Investigating Independent Assorment

INVESTIGATING INDEPENDENT ASSORTMENT: CONSIDERING MORE THAN ONE GENE AT A TIME

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 type that will be produced are AB, Ab, aB, ab.

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

The type that will be produced is ab.

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

All the genotypes that I found among the offspring are AaBa, Aabb, aaBb and aabb.

4. What is the expected phenotypic ratio of the offspring of an AaBb x aabb test cross? Show the punnett square you would use for predicting the outcomes

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5.What are the genotypic and phenotypic ratios from a AaBb x Aabb mating? Show the punnett square you would use for predicting the outcomes.

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

All the possible gametes from a trihybrid individual whose genotype is RrSsTt are RST, Rst, STr, and rst.