Chapter 01Overview of

Genetics

Genetics – Overview• Genetics is the study of heredity and variation

• Genetics is the unifying discipline in biology

• Explains why organisms have certain traits o How traits are passed from parent to offspringo Relationship between genes and traits

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Genetic Discoveries and Technology• Genetic discoveries are growing at a fast pace

• Example: The Human Genome Projecto Goal: Decipher the information in the human genome – the DNA of all

our chromosomeso First draft in 2001, completed 2003o 3 billion nucleotide base pairso Shed light on basic questions about human genes, traits, and possible

cures for disease

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mRNA

Chromosomes

Gene

Protein (composed of amino acids)

Cell

DNA

Amino acid

C G

C G

T A T

A

A T

A T

C G

T A

T A

T A G

DNA, the molecule of life

Trillions of cells

Each cell contains:

• 46 human chromosomes, found in 23 pairs

• 2 meters of DNA

• Approximately 3 billion DNA base pairs per set of chromosomes, containing the bases A, T, G, and C

• Approximately 20,000 to 25,000 genes coding for proteins that perform most life functions

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Genetic Technology• New genetic technologies are useful and

sometimes controversial

• Recombinant DNAo Can provide new medicineso Example: Human recombinant insulin

• Synthesized by E. coli that carry human genes• Provides insulin for diabetes patients

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• Mammalian cloningo 1997, Dolly the sheep was clonedo 2002, Carbon copy (or “Copycat”)o Cloned livestock could benefit farmerso Human cloning has been legally banned

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• Green fluorescent protein (GFP)o A jellyfish gene produces a bioluminescent proteino Under UV light it emits a green glowo 2008 Nobel Prize was awarded for the development of this powerful

toolo Examples:

• GFP gene expressed in mice• GFP expressed in the gonads of male mosquitoes

o Could be used to select sterile males for malaria control programs

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1.1 The Molecular Expression

of Genes

Biochemical composition of cells

How DNA stores the information to make proteins

Proteins are largely responsible for cells structure and function

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Genes and Traits• Geneticists study the relationship between genes

and traits

• Trait – any characteristic that an organism displays

• During growth and development, genes provide a blueprint that determine the organism’s traits

• Examples of human traits:o Eye coloro Hair textureo Heighto Blood Type

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Living Cells are Composed of Biochemicals• All cells are constructed from small organic

molecules

• These are linked together by chemical bonds to form larger molecules

• Cells contain four main types of large moleculeso Nucleic acidso Proteinso Carbohydrateso Lipids

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• Nucleic acids, proteins and carbohydrates can form macromoleculeso Polymers constructed from smaller molecules

• Cellular structures form as a result of the interaction of molecules and macromolecules

• Organelle – a membrane-bound compartment with a specialized function o ex: nucleus

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Proteins Determine Cell Structure and

Function• The characteristics of a cell depend on its proteins

• Proteins are the “workhorses” of cells

• They have diverse biological functionso Transporto Motor proteinso Signalingo Enzymes – proteins that speed up chemical reactions

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DNA Stores the Information for Protein

Synthesis• The genetic material in most living organisms is

deoxyribonucleic acid (DNA)

• DNA encodes the information required to synthesize all cellular proteinso It is able to do so because of its molecular structureo DNA is a polymer of nucleotides

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• Each nucleotide contains one nitrogenous base• Adenine (A)• Thymine (T)• Cytosine (C)• Guanine (G)

• Genetic information is stored in the linear sequence of bases

• Genes – segments of DNA that produce a functional product o The fundamental unit of heredityo Information in genes directs production of proteins

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For example:

• ATG GGC CTT AGC DNA Sequence• Met Gly Leu Ser Polypeptide Sequence

• TTT AAG CTT GCC DNA Sequence• Phe Lys Leu Ala Polypeptide Sequence

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• The DNA in living cells is contained within large structures termed chromosomes.

