Biology 105: Introduction to Genetics Page 1 Midterm Fall 2004

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1 Biology 105: Introduction to Genetics Page 1 Midterm Fall 2004 KEY Part I. Definitions Hemizygous: The genotype for genes present only in one copy in an otherwise diploid organism; e.g. X-linked genes in a male. Prophase I: Beginning of meiosis I; chromosomes condense and crossing-over occurs. Karyotype: Visual description of the complete set of chromosomes in one cell of an organism. Interference: A phenomenon where a cross-over in one part of the chromosome lowers the probability of a second cross-over nearby. Mitosis: A form of cell division where one diploid parent cell yields two genetically identical diploid daughter cells.

2 Part II Short Answer (Answer 5 out of the 6) 1. In humans, achondroplastic dwarfism and neurofibromatosis are extremely rare autosomal dominant conditions. If a woman with achondroplasia marries a man with neurofibromatosis, what genotypes and phenotypes could be predicted in their children and in what proportions? (these genes are unlinked) f A / + ; + / + x m + / + ; N / + m f A ; + + ; + 1/4 normal + / + ; + / + A / + ; + / + ; 1/4 dwarf A / + ; + / + + ; N + / N + / N 1/4 neuro + / + ; + / N 1/4 dwarf A / + ; + / N + neuro A / + ; + / + ; + ; + + / + + / + 2. A MU B MU C An AABBCC female is crossed with an aabbcc male and the resulting F1 progeny are backcrossed to aabbcc individuals. What fraction of the F2 will be heterozygous for all three genes? f I II A B C a b c a b c a b c m 0.80 x 0.95 x 0.5 = 0.38 NO cross overs in regions I AND II only want progeny with ABC gamete from female

3 3. A unique species of fish containing only one large chromosome is discovered. Describe the ratios and classes of progeny (with respect to ploidy) produced when a triploid member of this species is crossed to a diploid member of this species. 1/2 progeny will be triploid 1 /2 progeny will be diploid 4. For two genes far apart on the same chromosome, 50MU is the maximum distance. Why? Recombination occurs at the 4 strand stage and involves only 2 of 4 chromatids. 5. A couple went to see a genetic counselor because each has a sibling affected with albinism. Albinism is an autosomal recessive disease and neither member of the couple nor any of their four parents is affected. What are the chances that their child will be a carrier of albinism? chance that both parents are a/+ a/+ a/+ a/+ carriers + child is a carrier 2/3 x 2/3 x 1/2? add together a + a aa a+ + a+ ++ chance that female parent is a carrier + child is a carrier 2/3 x 1/3 x 1/2 chance that male parent is a carrier + child is a carrier 2/3 x 1/3 x 1/2 = 4/9

4 6. In genetically identical twin females heterozygous for a recessive X-linked eye disease gene, it was discovered that one individual was normal while the other expressed a mild form of the disease. How can these results be explained? This is the result of random X-chromosome inactivation. By chance, in one of the twins, the normal X-chromosome was preferentially inactivated in eye cells. Part III Multiple Choice 1. ( d ) A variable feature of some chromosomes in corn is the presence or absence of cytologically visible knobs. Suppose that one member of each of two pairs of chromosomes in a corn plane has a knob. If this plants were crossed with a knobless plant, what percentage of the offspring is expected to be knobless? a. 100% b. 75% c. 50% d. 25% e. 0% 2. ( e ) Which of the following events occurs during mitosis but not meiosis? a. Segregation of sister chromatics b. Pairing of homologous chromosomes c. Recombination between homologous chromosomes d. Alignment of chromosomes on the metaphase plate e. None of the above 3. ( a ) In the dihybrid cross AAbb x aabb (A and B are unlinked genes), what proportion of the F2 offspring will be homozygous for the dominant form of both genes? a. 1/16 b. 1/4 c. 3/16 d. 6/16 e. 9/16 4. ( c ) In four o clock plants, the allele for red flowers is incompletely dominant over the allele for white flowers so that the heterozygous are pink. What ratio of flower colors would you expect among the offspring, if pink flowers are crossed to red flowers? a. 100% pink b. 25% pink, 75% red c. 50% pink, 50% red d. 75% pink, 25% red e. none of the above

