In asexual reproduction, new combinations of genes arise by
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mutation
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Which reproductive strategy would allow the most rapid adaptation to a changing environment?
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sexual reproduction
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Asexually produced daughter cells are
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identical to each other and identical to the mother cell
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Sexual reproduction
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results in new combinations of genetic traits.
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Which process is absolutely necessary for sexual reproduction to occur in a life cycle, but is not necessarily required for organisms that only reproduce asexually?
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meiosis
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The essence of meiosis is that cells are formed that:
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receive one member of each pair of homologous chromosomes.
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If meiosis did not occur in sexually reproducing organisms, ____.
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the chromosome number would double in each generation
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A parent cell of a certain organism has 16 chromosomes and undergoes meiosis. The resulting cells will have how many chromosomes?
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8
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Homologous chromosomes may differ from each other in terms of:
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alleles
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B and D
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Identify the locations in Exhibit 12.3 where eggs (ova) are produced.
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A and C
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Identify the locations in Exhibit 12.3 where sperm are produced
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Copies of chromosomes linked together at their centromeres at the beginning of meiosis are called ____ chromatids.
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sister
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Meiosis typically results in the production of
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4 haploid cells
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During meiosis II
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sister chromatids of each chromosome are separated from each other.
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The chromosomal DNA is duplicated in which stage of the cell cycle?
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interphase
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The sister chromatids become separated during ____ of meiosis.
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anaphase II
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Each of the cells formed after meiosis I are:
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haploid.
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Sister chromatids are joined at the centromere in all of the stages of meiosis listed below except
a. metaphase II.
b. anaphase II.
c. telophase I.
d. metaphase I.
e. prophase II.
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b. anaphase II.
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Four of the five answers listed below concern cells with two chromosome sets. Select the exception.
a. zygote
b. somatic cells
c. gamete
d. diploid
e. skin cells
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c. gamete
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Four of the five answers listed below are characteristic of meiosis. Select the exception.
a. involves two divisions
b. reduces the number of chromosomes
c. results in producing genetically identical cells
d. produces haploid cells
e. occurs in the gonads
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c. results in producing genetically identical cells
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Four of the five answers listed below are haploid cells. Select the exception.
a. zygote
b. spore
c. egg
d. sperm
e. gamete
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a. zygote
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Homologous chromosomes separate during
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anaphase I.
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Crossing over usually occurs in which of the following stages of meiosis?
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prophase I
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The process of 'crossing over':
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involves breakages and exchanges being made between homologous chromosomes
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During which phase of meiosis will the chromosomes appear as closely associated packets of four chromatids?
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prophase I
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Crossing over is one of the most important events in meiosis because
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it produces new combinations of alleles on chromosomes.
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Which of the following is most probable at a metaphase I alignment?
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Each side has a mixture of maternal and paternal chromosomes.
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If a diploid organism has a genome consisting of 4 chromosomes, it can produce ____ different combinations of chromosomes (disregarding crossing over).
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16
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A child more strongly resembles the maternal grandmother than the maternal grandfather. One explanation could be due to random chromosome alignment during which stage of the mother's gamete formation?
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metaphase I
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Gamete formation
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involves the formation of sex cells.
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Compare mitosis and meiosis. Which statement is accurate?
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Meiosis II resembles mitosis
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The main evolutionary advantage of sexual over asexual reproduction is that it produces _____.
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more variation among offspring
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Meiosis functions in _____.
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sexual reproduction
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Meiosis _____ the parental chromosome number.
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halves
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Crossing over mixes up _____.
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alleles on homologous pairs
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The stage of meiosis that makes descendant cells haploid is _____.
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anaphase I
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Dogs have a diploid chromosome number of 78. How many chromosomes do their gametes have?
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39
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What is one of the very important differences between mitosis and meiosis?
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Homologous chromosomes pair up and exchange genetic material only in meiosis.
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DNA duplication occurs in this phase.
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interphase
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Homologous chromosomes separate; move to opposite poles
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anaphase I
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Spindle formation in two cells
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prophase II
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Chromatids separate, move to opposite poles.
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anaphase II
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As in mitosis, duplicated chromosomes align on the equator.
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metaphase I
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Tertrad of homologous chromosomes align on the equator.
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metaphase II
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Homologous chromosomes associate, crossing over occurs, chromosomes condense.
