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Unformatted text preview: Pictu re 4 0 EXAM 1 KEYS TO EXAM VERSIONS 1-4 (A-D): VERSION I OR A 1. A _____________ is a cross between an unknown and a homozygous recessive. a. testcross b. dihybrid c. monohybrid d. backcross e. controlled Vocabulary question 2. In a dihybrid cross AAbb aaBB, what proportion of homozygotes is expected among the F 2 offspring? a. 1/4 b. 1/2 c. 3/4 d. All are homozygotes. e. None are homozygotes. The most common mistake here was failure to recognize that progeny that are AA bb or aa BB are also homozygotes in addition to those that are AA BB and aa bb. Separate this problem into two monohybrid crosses. A mating between AA and aa individuals yields Aa in the F1 generation. Mating Aa individuals together produces 1/4 AA : 1/2 Aa : 1/4 aa. That is, 1/2 of the progeny will be AA or aa. Similarly, 1/2 of the progeny will be BB or bb for that monohybrid cross. Since these are independent events, they must be multiplied together to obtain the probabilities for both, i.e., P = 1/2 X 1/2 = 1/4. The homozygous genotypes are AABB, aabb, AAbb, and aaBB. 3. A diploid somatic cell from a rat has a total of 42 chromosomes (2 n = 42). What is the total number of chromosomes present in the cell during metaphase I of meiosis? a. 21 b. 42 c. 84 d. 126 e. 168 Reduction division has not occurred in metaphase I of meiosis. There will be 84 chromatids, but 42 chromosomes since the centromeres have not split at this stage. 4. The figure shows a chromosomal separation taking place. The letters stand for genes; capital and lowercase stand for different alleles. The diploid chromosome number in this organism is four. What process is shown? a. anaphase of mitosis b.metaphase of meiosis I c. anaphase of meiosis I d. telophase of mitosis e. anaphase of meiosis II The diploid number of chromosomes is 4. Here we see two chromosomes in the process of separating identical chromosomes (chromatids). Therefore this must be anaphase II. Had this been anaphase of meiosis I, homologs, each with sister chromatids intact, would be 1 segregating from one another. If this had been anaphase of mitosis, then you would have all four chromosomes separately undergoing chromatid separation. 5. If a trihybrid plant of genotype Aa Bb Cc was test-crossed, what proportion of the progeny would be aa Bb cc ? A. 3/16 B. 1/8 C. 1/6 D. 3/64 E. None of these answers are correct. There are 3 genes in this test cross, Aa Bb Cc X aa bb cc. In monohybrid test crosses 1/2 of the progeny will be homozygous recessive and 1/2 will be heterozygous. For example, Aa X aa will yield 1/2 Aa and 1/2 aa progeny. For the probability of genotype aa Bb cc, all independent events, 1/2 X 1/2 X 1/2 = 1/8. 6. Sickle cell anemia is an autosomal recessive trait in humans. In a cross between a father who has sickle cell anemia and a mother who is heterozygous for the gene, what is the probability that their first three children will be normal?...
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This note was uploaded on 10/03/2010 for the course BSC 315 taught by Professor O'donnell during the Fall '10 term at Alabama.
- Fall '10