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    Mastering Biology Ch.23 Flashcards

    Memorize flashcards and build a practice test to quiz yourself before your exam. Start studying the Mastering Biology Ch.23 flashcards containing study terms like Which of the following are basic components of the Hardy-Weinberg model?, Which of the following statements is not a part of the Hardy-Weinberg principle?, True of false? The Hardy-Weinberg model makes the following assumptions: no selection at the gene in question; no genetic drift; no gene flow; no mutation; random mating. and more.

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    Which of the following are basic components of the Hardy-Weinberg model?

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    Frequencies of two alleles in a gene pool before and after many random matings.

    (Why: Hardy and Weinberg were trying to determine how and whether allele frequencies in a population change from one generation to the next.

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    Which of the following statements is not a part of the Hardy-Weinberg principle?

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    The genotype frequencies in the offspring generation must add up to two.

    (Why: This statement is not true; the genotype frequencies in the offspring generation must add up to one.

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    Terms in this set (12)

    Which of the following are basic components of the Hardy-Weinberg model?

    Frequencies of two alleles in a gene pool before and after many random matings.

    (Why: Hardy and Weinberg were trying to determine how and whether allele frequencies in a population change from one generation to the next.

    Which of the following statements is not a part of the Hardy-Weinberg principle?

    The genotype frequencies in the offspring generation must add up to two.

    (Why: This statement is not true; the genotype frequencies in the offspring generation must add up to one.

    True of false? The Hardy-Weinberg model makes the following assumptions: no selection at the gene in question; no genetic drift; no gene flow; no mutation; random mating.

    True

    (Why: these are 5 assumption of the Hardy-Weinberg model)

    What is the frequency of the A1A2 genotype in a population compose of 20 A1A1 individuals, A1A2 individuals, and 100 A2A2 individuals?

    0.4

    (Why: the calculation to determine the frequency of the A1A2 genotype is: 80 A1A2 individuals / (20+80+100) total individuals=0.4, the frequency of the A1A2 genotype.

    What is the frequency of the A1 allele in a population compose of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?

    The frequency of the A1 allele is 0.3.

    (Why: the frequency of the A1 allele is p=(number of a1 alleles) / (total of all alleles) = [(2(20) + 80] / [(2 x 20) + (2 x 80) + (2 x100)]= 0.3

    Which of the following evolutionary forces results in adaptive changes in allele frequencies?

    Selection

    (Why: selection is the only evolutionary force that consistently results in adaptation. Mutation without selection and genetic drift are random processes that may lead to adaptive, maladaptive, or neutral effects on populations.)

    What genotype frequencies are expected under Hardy-Weinberg equilibrium for a population with allele frequencies of p=0.8 and q=0.2 for a particular gene?

    The expected genotype frequencies are 0.64, 0.32, and 0.04 for A1A1, A1A2, and A2A2, respectively.

    (Why: the expected frequency of the A1A1 genotype is p^2=(0.8)(0.8)=0.64; the expected frequency of the A1A2 genotype is 2pq=2(0.8)(0.2)=0.32; the expected frequency of the A2A2 genotype is q^2=(0.2)(0.2)=0.4. To verify your calculations, confirm that the three frequencies add up to one.

    Which of the following evolutionary forces could create new genetic information in population?

    Mutation

    (Why: mutations, which are changes in a cell's DNA, can introduce new genetic information in a population.)

    Generation-to-generation change in the allele frequencies in a population is _____.

    microevolution

    (Why: Generation-to-generation change in the allele frequencies in a population is the definition of microevolution)

    Which type of selection tends to increase genetic variation?

    Disruptive selection

    (Why: disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes, resulting in increased genetic variation in a population.)

    True or false? Heterozygote advantage refers to the tendency for heterozygous individuals to have better fitness than homozygous individuals. This higher fitness results in less genetic variation in the population.

    False

    (Why: Heterozygote advantage results in more genetic variation in the population.)

    Long necks make it easier for giraffes to reach leaves high on trees, while also making them better fighters in "neck wrestling" contests. In both cases, which kind of selection appears to have made giraffes the long-necked creatures they are today?

    Directional selection

    (Why: directional selection drives the average of the population in one direction, in this case, toward longer necks.)

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    Related questions

    QUESTION

    how many different kinds of gametes can be produced by a parent with the genotype GGRr?

    Source : quizlet.com

    Mechanisms of evolution (article)

    When a population is in Hardy-Weinberg equilibrium, it is not evolving. Learn how violations of Hardy-Weinberg assumptions lead to evolution.

