CELL DIVISION AND HEREDITY Student Packet SUMMARY IN EUKARYOTES, HERITABLE INFORMATION IS PASSED TO THE NEXT GENERATION VIA PROCESSES THAT INCLUDE THE CELL CYCLE, MITOSIS /MEIOSIS AND FERTILIZATION Mitosis alternates with interphase in the cell cycle. Interphase consists of three phases: growth, synthesis of DNA and preparation for mitosis. The cell cycle has specific checkpoints where the cell cycle stops until a go-ahead signal is received; both internal and external signals control these checkpoints via signal transduction pathways and key regulatory proteins include kinases and cyclins. Most cells exhibit density-dependent inhibition as well as anchorage dependence. When a cell specializes, it often enters into a stage where it no longer divides, but it can reenter the cell cycle when given appropriate clues. Nondividing cells may exit the cell cycle or hold at a particular stage in the cell cycle. Chromosome duplication and distribution during cell division Mitosis passes a complete genome from the parent cell to daughter cells. It occurs after DNA replication and is followed by cytokinesis. The resulting daughter cells are genetically identical. Mitosis plays a role in growth, repair and asexual reproduction. Mitosis is a continuous process with observable structural features. Meiosis, a reduction division, ensures genetic diversity in sexually reproducing individuals. During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles. This process is called independent assortment and contributes to genetic variation in populations. Separation of the homologous chromosomes ensures that each gamete receives a haploid set (1n) of chromosomes composed of both maternal and paternal chromosomes. During meiosis, homologous chromatids exchange genetic material via a process called crossing over, which also increases genetic variation in the resulting gametes. Following meiosis, fertilization involves the fusion of two gametes and increases genetic variation in populations by providing for new combinations of genetic information in the zygote. Fertilization also restores the diploid number (2n) of chromosomes. Genetic variation is the raw material for evolution. Mutations are the original source of this variation and recombination of genes (due to meiosis and fertilization) generates additional diversity. 1
THE CHROMOSOMAL BASIS OF INHERITANCE PROVIDES AN UNDERSTANDING OF THE PATTERN OF TRANSMISSION OF GENES FROM PARENT TO OFFSPRING Rules of probability can be used to analyze the passage of traits from parent to offspring. The laws of segregation and independent assortment can be applied to genes that are on different chromosomes. Genes that are located close to each other on the same chromosome are called linked genes and they tend to move as a unit; the probability that they will segregate is a function of the distance between them. The pattern of inheritance can often be predicted from data that gives the information about parent and/or offspring genotypes/phenotypes. Certain human genetic disorders can be attributed to the inheritance of single gene traits or specific chromosomal abnormalities. Many ethical, social and medical issues surround human genetic disorders. THE INHERITANCE PATTERNS OF MANY TRAITS CANNOT BE EXPLAINED BY SIMPLE MENDELIAN GENETICS Many traits are products of multiple genes and/or physiological processes. Patterns of inheritance of many traits do not follow ratios predicted by Mendel s laws and can be identified by quantitative analysis, where observed phenotypic ratios statistically differ from the predicted ratios. Some traits are determined by genes on sex chromosomes. Sex-linked genes reside on sex chromosomes (X in humans). In mammals and flies, the Y chromosome is very small and carries few genes. Some traits result for nonnuclear inheritance. Chloroplasts and mitochondria are randomly assorted to gametes and daughter cells and as a result, traits determined by chloroplast and mitochondrial DNA do not follow simple Mendelian rules. In animals, mitochondrial-determined traits are maternally inherited because mitochondrial DNA is transmitted by the egg and not by sperm. CHANGES IN GENOTYPE CAN RESULT IN CHANGES IN PHENOTYPE Errors in mitosis or meiosis can result in changes in phenotype. Changes in chromosome number often result in new phenotypes, including sterility caused by triploidy or increased vigor in some polyploids. The expression of genotype may be altered by environmental influences. For example, nutrition can affect height and weight in humans, soil ph determines flower color in some plants and increased UV light influences skin color in animals. 2
MULTIPLE CHOICE QUESTIONS 1. Researchers were studying how mitosis passes a complete genome from the parent cell to daughter cells. They observed cells of onion root tips during different stages of mitosis. In one of the cells, chromosomes appeared as sister chromatids dispersed in the cell and kinetochores were attached to the mitotic spindle. Which of the following statements best describes the process that will happen next? a. Homologous chromosomes, each with two sister chromatids, will move towards opposite poles of the cell. b. Sister chromatids will line up at the metaphase plate, a plane that is equidistant between the cell s two poles. c. Nuclear envelope will form and chromosomes will become less condensed. d. Sister chromatids will separate. 2. When nutrients are low, cells of the budding yeast Saccharomyces cerevisiae exit the mitotic cell cycle and enter meiosis. Researchers grew a culture of yeast cells in a nutrient-rich medium and then transferred them to a nutrient-poor medium to induce meiosis. At different times after induction, the DNA content per cell was measured in a sample cell and the average DNA content per cell was recorded in femtograms (1 fg = 10-15 gram). Time after Induction (hours) Average Amount of DNA per Cell (fg) 0 2 4 6 8 10 12 14 24 40 47.5 48 24 13 12 12 Which of the following processes are yeast cells undergoing at 8 hours? a. DNA Replication b. Mitosis c. Meiosis I d. Meiosis II 3. Which of the following statements correctly explains the process depicted in the picture below? a. Non-sister chromatids exchange genetic material and produce recombinant chromosomes in early meiosis. b. Maternal sister chromatids complete crossing over which leads to chromosomes with new combinations of alleles. c. Homologous chromosomes are randomly oriented at the metaphase plate and separation of the chromosomes ensures that each gamete receives a haploid set of chromosomes. d. The spindle fibers attach to chiasmata during an early stage of mitosis and then move the chromosomes to the equator of the cell. 3
