Chapter 3 : Genes, Genetics, and Geneticists

Graphical representation of the different models of inheritance: fluid blending and discrete particle.

The Principle of Segregation. Mendel observed that experimental crosses between pea plants with round or wrinkled seeds yielded plants with round seeds in the first generation and a 3:1 mix of plants with round and wrinkled seeds in the second generation. From this result and observations of other characteristics, he inferred that maternal and paternal hereditary information is packaged as discrete particles and that maternal and paternal particles separate from each other during gamete formation.

The Principle of Independent Assortment. By observing the hereditary patterns of two separate traits, seed shape and color, Mendel inferred that during gamete formation, the alleles for one trait segregate independently of the alleles for a trait located on a different chromosome. What would Mendel have seen if the genes for seed color and shape were adjacent on a chromosome?

Discovery of the “transforming factor” by Griffith. The strain of pneumonia-causing bacteria (smooth) is virulent because of a gene that codes for a protective outer covering. A different strain (rough) does not cause pneumonia because it lacks the gene for the protective covering. When nonvirulent bacteria are mixed with heat-killed virulent bacteria, the nonvirulent bacteria are transformed into virulent bacteria. The gene for the protective outer layer moves from the dead smooth bacteria to the living nonvirulent strain.

The experiments of Alfred Hershey and Martha Chase. One group of viruses containing protein labeled with the radioactive isotope 35S and a second group of viruses containing DNA labeled with the radioactive isotope 32P infected bacterial cells by injecting their genetic material into the cell. Hershey and Chase separated the viral coats from the bacterial cells and found 35S in the viral coats and 32P within the bacterial cells. Viral progeny that resulted from the infection also contained radioactive 32P. Hershey and Chase thus concluded that the genetic material of the virus was DNA and not protein.

The diagram illustrates the complex relationships among genes, the proteins they encode, and observable traits. The path from gene to trait is neither straightforward nor linear. One gene affects many traits, and one trait is the end result of the actions of many genes. Gene products produce feedback and alter the activities of other genes.

Gene changes in cystic fibrosis. Different mutations in the gene that encodes the chloride channel protein have different phenotypic effects.

Diagrams of the four types of chromosomal aberrations, or macromutations. The letters denote genetic loci.

Diagrams of the disruptive effect of a translocation on chromosome pairing in meiosis and the resulting gametes. The parental genotype possesses one normal version of each of the two chromosomes involved in the translocation event. The letters indicate genetic loci. Lowercase and uppercase letters denote different alleles for the same trait. Four of the six possible gametes derived from the parental genotype are nonviable because they do not have a complete complement of genes.

Replicative and nonreplicative transposons.

Crossing over between homologous chromosomes occurs during gamete production. As a result, the gametes differ from each other and from the parental genotype.

Sexual reproduction, genetic variation, and recombination. Genetic variation is created during three stages of sexual reproduction. When gametes are produced, they differ genetically from the parental genotype because they have half the amount of DNA and a random assortment of paternal and maternal chromosomes. During fertilization, genetic material from two sources is combined, creating an offspring that differs genetically from both parents. Finally, during gamete production, crossing over between maternal and paternal (homologous) chromosomes occurs, creating “within chromosome” genetic variation.

Asexual reproduction. Offspring are genetically identical to the parent. Genetic variation is created through mutation.

Bacterial conjugation. Genetic material is exchanged between F+ and F− cells.

Transformation. A cell takes up free DNA from its environment, integrates it into its chromosome, and expresses the encoded products.

Transduction. A virus serves as a conveyer of genetic material from one organism to another. In the example here, the organism is a bacterium, and the virus is a bacteriophage.