1). Fertilization results in
a). a zygote.
b). a diploid cell.
c). a cell with a new genetic combination.
d). All of these are correct.
2). The diploid number of chromosomes in humans is 46. The haploid number is
3). After chromosome replication and during synapsis,
a). homologous chromosomes pair along their lengths.
b). sister chromatids pair at the centromeres.
c). homologous chromosomes pair at their ends.
d). sister chromatids pair along their lengths.
4). During which stage of meiosis does crossing over occur?
a). prophase I
b). anaphase I
c). prophase II
d). telophase II
5). Synapsis is the process whereby
a). homologous pairs of chromosomes separate and migrate toward a pole.
b). homologous chromosomes exchange chromosomal material.
c). homologous chromosomes become closely associated.
d). the daughter cells contain half of the genetic material of the parent cell.
6). Terminal chiasmata are seen during which phase of meiosis?
a). anaphase I
b). prophase I
c). metaphase I
d). metaphase II
7). Which of the following occurs during anaphase I?
a). Chromosomes cluster at the two poles of the cell.
b). Crossing over occurs.
c). Chromosomes align down the center of the cell.
d). One version of each chromosome moves toward a pole.
8). Mitosis results in two ________ cells, while meiosis results in _______ haploid cells.
9). Genetic diversity is greatest in
b). sexual reproduction.
c). asexual reproduction.
d). binary fission.
10). Which of the following is not a hypothesis about the evolution of sex?
a). It evolved to repair damaged DNA.
b). It evolved as a way to eliminate individuals.
c). It evolved as a way to eliminate mutations.
d). It evolved as a way to "store" recessive alleles that may prove beneficial in the future.
Test Your Visual Understanding
1). Match the following labels with the correct numbers on the figure (Some labels may be used more than once.)
a). diploid stage (2n)
d). haploid stage (n)
2). Human cells spend most of their life cycles in a diploid stage, with only a selected number of cells undergoing meiosis to become haploid. Other organisms spend most of their life cycles in the haploid stage. Look ahead in this text to figures 28.14, 29.13, 29.15, and 30.10. Which of these organisms has a life cycle that is primarily haploid?
Answer: Figures 28.14 and 30.10 are organisms that spend a majority of their life cycles in a haploid stage of development. In figure 28.14, only the briefly formed zygospore is diploid. In figure 30.10, only the briefly formed zygosporangium is diploid.
Apply Your Knowledge
1). An organism has 56 chromosomes in its diploid stage. Indicate how many chromosomes are present in the following, and explain your reasoning:
a). somatic cells
b). metaphase (mitosis)
c). metaphase I (meiosis)
d). metaphase II (meiosis)
a). 56 chromosomes would be present in somatic cells, 28 homologous pairs?somatic cells are diploid and would have the full complement of chromosomes.
b). 112 chromosomes would be present in mitotic cells at metaphase. Prior to metaphase, the chromosomes replicated and so every chromosome is a pair of identical sister chromatids. So, 56 x 2 = 112 chromosomes.
c). 112 chromosomes would be present in meiotic cells in metaphase I. Prior to metaphase I, the chromosomes replicated and so every chromosome is a pair of sister chromatids. So, 56 x 2 = 112 chromosomes. These chromosomes have also undergone crossing over during prophase I so the sister chromatids are no longer identical as in metaphase of mitosis.
d). 56 chromosomes would be present in meiotic cells in metaphase II. During metaphase I and anaphase I, homologous pairs of chromosomes were separated so that the resulting cells contained one chromosome of a homologous pair still paired as sister chromatids.
e). 28 chromosomes would be present in the gametes. Each gamete would contain one chromosome from each homologous pair. During fertilization, the chromosome will be matched up with its homologue.
2). Humans have 23 pairs of chromosomes. Ignoring the effects of crossing over, what proportion of a woman's eggs contains only chromosomes she received from her mother?
Answer: The chances of a woman's egg receiving only maternal chromosomes is 1 in 223 or 8,388,608 and so it is quite improbable that this would occur. Because of independent assortment that occurs during metaphase I of meiosis, it is quite improbable that all of the maternal chromosomes would line up on the same side to be distributed to a single egg.
3). Many sexually reproducing lizard species are able to generate local populations that reproduce by parthenogenesis. Would the sex of these local parthenogenetic populations be male, female, or neuter? Explain your reasoning.
Answer: In parthenogenesis, a haploid egg, which contains the "female" chromosome, undergoes a nuclear division, which means it replicates its DNA but it doesn't undergo cytoplasmic division. The egg becomes diploid and because its is carrying only "female" chromosomes it develops into a female. Populations that arise from parthenogenesis contain only females.