Case 12 - Inheritance of Complex Phenotypes PDF

Case 12a - Inheritance of complex phenotypes 1. Objective of Population Genetics Population genetics aims to understand how evolutionary forces, including mutation, genetic drift, migration, inbreeding, and selection, influence the genetic structure of populations over time. It provides a mathemati...

Case 12a - Inheritance of complex phenotypes 1. Objective of Population Genetics Population genetics aims to understand how evolutionary forces, including mutation, genetic drift, migration, inbreeding, and selection, influence the genetic structure of populations over time. It provides a mathematical framework to study allele frequency changes and explores how genetic diversity is maintained or altered in populations. 2. Genetic Drift, Migration, Inbreeding, and Selection: Genetic Drift Refers to random fluctuations in allele frequencies due to chance events. It is more pronounced in small populations, leading to the potential fixation or loss of alleles, reducing genetic variation. → bottleneck effect Migration (Gene Flow) Involves the movement of individuals (or their genes) between populations. Migration introduces new alleles into a population, thereby increasing genetic diversity and counteracting the effects of genetic drift. Inbreeding Occurs when individuals mate with relatives, leading to an increase in homozygosity. This can reduce genetic diversity and lead to inbreeding depression by exposing deleterious recessive alleles. Selection Refers to differential survival and reproduction based on genotype. Natural selection favors alleles that confer a fitness advantage, while deleterious alleles may be purged from the population. This can either increase or decrease genetic variation, depending on the nature of selection (e.g., directional, stabilizing, or disruptive). 3. Effects on Genetic Variation if These Processes Were Eliminated Genetic Drift Without genetic drift, allele frequencies would be stable over time, and random loss or fixation of alleles would not occur. This would maintain higher genetic variation in small populations. Migration Without migration, populations would become more genetically isolated, potentially leading to divergence and reduced genetic diversity within individual populations. Inbreeding Inbreeding would not reduce heterozygosity, thus preserving genetic diversity, especially in small populations. Selection Without selection, all alleles would have an equal chance of being passed to the next generation, maintaining higher genetic variation but potentially allowing harmful alleles to persist. 4. Hardy-Weinberg Principle The Hardy-Weinberg principle is a fundamental concept in population genetics, stating that allele and genotype frequencies will remain constant in a population from generation to generation in the absence of evolutionary forces (i.e., genetic drift, migration, inbreeding, and selection). The principle assumes random mating, no mutation, no migration, infinite population size, and no natural selection. p^2 + 2pq + q^2 = 1 p+q=1 p = dominant q = recessive 5. Effect of Processes on Hardy-Weinberg Equilibrium: Genetic Drift Causes random deviations from Hardy-Weinberg equilibrium by changing allele frequencies in small populations. Migration Introduces new alleles, altering allele frequencies and causing a deviation from equilibrium. Inbreeding Increases homozygosity, violating the assumption of random mating and leading to deviations from expected genotype frequencies. Selection Alters allele frequencies by favoring certain alleles, leading to deviations from the equilibrium. 6. Apply to case Case 12b - Charles Darwin vs. Jean-Baptiste de Lamarck 1. What is Evolution? Evolution is the process by which populations of organisms change over time through alterations in allele frequencies. These changes result from natural selection, genetic drift, gene flow, and mutation. Evolution leads to adaptation, speciation, and the diversity of life. 2. Lamarck vs. Darwin - Lamarck's Theory (Lamarckism): Proposed that organisms evolve through the inheritance of acquired characteristics. According to Lamarck, traits acquired during an organism's lifetime (such as a giraffe stretching its neck) could be passed to offspring. - Darwin's Theory (Darwinism): Based on natural selection, Darwin proposed that individuals with advantageous traits are more likely to survive and reproduce. These traits are inherited by the next generation, leading to evolutionary changes over time. 3. Major Difference The key difference between Lamarck and Darwin's theories is the mechanism of inheritance. Lamarck believed in the inheritance of acquired traits, while Darwin argued for the inheritance of traits through genetic variation and natural selection. 4. Scientific Evidence Available to Lamarck and Darwin: - Lamarck: Had little empirical evidence, but his ideas were based on the observed progression of complexity in organisms. He lacked a mechanism for inheritance. - Darwin: Had access to fossil records, artificial selection (as observed in domestic animals), and biogeographical evidence (e.g., finches from the Galápagos Islands). Darwin also lacked a clear understanding of the genetic mechanism but observed the variation in populations that could lead to differential survival. 5. Has Lamarck’s Series Returned (Epigenetics)? While Lamarck's theory of the inheritance of acquired characteristics was largely dismissed for many years, the discovery of epigenetics has revived some aspects of his thinking. Epigenetics refers to changes in gene expression that do not involve alterations in the DNA sequence itself, often influenced by environmental factors. These changes can sometimes be inherited, which resembles Lamarck’s idea of traits acquired during an organism's lifetime being passed to offspring. However, epigenetics is still distinct from Lamarckism, as it does not involve the direct modification of genetic information but rather gene regulation mechanisms. 6. Apply to case