Biological Sciences
Phenotypic Frequencies
Phenotypic frequencies refer to the relative proportions of different observable traits within a population. These traits can include physical characteristics, behaviors, or other observable features. Understanding phenotypic frequencies is important in studying the distribution and variation of traits within a population, and it can provide insights into evolutionary processes and genetic diversity.
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5 Key excerpts on "Phenotypic Frequencies"
eBook - PDFDarwinian Dynamics
Evolutionary Transitions in Fitness and Individuality
- Richard E. Michod(Author)
- 2021(Publication Date)
- Princeton University Press(Publisher)
Consider a population of organisms composed of different genotypes. For each organism, we measure its individual fitness (expected reproductive success) and its genotype at a single diploid locus of interest (say with two alleles A and a) as represented by its individual frequency of allele a, q i = 0, 1/2, 1, for genotypes AA, Aa, and aa, respectively. Imagine plotting all organisms in the population on a graph, the abscissa being individual fre-quency, q t , and the ordinate being individual fitness. For any real popula-tion, there will be a distribution of points plotted for each q., the density of points depending upon the genotype frequencies in the population P, 2Q, and R for i = AA, Aa, and aa, respectively. The variance of fitness for each genotype class will depend on environmental and random factors as well as the canalization of fitness. In theory, we may consider the fitnesses of each genotype to be concentrated at a single value, W jy with weights P, 2Q, and R for i = AA, Aa, and aa, respectively. In practice, genotypic fitness W t will be an average value, but we ignore this complication, following Fisher (1941). As the population evolves, the frequencies of the genotypes change, as do the individual fitnesses W. (if there is frequency dependence), while the indi-vidual gene frequencies, q., remain fixed at their discrete values. For the pur-pose of understanding the effects of gene frequency change on fitness, Fisher (1930, 1941, 1958) defined a, the average effect of a gene substitution, as the slope of the linear regression fitted to these points by the technique of least squares, using the population genotype frequencies as weights (table 4-2). Fisher's approach embodies the covariance methods introduced above (for example, equation 4-11), since by definition a = RegfW^.].
eBook - PDF- Peter Donaldson, Ann Daly, Luca Ermini, Debra Bevitt(Authors)
- 2015(Publication Date)
- Garland Science(Publisher)
A large number of samples from a population are usually collected, and the genotype and allele frequencies are calculated. The genotype and allele frequencies of the sample popu-lation are then used to estimate the diversity of the population. To calculate a genotype frequency, the number of individuals having the same genotype is divided by the total number of individuals in the sample ( N ). For a locus with three genotypes, AA , Aa , and aa , the frequency ( f ) of each genotype is: f AA AA N ( ) = number of individuals Figure 1.11: Red blood cells in sickle cell disease. Sickle cells are shaped like a harvesting sickle and, unlike the normal doughnut-shaped red blood cells, these cells can be hard with sharp edges that can damage the wall of small blood vessels as they passage through the body. They will often clog the flow of blood and break up as they pass through the small blood vessels. CHAPTER 1 Genetic Diversity 20 f Aa Aa N ( ) = number of individuals f aa aa N ( ) = number of individuals The sum of all the genotype frequencies always equals 1 (or 100%). Genotypes are not permanent. They are disrupted in the processes of segregation and recombination that take place when individual alleles are passed to the next generation through the gametes. Alleles, in contrast, are not broken down and the same allele may be passed from one generation to the next. For this reason the calculation of allele frequencies is often the preferred choice when determining the genetic variability of a population. In addition, there are always fewer alleles than genotypes, e.g. for the gene with two alleles A and a above, there are two alleles, but there are three genotypes. By using alleles, popula-tion diversity can be described in fewer terms than by using genotypes. Finally, by using allele frequencies in case control population studies rather than genotype frequencies, no assumptions about the impact of homozygosity or of heterozygote advantage are being made.
