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?
cross, the phenotypic ratio is 9:3:4. There are only three different phenotypes
for hair color, agouti, black and albino. The individual with genotype recessive
for both traits, i.e. aabb, has the same albino phenotype as the aaBB and aaBb
individuals due to epistasis.
2. What type of gametes will be produced by a plant of genotype aabb?
might be caused by the recessive homozygous, recessive bb genotype.
3. List all the genotypes you would find among the offspring of an AaBb x aabb test cross.Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele.Both parents produce 25% each of SY, Sy, sY, and sy.
4. What is the expected phenotypic ratio of the offspring of an AaBb x aabb test cross?
These phenotypes will appear in a predicted 1:1s:1:1 ratio.
5. List all possible gametes from a trihybrid individual whose genotype is RrSsTt.
If only recessive traits were scored, we would only consider offspring with one
phenotype: dented, green seeds. A 9:3:3:1 ratio phenotypes would be impossible.
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?
cross, the phenotypic ratio is 9:3:4. There are only three different phenotypes
for hair color, agouti, black and albino. The individual with genotype recessive
for both traits, i.e. aabb, has the same albino phenotype as the aaBB and aaBb
individuals due to epistasis.
2. What type of gametes will be produced by a plant of genotype aabb?
might be caused by the recessive homozygous, recessive bb genotype.
Rather than the 9:3:3:1 seqregation of phenotypes normally seen with an AaBb x AaBb dihybrid
Same as stated above, only the albino phenotype masks any phenotype that
3. List all the genotypes you would find among the offspring of an AaBb x aabb test cross.Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele.Both parents produce 25% each of SY, Sy, sY, and sy.
4. What is the expected phenotypic ratio of the offspring of an AaBb x aabb test cross?
These phenotypes will appear in a predicted 1:1s:1:1 ratio.
5. List all possible gametes from a trihybrid individual whose genotype is RrSsTt.
If only recessive traits were scored, we would only consider offspring with one
phenotype: dented, green seeds. A 9:3:3:1 ratio phenotypes would be impossible.
20
