Chapter 1 Genetics: An Introduction

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.

• 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).

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).

This is onl a historical differentiation

Classic GeneticsClassic Genetics

From Mendel to the From Mendel to the Central Dogma of Central Dogma of Genetics (1866Genetics (1866--1941).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.

Mendel’s Garden Pea Pisum sativum has become the symbol of classic genetics.

What’s wrong with Mendel? Doesn’t he like cooked peas?

Modern GeneticsModern Genetics

From the Central From the Central Dogma of Genetics Dogma of Genetics (1941 to (1941 to todaystodaysGenomics).Genomics).

Beadle and Tatum (one Beadle and Tatum (one genegene--one polypeptide), one polypeptide), Watson and Crick Watson and Crick (DNA structure), Paul (DNA structure), Paul Berg (genetic Berg (genetic engineering) are engineering) are examples of examples of rosettarosettastones findings in stones findings in Modern Genetics.Modern Genetics.

The subdisciplines of Genetics

A more accurate differentiation of the different branches studied by genetics is looking at the Genetics subdisciplines

Transmission GeneticsMolecular Genetics

Quantitative GeneticsPopulation Genetics

Transmission GeneticsSometimes called classic Sometimes called classic genetics deals with how genetics deals with how genes are transmitted from genes are transmitted from generation to generation generation to generation and how they recombineand how they recombine

Molecular Genetics

It deals with the It deals with the molecular structure molecular structure and function of genes. and function of genes. Analyzing the Analyzing the sequence of the sequence of the nitrogen bases in a nitrogen bases in a gene (and how it may gene (and how it may change its expression) change its expression) is an example of is an example of molecular genetics.molecular genetics.

Population GeneticsIt studies heredity of It studies heredity of one or a few genes in a one or a few genes in a large group of large group of individuals individuals (population). For (population). For example the study of example the study of the distribution and the distribution and inheritance of lactose inheritance of lactose intolerance in different intolerance in different ethnic ethnic americanamericangroups.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)

95-100 %Asian Americans

95 %Native Americans

70-77 %African Americans

52 %Latinos

(Hispanic Americans)

2-19 %European Americans

U.S.

LACTOSEINTOLERANT

ADULTSPOPULATION

Quantitative GeneticsIt also studies heredity of It also studies heredity of a trait in a large group of a trait in a large group of individuals but the individuals but the individual trait is codified individual trait is codified by many genes by many genes simultaneously. For simultaneously. For example the example the discontinousdiscontinousdistribution of shell color distribution of shell color (a polygenic trait) in the (a polygenic trait) in the snail snail CepaeaCepaea nemoralusnemoralusfrom a population in from a population in England.England.

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.

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.

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

Basic Concepts of Genetics 1The 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.

• 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.

Basic Concepts of Genetics 2

In the cell, the genetic material (DNA) is organized in structures called CHROMOSOMESMany prokaryotes (not all) have a single, usually circular chromosome. This chromosome is made of DNA only.

• In eukaryotes, the DNA is located in the nucleus forming linear chromosomes. Each chromosome consists of a single DNA molecule complexed(associated) with histoneproteins.

Scanning micrograph of a

chromosome

Human female chromosomes shown by bright field G-banding

Human female karyotype shown by bright field G-banding of chromosomes

Basic Concepts of Genetics 3Each 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.

Basic Concepts of Genetics 4Mendel’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 Bb.)

Basic Concepts of Genetics 5

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.

• 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

Basic Concepts of Genetics 7• There are many differences between

organisms. These differences are the result of the different genes they carry.

• 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.