INVESTIGATING INDEPENDENT ASSORTMENT

INVESTIGATING INDEPENDENT ASSORTMENT

Mendel observed the effects of independent assortment when he carried out his dihybrid crosses. When he crossed two plants that bred true for different versions of two traits, the first-generation offspring all displayed the same phenotype (the dominant phenotype for both traits). However, when these plants were crossed, the second-generation offspring included four different phenotypes.

Mendel carefully recorded the numbers of phenotypes among the offspring of many dihybrid crosses. He found that certain combinations of phenotypes among the second-generation offspring occur in a 9:3:3:1 ratio, on the average. We now know that this pattern occurs because genes on pairs of homologous chromosomes are sorted out for distribution into one gamete or another independently of gene pairs of other chromosomes.

Activity

In this activity, you will first access the meiosis tutorial provided by the University of California, Santa Barbara. The tutorial demonstrates the random assortment of chromosomes into gametes. Next you will complete a tutorial quiz about independent assortment. This is part of The Biology Project from the University of Arizona.

Part 1.

Use your browser to go to the meiosis tutorial at

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Use the tutorial to learn how to determine which allele combinations are possible in two or even three trait crosses.

Part 2.

Use your browser to go to the independent assortment tutorial at

http://www.biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/dihybrid_cross.html

Go through questions 1-9 of the tutorial. If you answer any question incorrectly, review the tutorial material and try again.

When you have completed this much of the tutorial, answer the questions below:

1. What type of gametes will be produced by a plant of genotype AaBb?

            The gametes produced be a plant with genotype AaBb would be AB or Ab or aB or ab

2. What type of gametes will be produced by a plant of genotype aabb?

            The gametes produced be a plant with genotype aabb would be ab

3. List all the genotypes you would find among the offspring of an AaBb x aabb test cross.

            The genotypes would be AaBb, aaBb, Aabb aabb

4. What is the expected phenotypic ratio of the offspring of an AaBb x aabb test cross?

            The expected phenotypic ratio of the offspring of an AaBb x aabb would be 1:1:1:1

5. List all possible gametes from a trihybrid individual whose genotype is RrSsTt.

            The possible gametes for a trihybrid individual with a genotype RrSsTt would be RST or rst or RsT or RSt or rST or rsT or rSt