eBook - PDFHuman Heredity
Principles and Issues
- Michael Cummings(Author)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
It can also be used to detect when allele frequencies are shifting in the population. Changing allele frequencies in a population represent evolutionary change. One of the law’s most common uses is to measure the frequency of heterozygous carriers of deleterious reces-sive alleles in a population. This information can be used to calculate the risk of having an affected child. 19-4 Measuring Genetic Diversity in Human Populations ■ All genetic variants originate by mutation, but mutation is an insignificant force in bring-ing about changes in allele fre-quency. Other forces, including genetic drift, act on the genetic variation in the gene pool and are responsible for changing the frequency of alleles in the population (which is evolution). Drift is a random process that acts in small, isolated populations to change allele frequency from generation to generation. Examples include island populations and those separated from the general population by socioreligious practices. Natural selection acts on genetic diversity in populations to drive the process of evolution by changing allele frequencies. 19-5 Natural Selection Affects the Frequency of Genetic Disorders ■ Selection increases the reproductive success of fitter genotypes. As these individuals make a disproportionate contribution to the gene pool of succeeding generations, South Pacific Ocean North Atlantic Ocean South Atlantic Ocean South America North America Africa Tristan da Cuhna © Cengage Learning Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
eBook - PDF- Matthew B. Hamilton(Author)
- 2021(Publication Date)
- Wiley-Blackwell(Publisher)
Phenotype: The morphological, biochemical, physiological, and behavioral attributes of an individual; synonymous with character and trait. Dominant: Where the expressed phenotype of one allele takes precedence over the expressed phenotype of another allele. The allele associated with the expressed phenotype is said to be dominant. Dominance is seen on a continuous scale that includes “complete” dominance (one allele completely masks the phenotype of another allele so that the phenotype of a heterozygote is identical to a homozygote for the dominant allele) and “partial” or “incomplete” dominance (masking effect is incomplete so that the phenotype of a heterozygote is intermediate to both homozygotes) and includes over- and under-dominance (phenotype is outside the range of phenotypes seen in the homozygous genotypes). The lack of dominance (heterozygote is exactly intermediate to the phenotypes of both homozygotes) is when the effects of alleles are additive, a situation sometimes termed “codominance” or “semi-dominance.” Recessive: The expressed phenotype of one allele is masked by the expressed phenotype of another allele. The allele associated with the concealed phenotype is said to be recessive. 2.2 Hardy–Weinberg expected genotype frequencies • Hardy–Weinberg and its assumptions. • Each assumption is a population genetic process. • Hardy–Weinberg is a null model. • Hardy–Weinberg in haplo-diploid systems. Mendel’s “laws” could be called the original expecta- tions in population genetics. With the concept of par- ticulate genetics established, it was possible to make a wide array of predictions about genotype and allele frequencies as well as the frequency of phenotypes with a one-locus basis. Still, progress and insight into particulate genetics were gradual. Until 1914, it was generally believed that rare (infrequent) alleles would disappear from populations over time.
eBook - PDF- Armando Caballero(Author)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
2 Forces of Change in the Allele Frequencies Concepts to Study • Allele, gamete and genotype frequencies • Hardy–Weinberg equilibrium • Expected heterozygosity or gene diversity and allelic diversity • Gametic or linkage disequilibrium • Genetic drift • The ideal population of Wright–Fisher • Equilibrium between mutation and back-mutation • Migration models • Selection and dominance coefficients • Types of within-locus gene action • Stable and unstable equilibria • Antagonistic pleiotropy and marginal overdominance Objectives for Learning • To learn how to calculate allele frequencies from genotype frequencies • To know the conditions for Hardy–Weinberg equilibrium, its implications and the definition of expected heterozygosity and allelic diversity • To know how to calculate linkage disequilibrium • To understand the process of genetic drift • To learn the basic characteristics of the Wright–Fisher idealized population • To know how to calculate the changes in allele frequency by mutation and how an equilibrium between mutation and back-mutation is reached • To understand the homogenizing effect of migration and the different population models used for its description • To know the general model of fitness and the concepts of selection coefficient and dominance coefficient • To understand how the changes in allele frequency for deleterious or beneficial alleles take place with different types of within-locus gene action • To learn how to distinguish stable and unstable models of allele frequencies • To comprehend the concepts of antagonistic pleiotropy and marginal overdominance • To understand the impact of selection on the test for Hardy–Weinberg equilibrium 2.1 Allele, Gamete and Genotype Frequencies The genetic description of a population can be done at three different levels, the locus, the gamete or the individual genotype, by specifying the different variants in each case (allele, gamete or genotype) and their respective frequencies.
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