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prophase I
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Two cells formed; each has one of each duplicated chromosome.
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telophase I
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Four cells formed; Each cell has one of each unduplicated chromosome.
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telophase II
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Recombination via crossing over occurs during this stage.
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prophase I
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The number of homologous chromosomes is reduced by half at the end of this phase.
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anaphase I
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During this stage the sister chromatids separate.
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anaphase II
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. Following this phase, each individual cell is haploid.
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telophase I
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During this phase the homologous chromosomes separate.
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anaphase I
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may be none between meiosis I and meiosis II
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. interphase
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fusion of two gametes
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fertilization
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different molecular form of a gene
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allele
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haploid
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gamete
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First
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germ cell
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Second
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meiosis
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Third
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gamete
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Fourth
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fertilization
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Fifth
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zygote
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germ cell
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46, diploid
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gamete
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23, haploid
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daughter cell after meiosis I
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23, haploid
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daughter cell after meiosis II
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23, haploid
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daughter cell after mitosis
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46, diploid
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How do the products of meiosis differ from those of mitosis?
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Meiosis starts with a diploid cell and produces four genetically different haploid cells. Meiosis is used for gamete formation. Mitosis may start with a diploid cell and produces two cells that are genetically identical to each other and to the original cell. Mitosis is used for growth and repair.
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Why is meiosis necessary for sexual reproduction?
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If the chromosome number were not reduced from the diploid to the haploid number before gametes are produced, each succeeding generation would have twice the number of chromosomes as the generation before
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The cell in the diagram below is in anaphase I, not anaphase II. How do we know this?
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The sister chromatids are still attached.
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What adaptation have the freshwater bdelloid rotifers used to allow them to acquire genetic diversity while reproducing only via asexual reproduction?
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These rotifers can import genes from bacteria, fungi, protists, and even plants. Each rotifer is a genetic mosaic whose DNA spans almost all the major kingdoms of life: About 10 percent of its active genes have come from other organisms.
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Genes for specific traits (e.g., hair color) are located at specific spots on a chromosome. Is it possible for a maternal allele for blonde hair to be switched to the paternal chromosome of the homologous pair during meiosis?
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Yes, genes on maternal and paternal chromosomes of a homologous pair can be switched during the process of crossing over during prophase I of meiosis.
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If the process of 'zipping up' of chromosomes before crossing over were impaired in germ cells, would all of the resulting sperm have the exact same DNA, with no diversity? Why or why not?
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No, because the process of chromosome segregation would still produce sperm with different combinations of maternal and paternal chromosomes. The maternal and paternal chromosomes of a homologous pair randomly sort into gametes during the formation of each gamete.
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Imagine that allele "A" of a gene is present on the maternal chromosome and allele "a" is present on the paternal chromosome. When do the alleles "A" and "a" get separated into daughter cells? Does this occur after meiosis I or meiosis II, and why?
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Homologous chromosomes are separated during meiosis I. Therefore, alleles "A" and "a" would be separated into different daughter cells after meiosis I.
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Why can we predict that meiosis tends to give rise to greater genetic diversity among offspring in fewer generations than asexual reproduction?
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Meiosis produces recombination of genes leading to genetic differences by: (1) crossing over during prophase I, and (2) independent assortment of the chromosomes during metaphase I. In addition, random fertilization creates a limitless possibility of combinations resulting from the mating.
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The diploid chromosome number for the body cells of a frog is 26. What would that number be after three generations if meiosis did not occur before gamete formation?
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For the first generation, the frogs would have the usual 26 chromosomes in the diploid stage. The second generation would have twice this many - 52 chromosomes - if meiosis did not reduce the gamete number before chromosome number. Assuming two second generation frogs mated together, the third generation would have 104 chromosomes. You can see how this rapidly spirals out of control!
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Describe events that lead to genetic variety in sexual reproduction.
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1. Fertilization occurs between an egg and sperm from two genetically different individuals. The resulting zygote has a new combination of paternal and maternal homologous chromosomes.
2. Crossing over during meiosis. This allows new combinations of alleles to form on chromosomes. It mixes paternal and maternal chromosome contents of each homologous pair. Each time a germ cell undergoes meiosis in an individual, crossing over is different, leading to genetically diverse gametes in one individual.
3. Random assortment of chromosomes before segregation. A variety of combinations of paternal or maternal homologues separate into daughter cells each time meiosis occurs.
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