BIO 1331
Some plants are triploid. If a plant has a haploid chromosome number of 42, how many chromosomes would be present in a triploid plant's somatic cells?
126. If the haploid number is 42, then 3n is 3 x 42 or 126. Depending on the number of homologs present, polyploid species are called triploid (3n), tetraploid (4n), hexaploid (6n), octoploid (8n), and so on.
Which of the following organisms would most likely be affected by a mutation that occurred at a single locus on one chromosome?
A haploid yeast. Haploid organisms (n) have just one of each type of chromosome. If only one loci of a gene were altered, the haploid organism would have no "spare" copy to use instead. Humans are diploid (2n) as embryos and as adults; only their gametes are haploid. Tetraploid plants are 4n and likewise protected from the consequences of a single, non-functional allele.
What conclusion can be drawn from John Maynard Smith's mathematical model comparing asexual and sexual reproduction?
Asexual organisms can produce more offspring than sexually reproducing organisms. John Maynard Smith developed a mathematical model showing that because asexually reproducing individuals do not have to produce male offspring, their progeny, on average, can produce twice as many offspring as individuals that reproduce sexually. Asexual reproduction is much more efficient than sexual reproduction because no males are produced. However, if a gene is damaged or altered in a way that causes it to function poorly, it will be inherited by all of that individual's offspring when asexual reproduction occurs. Although it is true that organisms produced by sexual reproduction are more variable than those produced by asexual reproduction, that was not addressed in Smith's model.
If a cell has a diploid number of 50, how many chromosomes are present in the nucleus at the beginning of meiosis? How many chromosomes are present in each resulting nucleus at the end of meiosis?
Beginning: 50; End: 25 In most plants and animals, the original cell entering meiosis is diploid, and the four final daughter cells are haploid. If the diploid cell is 50, the cell is 2(n) and n = 25.
Currently, two extant elephant species (X and Y) are classified in the genus Loxodonta, and a third species (Z) is placed in the genus Elephas. Thus, which statement should be true?
Species X and Y share a greater number of homologies with each other than either does with species Z.
scientific dogma
a statement of opinion that is considered to be true
Linnaeus
developed a two-part naming system for organisms that is still used today.
Plato
greek philosopher who claimed that every organism was an example of a perfect essence, or type, created by God, and that these types were unchanging.
at the end of meiosis I, the cells are ______
haploid During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells. Notice, however, that each chromosome still consists of two sister chromatids—meaning that chromosomes are still replicated at the end of meiosis I. The letter n stands for the number of distinct types of chromosomes in a given cell and is called the haploid number. Triploid means (3n) whereas tetraploid means (4n).
Bacteria, archaea, and many algae and fungi have cells that contain just one of each type of chromosome. These cells and organisms are called __________.
haploid. Haploid organisms have only one copy of each chromosome, therefore, an individual has just one allele of each gene. Diploid organisms have two versions of each type of chromosome, whereas polyploidy (many-form) is when organisms have three or more types of chromosomes in each cell. The number of chromosome sets is termed a cell's ploidy.
If two modern organisms last shared a common ancestor 50 million years ago, we expect that they ______.
have fewer homologous structures than two modern organisms that shared a common ancestor one million years ago
Darwin and Wallace
independently developed the theory of evolution by natural selection
Aristotle
proposed that species were organized into a sequence based on increased size and complexity, with humans at the top.
Cuvier
published a detailed analysis of an extinct species-- that is, a species that no longer exists-- called the Irish "elk". Scientists accepted the fact of extinction because this gigantic deer was judged to be too large to have escaped discovery and too distinctive to be classified as a large-bodied population of an existing species
scientific hypothesis
testable statement that explains a phenomenon or a set of observations
The cells that produce sperm in humans contain 46 chromosomes. If one of these cells undergoes meiosis to form sperm cells and chromosomal nondisjunction occurs in chromosome 22 during meiosis II, what is the chromosome number in each of the resulting sperm?
23, 23, 22, and 24. Remember that n = 23 in humans. At the end of meiosis I, each daughter cell will have two copies of chromosome 22. One daughter cell undergoes normal meiosis II and yields sperm cells with 23 chromosomes each. The other daughter cell undergoes nondisjunction, resulting in a sperm cell with 24 chromosomes (because of an extra copy of chromosome 22) and another with 22 chromosomes (because it has no copies of chromosome 22).
The cells that produce sperm in humans contain 46 chromosomes. If one of these cells undergoes meiosis to form sperm cells and chromosomal nondisjunction occurs in chromosome 22 during meiosis I, what is the chromosome number in each of the resulting sperm?
24, 24, 22, and 22. Remember that n = 23 in humans. At the end of meiosis I, one daughter cell will have four copies of chromosome 22, whereas the other will have zero copies of chromosome 22. At the end of meiosis II, the former will divide into two sperm cells that have two copies of chromosome 22 (for a total of 24 chromosomes), whereas the latter will result in sperm cells that have zero copies of chromosome 22 (for a total of 22 chromosomes).
A plant species has a diploid number of 40. Which of the following chromosome counts represents monosomy in this species?
