# 3 when studying cell division in tissue samples

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3. When studying cell division in tissue samples, scientists often calculate a mitotic index, which is the ratio of dividing cells to the total number of cells in the sample. Scientists often calculate the mitotic index to compare the growth rates of different types of tissue. Which type of tissue would have a higher mitotic index, normal tissue or cancerous tissue? Explain . Since cancerous tissue has more dividing cells than normal tissue, cancerous tissue has a higher mitotic index. Lab 11: Punnett Squares Analysis The answers are intended only as samples. Answers also depend on how students completed the lab and on the random data generated by the computer. 1. For one of the monohybrid crosses you performed in this Investigation, describe how to use the phenotype ratios to determine the percentage of offspring displaying each trait. The percentage of offspring displaying each trait is determined by performing a monohybrid genetic cross using a Punnett square to determine the ratio of phenotypes that result. The ratio of phenotypes is determined by counting the number of offspring that display each trait and then reducing, if possible. For example, in Scenario 1, 2 graybodied flies:2 black-bodied flies result. This can be reduced to a 1:1 ratio. One out of two, or 50%, of the offspring are gray-bodied flies. One out of two, or 50%, of the

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offspring are black-bodied flies. 2. Can the genotype for a gray-bodied fly be determined? Why or why not? Describe all of the possible genotypes for a fly with that phenotype. No, it is difficult to determine the genotype for a gray-bodied fly because gray body color is a dominant trait in Drosophila. Thus, the fly could be homozygous or heterozygous for this trait. The possible genotypes for a gray-bodied fly are GG and Gg. The genotypes could be determined by doing back crosses. 3. Explain why an organism with a homozygous dominant genotype has the same phenotype as an organism with a heterozygous genotype. The dominant form of the trait always appears in heterozygous organisms. Therefore, an organism with a heterozygous genotype has the same phenotype as an organism with a homozygous dominant genotype. 4. What genetic information can be obtained from a Punnett square? What genetic information cannot be determined from a Punnett square? A Punnett square determines all of the ways in which alleles can combine. It may be used to predict ratios of offspring genotypes and phenotypes. Punnett squares can’t determine actual outcomes. They can only predict trends. Lab 12: Sex-Linked Traits Analysis The answers are intended only as samples. Answers also depend on how students completed the Investigation and on the random data generated by the computer. 1. Describe the phenotypes and genotypes of the parents that you chose on the Punnett squares.
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