    Key points:

    When a population is in Hardy-Weinberg equilibrium for a gene, it is not evolving, and allele frequencies will stay the same across generations.

    There are five basic Hardy-Weinberg assumptions: no mutation, random mating, no gene flow, infinite population size, and no selection.

    If the assumptions are not met for a gene, the population may evolve for that gene (the gene's allele frequencies may change).

    Mechanisms of evolution correspond to violations of different Hardy-Weinberg assumptions. They are: mutation, non-random mating, gene flow, finite population size (genetic drift), and natural selection.

    Introduction

    In nature, populations are usually evolving. The grass in an open meadow, the wolves in a forest, and even the bacteria in a person's body are all natural populations. And all of these populations are likely to be evolving for at least some of their genes. Evolution is happening right here, right now!

    To be clear, that doesn't mean these populations are marching towards some final state of perfection. All evolution means is that a population is changing in its genetic makeup over generations. And the changes may be subtle—for instance, in a wolf population, there might be a shift in the frequency of a gene variant for black rather than gray fur. Sometimes, this type of change is due to natural selection. Other times, it comes from migration of new organisms into the population, or from random events—the evolutionary "luck of the draw."

    In this article, we'll examine what it means for a population evolve, see the (rarely met) set of conditions required for a population not to evolve, and explore how failure to meet these conditions does in fact lead to evolution.

    [I'm totally new to population genetics! Where should I start?]

    Hardy-Weinberg equilibrium

    First, let's see what it looks like when a population is not evolving. If a population is in a state called Hardy-Weinberg equilibrium, the frequencies of alleles, or gene versions, and genotypes, or sets of alleles, in that population will stay the same over generations (and will also satisfy the Hardy-Weinberg equation). Formally, evolution is a change in allele frequencies in a population over time, so a population in Hardy-Weinberg equilibrium is not evolving.

    That's a little bit abstract, so let's break it down using an example. Imagine we have a large population of beetles. In fact, just for the heck of it, let's say this population is infinitely large. The beetles of our infinitely large population come in two colors, dark gray and light gray, and their color is determined by the A gene. AA and Aa beetles are dark gray, and aa beetles are light gray.

    In our population, let's say that the A allele has a frequency of

    0.3 0.3 0, point, 3

    , while the a allele has a frequency of

    0.7 0.7 0, point, 7

    . If a population is in Hardy-Weinberg equilibrium, allele frequencies will be related to genotype frequencies by a specific mathematical relationship, the Hardy-Weinberg equation. So, we can predict the genotype frequencies we'd expect to see (if the population is in Hardy-Weinberg equilibrium) by plugging in allele frequencies as shown below:

    p ^2 2 squared + 2pq + q ^2 = 1 2 =1 squared, equals, 1

    p = frequency of A, q = frequency of a

    Frequency of AA = p ^2 2 squared = 0.7 ^2 2 squared = 0.49

    Frequency of Aa = 2pq = 2 (0.7)(0.3) = 0.42

    Frequency of aa = 0.3

    ^2 2 squared = 0.09

    [What is the difference between allele and genotype frequency?]

    Let's imagine that these are, in fact, the genotype frequencies we see in our beetle population (

    9\% 9% 9, percent AA, 42\% 42% 42, percent Aa, 49\% 49% 49, percent

    aa). Excellent—our beetles appear to be in Hardy-Weinberg equilibrium! Now, let's imagine that the beetles reproduce to make a next generation. What will the allele and genotype frequencies will be in that generation?

    To predict this, we need to make a few assumptions:

    First, let's assume that none of the genotypes is any better than the others at surviving or getting mates. If this is the case, the frequency of A and a alleles in the pool of gametes (sperm and eggs) that meet to make the next generation will be the same as the overall frequency of each allele in the present generation.

    Second, let's assume that the beetles mate randomly (as opposed to, say, black beetles preferring other black beetles). If this is the case, we can think of reproduction as the result of two random events: selection of a sperm from the population's gene pool and selection of an egg from the same gene pool. The probability of getting any offspring genotype is just the probability of getting the egg and sperm combo(s) that produce that genotype.

    We can use a modified Punnett square to represent the likelihood of getting different offspring genotypes. Here, we multiply the frequencies of the gametes on the axes to get the probability of the fertilization events in the squares:

    Source : www.khanacademy.org

    Hardy

    Hardy-Weinberg Principle

    Hardy–Weinberg Equilibrium (HWE) is a null model of the relationship between allele and genotype frequencies, both within and between generations, under assumptions of no mutation, no migration, no selection, random mating, and infinite population size.