39. Rather than having 20 pairs of chromosomes, the cell has 19 pairs of chromosomes and one unpaired chromosome, or 39 chromosomes in all. (Monosomy is when a cell has 2n - 1.) If the diploid number is 40, that means 2n = 40 and n = 20.
What is the end result of meiosis and the subsequent fertilization of gametes?
An offspring with chromosome complement unlike that of either parent. Sexual reproduction results in offspring that have chromosome complements unlike those of their siblings or their parents. Asexual reproduction in eukaryotes is based on mitosis. The chromosomes in the cells produced by mitosis are identical to the chromosomes in the parental cell. Through meiosis, followed by fertilization, the number of chromosome pairs is maintained between generations. Meiosis alone would result in an offspring with half the number of the parental cells.
Which hypothesis explains why many instances of aneuploidy in humans involve chromosome 21?
Aneuploidy tends to be lethal in embryos if it involves chromosomes with a large number of genes. This hypothesis explains why many instances of aneuploidy in humans involve chromosome 21. Trisomy-21 is most likely common because it involves a small chromosome with a correspondingly small number of genes. Trimomy 21 can occur regardless of maternal age. The frequency increases as maternal age increases, but maternal age alone does not cause trisomy 21. The frequency of nondisjunction is about equal among chromosomes, but aneuploidy tends to be lethal to embryos if it involves chromosomes that contain a large number of genes.
During a study session about evolution, one of your fellow students remarks, "The giraffe stretched its neck while reaching for higher leaves; its offspring inherited longer necks as a result." Which of the following statements will most likely be helpful in correcting this student's misconception?
Characteristics acquired during an organism's life are generally not passed on through genes.
lamarck
claimed that simple organisms originate at the base of the chain by spontaneous generation and then evolve by moving up the chain over time
If Charles Darwin had been aware of genes and their typical mode of transmission to subsequent generations, with which statement would he most likely have been in agreement?
If natural selection can change gene frequency in a population over generations, then given enough time and genetic diversity, natural selection can cause sufficient genetic change to produce new species from old ones.
A biologist studied a population of squirrels for 15 years. During that time, the population was never fewer than 30 squirrels and never more than 45. Her data showed that over half of the squirrels born did not survive to reproduce, because both competition for food and predation. In a single generation, 90 percent of the squirrels that were born lived to reproduce, and the population increased to 80. Which inferences about this most recent surge in the population size might be true?
The amount of available food may have increased and/or the predators that prey on squirrels may have decreased.
If a fatal disease were to infect a grove of aspen trees that reproduced asexually (e.g., the trees are clones), which of the following effects on the aspen grove would be most likely?
The entire grove would die as a result of infection. If a gene is damaged or altered in a way that causes it to function poorly, it will be inherited by all of that individual's offspring when asexual reproduction occurs. Thus, if any members of the grove were susceptible to the fatal infection, the entire grove would die as a result of the infection.
Which of the following statements is a leading hypothesis to explain why many organisms reproduce sexually?
The offspring that result from sexual reproduction are genetically variable, and some of them can fight off diseases more easily than genetically identical offspring. Sexually reproducing parents can produce offspring that are unique and lack deleterious alleles. If a new strain of disease-causing agent evolves, then all the asexually produced offspring are likely to be susceptible to that new strain. But if the offspring are genetically varied, then it is likely that at least some offspring will have combinations of alleles that enable them to fight off the new strain of pathogen or parasite and produce offspring of their own. The amount of energy invested and the numbers of offspring produced vary for different species. Crossing over and the independent assortment of maternal and paternal chromosomes ensure that each gamete is genetically unique. Fertilization adds another layer of genetic diversity, ensuring that each individual is genetically unique.
A segment of DNA that is found at a specific place in a chromosome and influences hereditary traits is called ________.
a gene. A gene is a segment of DNA that is found at a specific place on a chromosome and influences hereditary traits. For example, each copy of chromosome 2 in Drosophila carries many different genes, including one that influences eye color. However, this gene comes in two different forms—one that contributes to normal-colored red eyes and another that can result in purple eyes. Biologists use the term allele to denote different forms of a gene. Homologs are chromosomes that are the same size and shape. Homologous pairs also contain the same genes in the same positions along the chromosomes.
scientific theory
an explanation for a broad class of phenomena that is supported by a wide body of evidence. A theory serves as a framework for the development of new hypothesis.
Crossing over occurs .
between chromatids of homologous chromosomes. Crossing over results in an exchange of segments between maternal and paternal (homologous) chromosomes. The chromatids that meet to form a chiasma in prophase I are non-sister chromatids. At each chiasma there is an exchange of parts of chromosomes between paternal and maternal homologs. These reciprocal exchanges between different homologs create non-sister chromatids that have both paternal and maternal segments. This process of chromosome exchange is called crossing over. Sister chromatids contain identical copies of the DNA double helix. Any crossing over that occurs between sister chromatids is undetectable and will not change the genetic sequence. Crossing over is an intrinsic part of prophase I.
