BISC 160 Test Final (Chapters 7-8, 15) PDF

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Extensive lecture notes on Chapters 7-8 and 15 of course textbook....

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CHAPTER 7 ●

Meiosis halves the nuclear chromosome content and generates diversity. ○

There are two nuclear divisions to reduce the number of chromosomes to the haploid number, but DNA only replicates once.

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Overall Functions of Meiosis: ○

to reduce the number of chromosomes from diploid to haploid

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to ensure that each of the haploid products has a complete chromosome set

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to generate genetic diversity among the products (gametes)

The Process: ○

Early Prophase I: the chromatin begins to condense following interphase

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Mid Prophase I: synapsis aligns homologs, and chromosomes condense further

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Late Prophase I-Prometaphase: chromosomes continue to coil and shorten, crossing over occurs, genetic material is exchanged between nonsister chromatids in a homologous pair, nuclear envelope breaks down

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Metaphase I: homologous pairs line up on the equatorial plate (tetrads crossing over the center plate)

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Anaphase I: homologous chromosomes move to opposite poles of the cell (not separated from the centromere)

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Telophase I: chromosomes gather into nuclei, and the original cell divides

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Prophase II: chromosomes condense again

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Metaphase II: centromeres of paired chromatids line up on the equatorial plate

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Anaphase II: chromatids separate and are pulled to opposite poles; due to crossing over and independent assortment, each cell has unique genetic makeup

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Telophase II: chromosomes gather into nuclei, and the cells divide

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Products: four haploid cells

Meiotic division reduces the chromosome number. ○

Meiosis I is characterized by homologous chromosomes coming beside each other and lining up along their lengths (this doesn't happen in mitosis).

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Also unlike mitosis, these homologous chromosome pairs separate, but the individual chromosomes (sister chromatids attached by a centromere) stay intact.

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Like mitosis, meiosis is preceded by interphase, in which the S phase with DNA replication occurs.

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At the end of meiosis I, two nuclei form, each with half the original chromosomes.

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Meiosis II separates the sister chromatids (like in mitosis), and meiosis II results in four cells, each containing the haploid number of chromosomes with genetic diversity.

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Crossing over and independent assortment generate diversity. ○

Crossing over occurs during Prophase I through to Metaphase I, in which the homologous chromosomes pair by adhering along their lengths (synapsis). The four chromatids of the pair for a tetrad, which at some point begins to repel each other leaving the remaining attached part, the chiasma. This point is where genetic material is exchanged between non sister chromatids. Any of the four chromatids can do this with one another. Chiasmata resolve to form: ■

Crossing over results in recombinant chromatids, increasing the genetic variation by reshuffling genetic information between homologous pairs.

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Independent assortment: it's a matter of chance which member of a homologous pair goes with which daughter cell at anaphase I. Each pair could come from the mother or each pair from the father or two of each or one with three, etc. This all depends on how the chromosome pairs line up at metaphase I.

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Meiotic errors lead to abnormal chromosome structure and numbers. This leads to abnormal chromosomes in the daughter cells. Consequences for offspring can be significant. ○

Nondisjunction: homologous pair fails to separate at anaphase I or chromatid pair fails to separate at anaphase II ■

Anaphase I mistakes result in two of the four daughter cells carrying both members of the homologous pair and the other two having neither member of the pair.

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Anaphase II mistakes result in one daughter cell having an extra chromosome, one daughter cell having one less chromosome than the full set, and two unaffected daughter cells. ●

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Aneuploidy: having an abnormal number of chromosomes

Polyploidy: higher order nuclei (3n triploid or 4n tetraploid, etc), could occur due to an extra round of DNA replication before meiosis or because there is no spindle formation in meiosis II ■

Triploid fruits: infertile, therefore lacking seeds

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Translocation: during crossing over in meiosis I, chromatids break and rejoin to other homologous chromosomes. Sometimes this might happen between non-homologous chromosomes, resulting in translocation.

CHAPTER 8 ●

Genes are particulate and are inherited according to Mendel's Laws. ○

The blending inheritance hypothesis theorized that the parental determinants would be lost within the offspring creating a mix.

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The particulate inheritance hypothesis theorized that the offspring would still contain the determinants, no matter what phenotype it showed.

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Mendel used the scientific method to test his hypothesis with pea plants, which had both female and male sex organs, allowing them to self fertilize. The male organs, though,

could be removed, so that Mendel could manually fertilize with the pollen from a different flower.

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Character: observable physical feature (pea shape)

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Trait: particular of a character (round or wrinkled)

Mendel removed the stamens to emasculate a flower, then using pollen from another flower, fertilized its pistils. These two plants were the parental generation. (pollen provider: father, pollen receiver: mother)

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Seeds formed, then were planted, resulting in the first filial generation (F1). Traits were examined and recorded quantitatively.