5 5. ( d ) Sister chromatids are genetically, while homologous chromosome pairs are genetically. a. similar, identical b. distinct, identical c. identical, identical d. identical, similar e. none of the above 6. ( c ) D. melanogaster has four pairs of chromosomes. Sperm from this species are formed by a meiotic process in which homologous chromosomes pair and segregate but do not undergo recombination. How many genetically different kinds of sperm could be produced by a D. melanogaster male? a. 4 b. 8 c. 16 d. 64 e ( e ) A trait which is passed father-to-son generation after generation is most likely to be a. X-linked recessive b. X-linked dominant c. Autosomal recessive d. Autosomal dominant e. none of the above 8. ( d ) All of these statements are true about mitosis except a. results in two genetically identical products b. occurs in the somatic cells c. although recombination is rare, it occurs d. homologs pair e. sister chromatids segregate at anaphase 9. ( b ) A species of insect is discovered in which the rate of meiotic recombination is three times higher in females than in males. By exclusively mapping in males, genes A and C are found to be separated by 12MU. If the mapping were performed in females the expected map distance would be: a. 12MU b. 36MU c. 4MU d. 3MU e. need more information

6 10. ( c ) In the following pedigree, the indicated trait is caused by what type of allele? a. autosomal recessive b. autosomal dominant c. X-linked recessive d. X-linked dominant e. Y-linked 11. ( a ) In the following pedigree, the indicated trait is caused by what type of allele? a. autosomal recessive b. autosomal dominant c. X-linked recessive d. X-linked dominant e. Y-linked 12. ( e ) What biological process did the Punnett Square represent? a. Segregation of sister chromatids b. Segregation of homologous chromosomes c. Crossing over between homologous chromosomes during prophase I of meiosis d. Independent alignment of different homologous pairs on the metaphase I of spindle e. Random fusion of sperm and eggs during fertilization 13. ( b ) Mitotic recombination results in loss of heterozygosity from a. the point of recombination to the centromere b. the point of recombination to the telomere c. the centromere to the telomere d. 15 MU past the point of recombination e. 50 MU past the point of recombination

7 Part IV I. Sparkled eyes (sp) and wavy wings (wy) are autosomal recessive traits of Drosophila. A homozygous wild-type (normal) females was crossed to a sparkled, wavy male and the F1 males were then crossed to the F1 females to obtain the F2. Of 613 F2 obtained, 449 were wildtype, 14 were sparkled, 15 were wavy and 135 were sparkled, wavy. Remember, there is no recombination in Drosophila males. A) Using a diagram, explain the origin of these four classes P: f sp+ wy+ sp wy m sp+ wy+ sp wy m F1: sp+ wy+ sp+ wy+ f m No recombination sp wy sp wy parental recombinant gametes sp+ wy+ sp wy sp+ wy sp wy+ sp+ wy+ Wild type Wild type Wild type Wild type sp wy Wild type sp wy (135) wy (15) sp (14) f in males B) Determine the map distance between these two genes (show reasoning) RF sp-wy = # recombinants / # total progeny = (15+14)x2 / 613 = m.u. C) If recombination did occur in Drosophila males what additional phenotypic and genotypic classes (if any) would be present in the F2. Explain your reasoning. M Recombinant gametes F gametes gametes sp+ wy+ sp wy sp+ wy sp wy+ sp+ wy sp+ wy+/ sp+ wy sp wy / sp+ wy sp+ wy / sp+ wy sp wy+ / sp+ wy sp wy+ sp+ wy+ / sp wy+ sp wy / sp wy+ sp+ wy / sp wy+ sp wy+ / sp wy+ No additional phenotypic classes would be observed. Additional genotypic classes: sp+wy / sp+wy, sp wy+ / sp+ wy, sp+ wy / sp wy+, sp wy+ / sp wy+

8 Part IV (continued) II. Large numbers of offspring from a cross of true-breeding no-wing female Drosophila to normal winged Drosophila males are scored (normal wing is dominant to no-wing). The following results are obtained: 1005 normal-winged females, 999 no-wing males, 3 no-wing females, 2 normal wing males. A) What chromosome is the no-wing gene likely to reside on? Explain your reasoning. The gene resides on the X-chromosome. It s the only way in which virtually all the male progeny could inherit the mutation while none of the female did. B) Using a diagram (Punnett Square), explain the origin of the two rare progeny classes described above. The rare progeny classes arise due to nondisjunction: X Y X nw X nw X nw X nw X X nw X nw Y (dies) (nw f) O XO OY (wt m) (dies) C) Describe a genetic or cytological experiment to test your proposed answer for part B. Cytological experiment: karyotype the rare progeny and see if it s possible to detect the presence Y chromosome in the no-wing female and the absence of the Y chromosome in the normal winged male. Genetic experiment: test-cross rare wild-type males to parental nowing females (or any other fly). The XO male should be sterile and produce no progeny.

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