4. If these four cells resulted from cell division of a single cell with diploid chromosome number 2n = 4, what process has just occurred? a. Normal meiosis b. Translocation c. Nondisjunction d. Inversion 5. In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous offspring of homozygous red and homozygous white cattle. When two roan cattle are crossed, the phenotypes of the offspring are found to be in the ratio of 1 red : 2 roan : 1 white. Which of the following crosses could produce the highest percentage of roan cattle? a. Roan cattle x roan cattle b. White cattle x roan cattle c. Red cattle x roan cattle d. Red cattle x white cattle 6. In rabbits, the trait for short hair (S) is dominant and the trait for long hair (s) is recessive. The trait for green eyes (G) is dominant and the trait for blue eyes (g) is recessive. A cross between two rabbits produces a litter of six short-haired rabbits with green eyes and two short-haired rabbits with blue eyes. What is the most likely genotype of the parent rabbits in this cross? a. ssgg x ssgg b. SsGg x SSGg c. SSGG x SSGG d. SsGg x SsGg 7. In the pedigree below, circles represent females, squares represent males and shaded figures represent individuals expressing a specific trait. The expression of this trait is most likely due to which of the following? a. Autosomal dominant inheritance b. Autosomal recessive inheritance c. Sex-linked dominant inheritance d. Sex-linked recessive inheritance 4
8. Some traits result from expression of DNA that is not located in the nucleus. Which of the following is an incorrect statement about nonnuclear inheritance? a. Mitochondrial-determined traits are maternally inherited because the male gametes deliver very little cytoplasm. b. Maternal inheritance can be used to trace a specific genome from progeny back through multiple generations to its original mother. c. Traits determined by chloroplast and mitochondrial DNA follow Mendelian rules. d. Chloroplasts and mitochondria are randomly distributed to gametes and daughter cells. 9. Seasonally, many mammals change color of their coats, such as the arctic fox, long-tailed weasel, snowshoe hare, white-tailed jackrabbit, Siberian hamster, etc. Which of the following is most likely responsible for the seasonal change in coat color? a. The amount of daylight and temperature influences the expression of the coat color genes. b. High spring and summer temperatures denature the pigment molecules in the lightly colored winter coat, causing the coat to change color. c. The diet of animals changes with seasons and this contributes to changes in coat color. d. Sexual selection favoring a darker spring coat causes animals to change their coat color. MATH GRID IN 1. Karen and Steve each have a sibling with cystic fibrosis. Neither Karen nor Steve nor any of their parents have the disease and none of them have been tested to see if they have the cystic fibrosis gene. Based on this incomplete information, calculate the probability that if this couple has a child, the child will have cystic fibrosis. Give your answer as a value between 0 and 1, to three decimal places. 5
2. What is the probability of producing offspring with the genotype AaBBCcdd if the father s genotype is AaBbCcDd and the mother s genotype is AaBBccDd? Assume independent assortment of all gene pairs. Give your answer as a fraction. SHORT FREE RESPONSE QUESTIONS 1. Fruit flies (Drosophila melanogaster) with a wild-type phenotype have gray bodies and red eyes. Certain mutations can cause changes to these traits. Mutant flies may have a black body and/or cinnabar eyes. To study the genetics of these traits, a researcher crossed a true-breeding wild-type male fly (with gray body and red eyes) with a true-breeding female fly with a black body and cinnabar eyes. All of the F 1 progeny displayed a wildtype phenotype. Female flies from the F 1 generation were crossed with true-breeding male flies with black bodies and cinnabar eyes. The table below represents the predicted outcome and the data obtained from the cross. Explain the difference between the expected data and the actual numbers observed. F 2 Generation Phenotypes Body Color Eye Color Number Predicted Number Observed Gray Red 244 455 Black Cinnabar 244 432 Gray Cinnabar 244 42 Black Red 244 47 6
2. The cell cycle is fundamental to the reproduction of eukaryotic cells. Describe how the cell cycle is regulated and discuss ONE consequence of abnormal regulation. LONG FREE RESPONSE QUESTION In fruit flies, the phenotype for eye color is determined by a certain locus. E indicates the dominant allele and e indicates the recessive allele. The cross between a male wild-type fruit fly and a female white-eyed fruit fly produced the following offspring. Wild-type Wild-type White-eyed White-eyed Brown-eyed Male Female Male Female Female F 1 0 45 55 0 1 The wild-type and white-eyed individuals from the F1 generation were then crossed to produce the following offspring. F 2 23 31 22 24 0 a. Determine the genotypes of the original parents and explain your reasoning. You may use Punnett squares to enhance your description, but the results from the Punnett squares must be discussed in your answer. b. Use a Chi-squared test on the F 2 generation data to analyze your prediction of the parental genotypes. Show all your work and explain the importance of your final answer. c. The brown-eyed female in the F 1 generation resulted from a mutational change. Explain what a mutation is and discuss two types of mutations that might have produced the brown-eyed female in the F 1 generation. 7
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