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Sometimes the F1 generation was allowed to self pollinate, producing the F2 generation, which was characterized and recorded.

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Mendels's first experiments involved monohybrid crosses. ○

Crossing two strains of peas, the F1 generation would come out with only one trait, the other seeming to have disappeared. A theory that the F1 offspring were not a blend of the two parent traits appeared.

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But with self pollination the F2 generation showed a reappearance of that second characteristic, leading to the second theory: the trait does not disappear.

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This concluded that the blending theory is false, and provided support for the particulate theory. (Not really particulates, we know know, but they are physically distinct quantities, sequences of DNA carried on chromosomes.)

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Genes (determinants) occur in pairs and segregate from one another during the formation of gametes; ultimately there are two copies of the gene for each character, one inherited from each parent.

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There are multiple alleles for a gene. They can occur in different forms: homozygous or heterozygous.

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Phenotype: physical appearance of an organism

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Genotype: genetic constitution of the phenotype, causes the phenotype

Mendel's first law states that the two copies of a gene segregate. ○

Law of Segregation: when any individual produces gametes, the two copies of a gene separate, so each gamete receives only one copy, so half of the generation will have the dominant allele, while the other half has the recessive.

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Genes determine phenotypes by producing proteins with particular functions. Dominant genes will be expressed to produce a functional protein, while

recessive genes mutate so that it is no longer expressed or it codes a nonfunctional protein. ●

Mendel verified his hypotheses by performing test crosses. Mendel hypothesized that there were two possible allele combination for a dominant phenotype (round, Rr or RR). A test cross is used to determine whether an individual showing a dominant trait is homo or heterozygous. The individual is crossed with a homozygous recessive individual, so that the results are easy to interpret. ○

With a homozygous dominant individual, offspring will be Rr and show the dominant train. With a heterozygous individual, offspring will be half heterozygous (Rr) and half recessive (rr).

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Mendel's second law states that copies of different genes assort independently. ○

Studied via a series of experiments with two differing characteristics; would one offspring consist of all dominant traits and one all recessive traits, or could they mix? Were they dependent of the other traits or independent?

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Mixing an RRYY with an rryy produced an F1 generation with RrYy. When the F1 gen self pollinates, the gametes combine randomly to produce the F2 generation with a 9:3:3:1 ratio (RY, Ry, rY, ry → RRYY, RrYY, etc.) Shows independent segregation.

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The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation.

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Probability is used to predict inheritance. If something is certain to happen the probability is one; if something is impossible the probability is zero. All other events have a probability lying between zero and one. ○

If two coins are tossed, each acts independently of the other. One could land on heads, and it would have no effect on the other coins outcome.

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Mendel's laws can be observed in human pedigrees. ○

Pedigrees are family trees that show the occurrence of inherited phenotypes in several generations of related individuals. Pedigrees are limited, because families are unlikely to have the number of kids that would show the proportions that Mendel discovered.

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Something to notice for dominant inheritance in a pedigree is that each person affected has an affected parent and about half the offspring of an affected parent

are affected as well. Should a parent be homozygous dominant, all the children would be affected. ○

Something to notice for recessive inheritance is that most affected people have two unaffected parents. Two heterozygous carrier parents only affect about one fourth of their children. If one parent it heterozygous, the recessive allele will be passed on to about half the unaffected offspring (also heterozygous).

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Alleles and genes interact to produce phenotypes. ○

Alleles can be changed by mutations and give rise to new alleles; a single gene might have a plethora of alleles.

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New alleles arise by mutation. An allele can mutate or change to become a different allele by rare, stable, and inherited changes in the genetic material. Recessive alleles could be the result of a mutated dominant allele being passed down by generations. ○

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Multiple alleles can increase the number of possible phenotypes.

Dominance is not always complete, meaning that many genes have alleles that aren't dominant nor recessive to one another. ○

Incomplete Dominance: there is an intermediate phenotype, one that looks like blending, but the original phenotypes show back up in later generations, following Mendel's law of inheritance. (We would say that the allele for red is incompletely dominant over the allele for white.)

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Codominance: two alleles of a gene both present their phenotypes when present in a heterozygote, such as in the ABO blood group.

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Genes interact when they are expressed. ○

Epistasis occurs when the phenotypic expression of one gene is affected by another gene.

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For example, two genes determine coat color in labs: B (black) is dominant to b (brown) and E (pigment deposition) is dominant to e (no pigment deposition). So a dog with a Be will be yellow, because the allele that would allow pigment to be deposited is lacking. Gene E is epistatic to Gene B.

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The environment affects gene action. Genotype and environment often interact to determine the phenotype of an organism. ○

Light, temperature, and nutrition can affect the phenotypic expression of a genotype. For example, body weight is not only determined by multiple genes, but also by nutrition and activity.

