CHAPTER 1 Genetics: An Introduction


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CHAPTER 1 Genetics: An Introduction

  1. 1. Chapter 1 Genetics: An Introduction
  2. 2. What is Genetics? • Genetics is the fraction of biology that studies heredity. • Genetics is the center of all biology because gene activity underlies all biological processes.
  3. 3. • The study of Genetics involves the understanding of biological properties that are transmitted from parents to offspring. • It studies genes, the molecular nature of these genes, how these genes are transmitted from generation to generation. It includes the study of how these genes are expressed and how these genes’ activity is regulated. • An understanding of gene structure, activity and regulation helps to understand how these genes control all biological processes (from cell structure to reproduction).
  4. 4. Classic and Modern Genetics • The terms of “classic” and “modern genetics” are used to separate the scientific research that has been done in Genetics since its beginnings. • The abbreviated timeline of events (or the expansive timeline in your textbook) shows that generally classic genetics involve the studies and discoveries previous to the Central Dogma of genetics proposed by Beadle and Tatum in 1941 (these studies encompassed mainly transmission genetics). After 1941, we talk about Modern Genetics (mainly Molecular genetics).
  5. 5. Classic Genetics From Mendel to the Central Dogma of Genetics (1866-1941). Mendel’s published work, Experiments in Plant Hybridization (1865), languished with no discernable impact until in 1900 three other investigators independently discovered the same genetic principles.
  6. 6. Mendel’s Garden Pea Pisum sativum has become the symbol of classic genetics. What’s wrong with Mendel? Doesn’t he like cooked peas?
  7. 7. Modern Genetics From the Central Dogma of Genetics (1941 to todays Genomics). Beadle and Tatum (one gene-one polypeptide), Watson and Crick (DNA structure), Paul Berg (genetic engineering) are examples of rosetta stones findings in Modern Genetics.
  8. 8. The subdisciplines of Genetics A more accurate differentiation of the different branches studied by genetics is looking at the Genetics subdisciplines Transmission Genetics Molecular Genetics Quantitative Genetics Population Genetics
  9. 9. Transmission Genetics Sometimes called classic genetics deals with how genes are transmitted from generation to generation and how they recombine
  10. 10. Molecular Genetics It deals with the molecular structure and function of genes. Analyzing the sequence of the nitrogen bases in a gene (and how it may change its expression) is an example of molecular genetics.
  11. 11. Population Genetics It studies heredity of LACTOSE one or a few genes in a POPULATION INTOLERANT ADULTS large group of U.S. individuals European Americans 2-19 % (population). For Latinos (Hispanic 52 % example the study of Americans) the distribution and African Americans 70-77 % inheritance of lactose Native Americans 95 % intolerance in different Asian 95-100 % ethnic american Americans groups. Source: Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution," current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517) and Norman Kretchner, "Lactose and Lactase," Scientific American 277 (Oct. 1972, pp. 71-78)
  12. 12. Quantitative Genetics It also studies heredity of a trait in a large group of individuals but the individual trait is codified by many genes simultaneously. For example the discontinous distribution of shell color (a polygenic trait) in the snail Cepaea nemoralus from a population in England.
  13. 13. How do we study genetics • We can gain knowledge of genetic developments by performing research using the scientific method or hypothetico- deductive method of investigation.
  14. 14. Basic Research: Research done to gain an understanding of fundamental phenomena. For example studying the chemical composition of the cell membrane. Applied Research: Experiments done with the idea of solving an specific problem.
  15. 15. Tools used in Genetic Research • Classic techniques of breeding such as crosses, backcrosses and testcrossing. • Microscopic techniques, such as SEM, TEM, and others. • Molecular techniques such as DNA sequencing, PCR, cloning, etc… Use of genetic model organisms • Construction of Genetic maps • Genetic Databases
  16. 16. Basic Concepts of Genetics 1 The biological information fundamental to life is encoded in the molecule of DNA. DNA (deoxyribonucleic acid) is the genetic material in all prokaryotes, eukaryotes and some viruses. Other viruses have RNA.
  17. 17. • The molecule of DNA is made of two strands (chains). Each strand is a chain of NUCLEOTIDES. • Each nucleotide is formed of three components: A phosphate group (PO4-3), a pentose (5C sugar) and a nitrogen base (A, G, C, T). The arrangement of the nucleotides in the chain forms a double helix. • GENES (which Mendel called factors) are specific sequences of nucleotides. • The full sequence of the DNA or all the genes of an organism is called its GENOME.
  18. 18. Basic Concepts of Genetics 2 In the cell, the genetic material (DNA) is organized in structures called CHROMOSOMES Many prokaryotes (not all) have a single, usually circular chromosome. This chromosome is made of DNA only.
  19. 19. • In eukaryotes, the DNA is located in the nucleus forming linear chromosomes. Each chromosome consists of a single DNA molecule complexed (associated) with histone proteins.
  20. 20. Scanning micrograph of a chromosome
  21. 21. Human female chromosomes shown by bright field G-banding
  22. 22. Human female karyotype shown by bright field G-banding of chromosomes
  23. 23. Basic Concepts of Genetics 3 Each organisms contains 2 copies of a gene (diploid), one maternal and one paternal. The alternative versions of the gene are called ALLELES. An organism having a pair of identical alleles is said to be HOMOZYGOUS. An organism having a pair of different alleles is said to be HETEROZYGOUS. The complete genetic makeup of an organism is the GENOTYPE. The physical expression of the genotype or the observable properties of an individual is the PHENOTYPE.
  24. 24. Basic Concepts of Genetics 4 Mendel’s 1st Law or Principle of segregation: The factors (alleles) segregate independently into the gametes during Meiosis. (i.e. An organism Aa will produce gametes A and a.) Mendel’s 2nd or Principle of independent assortment: The two factors (alleles) controlling one trait segregate independently from the two factors (alleles) controlling another trait. (i.e. Aa will segregate independently from
  25. 25. Basic Concepts of Genetics 5
  26. 26. Basic Concepts of Genetics 6 • The process by which a gene produces its product is called GENE EXPRESSION. • Beadle and Tatum hypothesis, One-gene- one- polypeptide, explains how gene expression is accomplished. Each gene is expressed as a protein. This protein can be the final product (such as hair protein) or an enzyme that catalyzes the formation of the final product.
  27. 27. • The expression of a DNA gene is accomplished by the process of TRANSCRIPTION (from DNA to mRNA) and TRANSLATION (from mRNA to protein), thus providing the CENTRAL DOGMA of GENETICS: DNA mRNA Protein
  28. 28. Basic Concepts of Genetics 7 • There are many differences between organisms. These differences are the result of the different genes they carry.
  29. 29. • These differences have resulted from the evolutionary process of: Mutations: Change in the genetic material. Recombination: The exchange of genetic material between chromosomes. Selection: Particularly favorable gene combinations in a given environment.