Which of these conditions are always true of populations evolving via natural selection? condition 1: the population must vary in traits that are heritable condition 2: some heritable traits must increase reproductive success condition 3: individuals pass on most traits that they acquire during their lifetime
conditions 1 and 2. Traits that are acquired during one's lifetime are not heritable traits. Jean-Baptiste Lamarck contended that species change through time via the inheritance of acquired characteristics. The idea is that as an individual develops, its phenotype changes in response to challenges posed by the environment, and it passes on these phenotypic changes to offspring. This is not evolution by natural selection.
Gametes ______.
fuse to form a zygote. When two haploid gametes fuse during fertilization, a full complement of chromosomes is restored; the diploid cell that results is called a zygote. The process of sperm and egg formation is called gametogenesis.
Which of the following must exist in a population before natural selection can act upon that population?
genetic variation among individuals. Evolution by natural selection occurs when (1) heritable variation leads to (2) differential reproductive success. Heritable variation means genetic variation. Traits that are acquired during one's life are not heritable traits. Many species reproduce asexually but still evolve. The mode of reproduction does not determine whether or not evolution can occur. Selective pressures may have many forms, and they are not necessarily limited to the presence of predators.
When a sperm and egg combine, the resulting embryo has _______.
half of the mother's genetic information and half of the father's. Each diploid individual receives a haploid chromosome set from its mother and a haploid set from it father. When two haploid gametes fuse during fertilization, a full complement of chromosomes is restored. The cell that results from fertilization is diploid and is called a zygote.
In 1883, German biologist, August Weismann, formally proposed a hypothesis; during the formation of gametes, if sperm and egg contribute equal number of chromosomes to the fertilized egg, then each must contain .
half of the usual number of chromosomes. In 1883, German biologist, August Weismann, formally proposed a hypothesis; during the formation of gametes, if sperm and egg contribute equal number of chromosomes to the fertilized egg, then each must contain half the usual number of chromosomes. This way, when sperm and egg combine, the resulting cell has the same chromosome number as its mother's cells and its father's cells have.
An individual plant that exhibits self-fertilization can produce offspring different from itself primarily as a result of _____.
independent assortment and crossing over during the formation of spores. Crossing over and the independent assortment of maternal and paternal chromosomes ensure that each gamete is genetically unique. Even if two gametes produced by the same individual fuse to form a diploid offspring--in which case self-fertilization is taking place--the offspring are very likely to be genetically different from the parent. Meiosis, not mitosis, leads to the formation of gametes. Crossing over is an intrinsic part of meiosis and not limited to one type of gamete.
Some males have an abnormal karyotype consisting of 47 chromosomes with an extra Y chromosome. This condition is most likely caused by .
nondisjunction in the production of the male gamete. Males have one X and one Y sex chromosome, making the Y the "male" chromosome. (Females have two X chromosomes). During prophase I in a male, the two X sister chromatids and two Y sister chromatids form a tetrad. The homologous chromosomes (X and Y in this case) separate during meiosis I, whereas the sister chromatids (X or Y) separate in meiosis II. In order to get two copies of the Y chromosome into a single gamete, nondisjunction must have involved the Y chromosome. Either the tetrad failed to separate in meiosis I or the Y sister chromatids failed to separate in meiosis II; either one would yield a gamete with an extra (male) Y chromosome.
Pseudogenes are __________.
nonfunctional, vestigial genes. Pseudogenes are members of a gene family that resemble a working gene but do not code for a functional product because of a mutation. Like vestigial traits, vestigial genes are clearly similar to functioning genes in closely related species. (A vestigial trait is a reduced or incompletely developed structure that has no function, or reduced function, but is clearly similar to functioning organs or structures in closely related species.) With a few minor exceptions, all organisms use the same rules for transferring the information coded in DNA into proteins. Introns are sequences that are transcribed into RNA but are removed as the exons are spliced together prior to translation. Unrelated genes coding for the same product would be similar to the independent development of similar traits due to convergent evolution.
Developmental homologies most likely occur because ______.
of genetic homologies....Homology is a similarity that exists in species because of inherited traits from a common ancestor. The most fundamental of all homologies is the genetic code. The three levels of homology (genetic, developmental, and structural) interact. Developmental homologies are similarities in embryonic form or developmental processes due to inheritance inherited from a common ancestor. Genetic homologies cause the developmental homologies observed in embryos, which then lead to the structural homologies recognized in adults. Organisms that live in different environments may have homologies if they share a common ancestor. Organisms that have common habitats or reproductive behaviors may have homologies, but the homology is the result of common ancestry, not the habitat or reproductive behavior.
Fertilization was first observed in ______.
sea urchins, because the eggs are large and semi-transparent. The first biologists to observe fertilization studied the large, translucent eggs of sea urchins. Owing to the semitransparency of the sea urchin egg cell, researchers were able to see the nuclei of a sperm and an egg fuse. The number of chromosomes present did not aid or hinder the observations of fertilization.
During sexual reproduction, a male reproductive cell, a , and a female reproductive cell, a , unite in a process called to form a new individual.
sperm, egg, fertilization The diploid cell that results from fertilization is called a zygote, the first cell of the new individual.