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Penetrance is the proportion of individuals in a group with a given genotype that actually show the expected phenotype.

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Expressivity is the degree to which a genotype is expressed in an individual. How does the gene effect different individuals?

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Heritability is the relative contribution of genetic versus environmental factors to the variation in that character of a particular population.

CHAPTER 15 ●

Evolution is both factual and the basis of broader theory. Evolution is the change in the genetic composition of populations over time. ○

Evolutionary Theory is the understanding and application of the process of evolutionary change to biological problems; this is based on what we have learned about how changes occur and what changes have occurred in the past.

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Darwin and Wallace introduced the idea of evolution by natural selection.

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Darwin noticed in his travel of the world that species looked different in different areas, or possibly there was a species in an area that didn't exist anywhere else. He wondered what made the species change like they did.

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Darwin asserted three things: species are not immutable (they change over time), divergent species share a common ancestor and have diverged from one another gradually over time (descent with modification), and changes in species over time can be explained by natural selection.

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Natural selection is the increased survival and reproduction of some individuals compared with others, based on differences in their traits. Darwin and Wallace were both credited with the idea of natural selection.

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Mutation, selection, gene flow, genetic drift, and nonrandom mating result in evolution.

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Mutation generates genetic variation: mutations are any change in the nucleotide sequences of an organism's DNA. ○

Natural selections acts on random variation in genes resulting in adaptation.

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Some mutations are deleterious and some are neutral, but others may be advantageous in certain environmental conditions or can restore genetic variation where other processes have removed it.

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Mutation both creates and helps maintain genetic variation.

At a particular locus on a chromosome, there are different forms of a gene called alleles. An individual will have no more than 2, but in the population there may be many. ○

The sum of all copies of all alleles at all loci found in a population constitutes its gene pool. All copies of all alleles at one locus is the gene pool for that locus. The gene pool ultimately is the sum of all the genetic variation in a population. ■

The proportion of each allele in a gene pool: allele frequency.

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The proportion of each genotype among individual in the population: genotype frequency.

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Selection on genetic variation leads to new phenotypes. ○

Artificial selection is where people choose the traits they want to be present and breed to achieve those results (preferred by human breeders).

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Natural selection results in traits that helps organisms survive and reproduce more effectively. ■

In both cases, selection increased the frequency of the favored trait from one generation to the next.

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Natural selection increases the frequency of beneficial mutations in populations. ○

Slight differences between individuals affect the chance that a given individual will survive and reproduce, subsequently increasing the frequency of the favored trait in the next generation.

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A favored trait that evolves through natural selection is known as adaptation.

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Natural selection also acts to remove deleterious mutations from populations.

Gene flow may change allele frequencies. Migration of individuals and movements of gametes between populations can change allele frequency in a population.

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Genetic drift may cause large changes in small populations. ○

Genetic drift is random changes in allele frequencies from one generation to the next that may cause large changes in allele frequencies over time. Harmful alleles may increase, while rare advantageous alleles may be lost.

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Population bottleneck occurs when a population passes through an environmental event that only a small number of individuals survive. In future generations, allele frequency changes become very visible.

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Founder effect: the change in genetic variation caused by a small population of individuals founding a new population in a new region. That small group cannot account for the diversity in the entire source group.

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Nonrandom mating can change a genotype or allele frequencies. Any time individuals mate preferentially with other individuals of the same genotype, homozygous genotypes will increase, while heterozygotes decrease. (vice versa) ○

Sexual selection occurs when individuals of one sex mate preferentially with particular individuals of the opposite sex rather than at random. This affects reproductive success among the individuals of a population affecting allele frequency.

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Natural selection favors survival, while sexual selection favors reproduction. If you are surviving but not producing, you are not passing on to the next generation. Sexual selection favors traits that enhance an individual's chance of reproduction, even when that particular trait decreases its survival chance.

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Evolution can be measured by changes in allele frequencies. ○

Much of evolution occurs through gradual changes in the relative frequencies of different alleles in a population from one generation to the next.

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Allele Frequency Formula:

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N AA + N Aa + N aa = N (total number of individuals in a population)

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The number of alleles in that population would be 2N. (diploid organisms)

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p is frequency of A (dominant); q is frequency of a (recessive).

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For each population, q + p = 1.

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Allele frequencies can be equal but genotypic frequencies can vary depending on the distribution, AA, Aa, aa (genetic structure).

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Genetic variation is measured by allele frequency; genetic variation distribution is measured by genotype frequency.

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Evolution will occur unless certain restrictive conditions exist. ○

The Hardy-Weinberg equilibrium describes a model in which allele frequencies do not change and genotypic frequencies can be predicted across generations based on the allele frequency.

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5 Conditi...