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Chapter 4. Heredity and Evolution. Chapter Outline. Genetic Principles Discovered by Mendel Mendelian Inheritance in Humans Polygenic Inheritance Genetic and Environmental Factors Mitochondrial Inheritance. Chapter Outline. Modern Evolutionary Theory
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Chapter 4 Heredity and Evolution
Chapter Outline • Genetic Principles Discovered by Mendel • Mendelian Inheritance in Humans • Polygenic Inheritance • Genetic and Environmental Factors • Mitochondrial Inheritance
Chapter Outline • Modern Evolutionary Theory • Factors That Produce and Redistribute Variation • Natural Selection Acts on Variation • Review of Genetics and Evolutionary Factors
Genetic PrinciplesDiscovered by Mendel • Gregor Mendel (1822-1884) laid down the basic principles of heredity. • Crossed different strains of purebred plants and studied their progeny. • Worked with common garden peas and considered only one trait at a time. • His work illustrates the basic rules of inheritance.
Principle of Segregation • Genes occur in pairs because chromosomes occur in pairs. • During gamete production, members of each gene pair separate so each gamete contains one member of a pair. • During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.
Dominance And Recessiveness • Recessive traits are not expressed in heterozygotes. • For a recessive allele to be expressed, there must be two copies of the allele. • Dominant traits are governed by an allele that can be expressed in the presence of another, different allele. • Dominant alleles prevent the expression of recessive alleles in heterozygotes.
Principle of Independent Assortment • The distribution of one pair of alleles into gametes does not influence the distribution of another pair. • The genes controlling different traits are inherited independently of one another.
Mendelian Inheritance in Humans • Over 4,500 human trains are known to be inherited according to Mendelian principles. • The human ABO blood system is an example of a simple Mendelian inheritance. • The A and B alleles are dominant to the O allele. • Neither the A or B allele are dominant to one another; They are codominant and both traits are expressed.
Polygenic Inheritance • Polygenic traits are continuous traits governed by alleles at more than one genetic locus. • Continuous traits show gradations, there is a series of measurable intermediate forms between two extremes. • Skin color is a common example of a polygenic trait it is governed by 6 loci and at least 12 alleles.
Discontinuous Distribution of Mendelian Traits • Shows the discontinuous distribution of ABO blood type in a hypothetical population. • The expression of the trait is described in terms of frequencies.
Continuous Expressionof a Polygenic Trait • Represents the continuous expression of height in a large group of people.
Mitochondrial Inheritance • All cells contain mitochondria that convert energy into a form that can be used by the cell. • Each mitochondrion contains several copies of a ring-shaped DNA molecule, or chromosome. • Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers. • All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time.
Heredity and Evolution • Evolution works at four levels: • Molecular • Cellular • Individual • Population • The levels reflect different aspects of evolution and are integrated in a way that produces evolutionary change.
The Modern Synthesis Evolution is a two-stage process: • The production and redistribution of variation (inherited differences between individuals). • Natural selection acting on this variation (whereby inherited differences, or variation, among individuals differentially affect their ability to reproduce successfully).
A Current Definition Of Evolution • From a modern genetic perspective, we define evolution as a change in allele frequency from one generation to the next. • Allele frequencies are indicators of the genetic makeup of an interbreeding group of individuals known as a population.
Mutation • Mutation is a molecular alteration in genetic material: • For a mutation to have evolutionary significance it must occur in a gamete (sex cell). • Such mutations will be carried on one of the individual's chromosomes. • During meiosis the chromosome carrying the mutation will assort giving a 50% chance of passing the allele to an offspring.
Gene Flow • Gene flow is the exchange of genes between populations. • If individuals move temporarily and mate in the new population (leaving a genetic contribution), they don’t necessarily remain in the population. • Example: The offspring of U.S. soldiers and Vietnamese women represent gene flow, even though the fathers returned to their native population.
Genetic Drift • Genetic drift is directly related to population size. • Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations. • Drift may also occur solely because the population is small: • Alleles with low frequencies may simply not be passed on to offspring, so they eventually disappear from the population.
Founder Effect • Genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations. • A new population will be established, and as long as mates are chosen only within this population, all the members will be descended from the founders. • An allele that was rare in the founders’ parent population but is carried by even one of the founders can eventually become common.
Recombination • In sexually reproducing species both parents contribute genes to offspring. • The genetic information is reshuffled every generation. • Recombination doesn’t change allele frequencies, however, it does produce different combinations of genes that natural selection may be able to act on.
Natural Selection • Natural selection provides directional change in allele frequency relative to specific environmental factors. • If the environment changes, selection pressures also change. • If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.
New Technologies • Polymerase chain reaction (PCR) makes it possible to analyze and identify DNA as small as one molecule and produce multiple copies of the original DNA. • RecombinantDNA techniques allow scientists to transfer genes from the cells of one species into the cells of another. • Geneticmanipulation is controversial due to safety and environmental concerns.
1. Mendel used the term dominant for • plants that were larger than others of the same variety. • a trait that prevented another trait from appearing. • a variety of pea plants that eliminated a weaker variety. • a trait that "skipped" a generation.
Answer: b • Mendel used the term dominant for a trait that prevented another trait from appearing.
2. Genes exist in pairs in individuals; during the production of gametes, the pairs are separated so that a gamete has only one of each kind. This is known as the • principle of segregation. • principle of independent assortment. • mitosis. • unification theory.
correct: a • Genes exist in pairs in individuals; during the production of gametes, the pairs are separated so that a gamete has only one of each kind. This is known as the principle of segregation.
3. Traits that have a range of phenotypic expressions and show a continuum of variation are termed • co-dominant. • polygenic. • polymorphic. • sex-linked.
Answer: b • Traits that have a range of phenotypic expressions and show a continuum of variation are termed polygenic.
4. When alleles are introduced into a population from another population, this is known as • genetic drift. • gene flow/migration. • founder effect. • bottleneck effect.
Answer: b • When alleles are introduced into a population from another population, this is known as gene flow/migration.
5. The most complete definition of biological evolution is • change. • mutation. • survival of the fittest. • a change in allele frequency from one generation to the next.
Answer: d • The most complete definition of biological evolution is a change in allele frequency from one generation to the next.
