypothesis that there is no interaction between the genes and that the distribution can be explained by chance. It suggests that the genes for color and smoothness are not linked and indicates that they are on different chromosomes and independently assorted.
• Linked - A ChiSquare probability value greater than 11.3 [Chi Square probability of .01 (1%) ] indicates that we should reject the null hypothesis that there is no interaction between the genes. It suggests that the genes for color and smoothness are linked. It suggests that the genes are on the same chromosome.

Analyze class data - Analyze the entire set of class data.

• Analsysis - What percent of the class data suggests that the genes are linked, and what percent suggests that the genes are independently assorted?
• What is the value of looking at large data sets from your colleagues as opposed to a single data set collected by your lab group?

Examine summary data - Examine summary data. Does the data suggest a 9:3:3:1 Mendelian ratio?