    From: American Trypanosomiasis Chagas Disease (Second Edition), 2017

    Related terms:

    Allele FrequencyDNA ProfilingHuman Leukocyte AntigenGenetic DriftPopulation GeneticsHaplotypePhenotypeGenetic VariationGene StructureDNASialic AcidDominant Gene

    View all Topics

    Genetic Variation in Populations

    Robert L. Nussbaum MD, FACP, FACMG, in Thompson & Thompson Genetics in Medicine, 2016

    The Hardy-Weinberg Law

    The Hardy-Weinberg law rests on these assumptions:

    The population under study is large, and matings are random with respect to the locus in question.

    Allele frequencies remain constant over time because of the following:

    There is no appreciable rate of new mutation.

    Individuals with all genotypes are equally capable of mating and passing on their genes; that is, there is no selection against any particular genotype.

    There has been no significant immigration of individuals from a population with allele frequencies very different from the endogenous population.

    A population that reasonably appears to meet these assumptions is considered to be inHardy-Weinberg equilibrium.

    View chapter on ClinicalKey

    Hardy–Weinberg Equilibrium and Random Mating

    J. Lachance, in Encyclopedia of Evolutionary Biology, 2016

    The Hardy–Weinberg Principle

    The HardyWeinberg principle relates allele frequencies to genotype frequencies in a randomly mating population. Imagine that you have a population with two alleles (A and B) that segregate at a single locus. The frequency of allele A is denoted by p and the frequency of allele B is denoted by q. The Hardy–Weinberg principle states that after one generation of random mating genotype frequencies will be p2, 2pq, and q2. In the absence of other evolutionary forces (such as natural selection), genotype frequencies are expected to remain constant and the population is said to be at Hardy–Weinberg equilibrium. The Hardy–Weinberg principle relies on a number of assumptions: (1) random mating (i.e, population structure is absent and matings occur in proportion to genotype frequencies), (2) the absence of natural selection, (3) a very large population size (i.e., genetic drift is negligible), (4) no gene flow or migration, (5) no mutation, and (6) the locus is autosomal. When these assumptions are violated, departures from Hardy–Weinberg proportions can result.

    One useful way to think about the Hardy–Weinberg principle is to use the metaphor of a gene pool (Crow, 2001). Here, individuals contribute alleles to an infinitely large pool of gametes. In a randomly mating population without natural selection, offspring genotypes are found by randomly sampling two alleles from this gene pool (one from their mother and one from their father). Because the allele that an individual receives from their mother is independent of the allele they receive from their father, the probability of observing a particular genotype is found by multiplying maternal and paternal allele frequencies. Mathematically this involves the binomial expansion: (p + q)2 = p2 + 2pq + q2 (see the modified Punnett Square in Figure 1 for a graphical representation). Note that there are two ways that an individual can be an AB heterozygote: they can either inherit an A allele from their mother and a B allele from their father or they can inherit a B allele from their mother and an A allele from their father.

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    Figure 1. Graphical representation of the Hardy–Weinberg principle. The frequency of A alleles is denoted by p and the proportion of B alleles by q. AA homozygotes are represented by white, AB heterozygotes by gray, and BB homozygotes by gold. Shaded areas are proportional to the probability of observing each genotype.

    Additional insight can be found by considering an empirical example (Figure 2). Consider a population that initially contains 18 AA homozygotes, 4 AB heterozygotes, and 3 BB homozygotes. The alleles in the gene pool, 80% are A and 20% are B. After a single generation of random mating we observe Hardy–Weinberg proportions: 16 AA homozygotes, 8 AB heterozygotes, and 1 BB homozygote. Note that allele frequencies remain unchanged.

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    Figure 2. Hardy–Weinberg example. AA homozygotes (black circles), AB heterozygotes (black and gold circles), and BB homozygotes (gold circles) contribute to the gene pool. A alleles are shown as black half-circles and B alleles are shown as gold half-circles. After a single generation of random mating Hardy–Weinberg proportions are obtained.

    There are a number of evolutionary implications of the Hardy–Weinberg principle. Most importantly, genetic variation is conserved in large, randomly mating populations. A second implication is that the Hardy–Weinberg principle allows one to determine the proportion of individuals that are carriers for a recessive allele. Third, it is important to note that dominant alleles are not always the most common alleles in a population. Another implication of the Hardy–Weinberg principle is that rare alleles are more likely to be found in heterozygous individuals than in homozygous individuals. This occurs because q2 is much smaller than 2pq when q is close to zero.

    Source : www.sciencedirect.com

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