1. The diagrams below show two different scenarios for a pair of homologous chromosomes, known as a tetrad, undergoing a change where segments of DNA switch on parts of the chromosomes. In each scenario, the cell containing the tetrad is about to undergo a meiotic division. The four chromosomes are labeled 1 through 4 in the diagrams. Which of these best predicts the outcome of the changes illustrated in the diagrams? (A) In scenario A, DNA is switching in two different chromosomes, which may result in a missing sex cell or sex cells with duplicate genes, whereas in scenario B, DNA is switching on homologous chromosomes, which will likely result in no change since each chromosome still contains the same set of genes. (B) There will be no change in the chromosomes in either scenario because, in scenario A, the DNA is switching in homologous chromosomes, which have the same set of genes, and in scenario B, DNA is switching on sister chromatids, which are identical chromosomes. (C) In scenario A, DNA is switching in homologous chromosomes, which will create new combinations of alleles in the gametes, whereas in scenario B, DNA is switching on sister chromatids, which will result in no change since the chromosomes are identical. (D) There will be some genetic change in the gametes produced in scenario A because the DNA is switching in two different chromosomes; however, there will be significant genetic changes in the gametes produced in scenario B because genetic recombination is occurring in two sets of chromosomes. Page 1 of 5
2. The graphs below show changes in chromosome number for two different cells undergoing meiosis. Which description best explains the differences observed in the graphs? (A) Cell 1 is undergoing normal meiosis, whereas cell 2 is not undergoing normal meiosis because either homologous chromosomes or sister chromatids fail to separate to produce haploid cells. (B) Cell 2 is undergoing normal meiosis, whereas cell 1 is not undergoing normal meiosis because it has too few chromosomes after replicating its chromosomes and completing the second division. (C) Cell 1 is undergoing normal meiosis, whereas cell 2 is completing meiosis but with too many chromosomes because a pair of homologous chromosomes failed to separate and resulted in a trisomy. (D) Cell 2 is undergoing normal meiosis, whereas cell 1 is undergoing mitosis followed by meiosis because the cell first divided normally and then divided again to reduce the chromosome number by half in the daughter cells. Page 2 of 5
3. In fruit flies (Drosophila melanogaster), the allele for miniature wings (m) is recessive to the allele for normal wings (M), and the gene for vermilion eyes (v) is recessive to the allele for normal eyes (V). A female who is heterozygous for both traits is crossed with a vermilion-eyed, miniature-winged male. The phenotypes of their offspring are described in the table below. Which statement best explains the results observed in the Drosophila cross? (A) The data support Mendel s Rule of Independent Assortment because the outcome is close to the expected 50% for both dominant traits and 50% for both recessive traits for a cross involving two independent traits. (B) The data suggest that the two genes are linked since the dominant traits (normal eyes and wings) normally appear together and the recessive alleles (miniature wings and vermilion eyes) normally appear together in the offspring. (C) The data indicate that the allele is most likely sex-linked because, in the F 1 generation, female offspring express different phenotypes than male offspring, which occurs when genes are found on the X chromosome. (D) The data show that the two genes are linked and that they segregate only when crossing over occurs at a location between them on the chromosome, resulting in a small number of offspring showing one dominant and one recessive phenotype. Page 3 of 5
4. Sordaria fimicola is a fungus that exists as a haploid organism through most of its life cycle and becomes diploid when the fusion of two haploid nuclei forms a diploid nucleus. The diploid nucleus must then undergo meiosis to restore its haploid state. Meiosis, followed by mitosis, in Sordaria results in the formation of eight haploid ascospores contained within a sac called an ascus (plural, asci). Each spore can develop into a new haploid fungus. The diagram below shows meiosis in a nucleus created by the fusion of a wild-type Sordaria with black ascospores (+) and a mutant strain with tan spores (tn). The ascus that develops contains four black ascospores and four tan ascospores. The normal arrangement of the eight ascospores in the ascus after meiosis and mitosis is shown below. Crossing over often occurs in Sordaria during meiosis. A set of asci is shown below. Page 4 of 5
Which statement best explains the meiotic process that produced these asci? (A) The evidence shows that crossing over has not occurred in the asci since all the ascospores are either black or tan, but not both at the same time. (B) The evidence shows that crossing over has occurred in two asci since there is an alternating pattern of two black and two tan ascospores in the ascus, indicating that crossing over occurred during meiosis I. (C) The evidence shows that crossing over has occurred in six asci since those asci have black and tan phenotypes that are not in the expected location given a normal meiotic division. (D) The evidence shows that crossing over has occurred in fourteen of the asci since those asci do not have four black ascospores on the top and four tan ascospores on the bottom of the ascus. Page 5 of 5