Heredity and Evolution 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 Principles Discovered 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.

Traits Mendel Studied: Peas

Results: One Trait at a Time

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.

Alleles

Punnett square

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.

Some Mendelian Traits in Humans: Dominant Condition Manifestations Achondroplasia Dwarfism due to growth defects involving the long bones of the arms and legs; trunk and head size usually normal. Brachydactyly Shortened fingers and toes. Familial hyper- cholesterolemia Elevated cholesterol levels and cholesterol plaque deposition; a leading cause of heart disease, with death frequently occurring by middle age.

Some Mendelian Traits in Humans: Recessive Condition Manifestations Cystic fibrosis Among the most common genetic disorders among European Americans; abnormal secretions of the exocrine glands, with pronounced involvement of the pancreas; most patients develop obstructive lung disease. Tay-Sachs disease Most common among Ashkenazi Jews; degeneration of the nervous system beginning at about 6 months of age; lethal by age 2 or 3 years.

ABO Genotypes and Associated Phenotypes Antigens on Red Blood Cells ABO Blood Type (Phenotype) AA, AO A BB, BO B AB A and B OO None O

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 Expression of 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.

Sickle-cell Distribution in the Old World

Malaria Distribution in the Old World

Levels of Organization in the Evolutionary Process Factor Level Evolutionary Process Mutation DNA Storage of genetic information; ability to replicate; influences phenotype by production of proteins Chromosomes A vehicle for packaging and transmitting DNA

Levels of Organization in the Evolutionary Process Factor Level Evolutionary Process Recombination (sex cells only) Cell Basic unit of life, contains chromosomes, divides for growth and production of sex cells Natural selection Organism The unit that reproduces and which we observe for phenotypic traits Drift, gene flow Population Changes in allele frequencies between generations

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. Recombinant DNA techniques allow scientists to transfer genes from the cells of one species into the cells of another. Genetic manipulation is controversial due to safety and environmental concerns.

Quick Quiz

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.