• Each chromosome is a complex of DNA and proteins

• An average human chromosome containso More than a 100 million nucleotides o about 1,000 different genes

• Human cells have a total of 46 chromosomes

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DNA Information is Accessed During the Process of Gene Expression

• Gene expression occurs in two stepso Transcription

• The genetic information in DNA is copied into a nucleotide sequence of ribonucleic acid (RNA)

o Translation• The nucleotide sequence in RNA provides the

information (using the genetic code) to produce the amino acid sequence of a polypeptide

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DNA

Gene

Transcription

Translation

RNA (messenger RNA)

Protein(sequence ofamino acids)

Functioning of proteins within livingcells influences an organism’s traits.

1.2 The Relationship Between

Genes and Traits

How expression of genes leads to an organism’s traits

Genetic variation The relationship between genes, traits and the

environment How genes are transmitted in sexually reproducing

species The process of evolution

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Traits

• Trait – any characteristic that an organism displays

• Morphological traitso Affect the appearance of the organismo Example: The color of a flower

• Physiological traitso Affect the function of the organismo Example: Ability to metabolize a sugar

• Behavioral traitso Affect the ways an organism responds to the environmento Example: Mating calls of bird species

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Molecular Expression of Genes Leads to an Organism’s Traits

• The relationship between genes and traits spans four levels of biological organization:o Molecular level

• Transcription and translation produce proteinso Cellular level

• Proteins function within the structures of the cello Organism level

• Traits seen at the organism level arise from molecular and cellular properties

o Population level

• Species traits are selected based on survival and reproduction

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Example: Dark and Light Butterflies• Molecular level

o The pigmentation gene exists in two different alleles – variant forms of a gene with different sequences

• Cellular levelo One version of the pigmentation enzyme functions well;

the other enzyme functions poorly

• Organism levelo Butterflies with much pigment look dark; little pigment look

light

• Population levelo Light or dark butterflies are selected in different

environments21

Dark butterflies are usuallyin forested regions.

Light butterflies are usuallyin unforested regions.

d. Populational level

a. Molecular level

The relationship between genes and traits spans four levels of biological organization

Pigmentation gene,dark allele

Pigmentation gene,light allele

Transcription and translation

Highly functionalpigmentation enzyme

Poorly functionalpigmentation enzyme

Wing cells

Lots of pigment made Little pigment made

Pigmentmolecule

Dark butterfly Light butterfly

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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c. Organism level

b. Cellular level

Inherited Differences in Traits Are Due to

Genetic Variation

• Genetic variation refers to differences in inherited traits among individuals within a populationo Ex: White vs. purple flowerso Ex: Black vs. brown hair

• In some cases, genetic variation is very strikingo Members of the same species may be misidentified

as belonging to different specieso Morphs – contrasting forms within a single species

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• Genetic variation results from different kinds of changes at the molecular level

o Gene mutations• Heritable changes in gene sequence• One nucleotide change can have big effect

o Changes in chromosome structure• Large segments of the chromosome may be lost

or rearranged

o Changes in chromosome number• One chromosome lost of gained• Whole set of chromosomes lost or gained

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Traits Are Governed by Genes and the Environment

• Traits cannot be explained by genes alone

• Traits result from the interaction between genes and the environmento Called the norm of reactiono Ex: Diet has an effect on height, weight and even intelligenceo Environment may control whether a genetic disease is manifestedo Ex: Phenylketonuria

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• Phenylketonuria (PKU)

o Phenylalanine hydroxylase enzyme• Converts phenylalanine to tyrosine

o Humans need one or two functional copies of the gene to metabolize phenylalanine

o Humans with two copies of a rare inactive allele cannot metabolize phenylalanine

• Phenylalanine accumulates, causing a number of detrimental effects

• Can be mentally impaired

o Newborns are now screened for PKU

o Individuals with PKU are put on a strict dietlow in phenylalanineo Controlled diet allows normal development

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During Reproduction, Genes are Passed from Parent to Offspring

• Gregor Mendel, in the mid-19th century, provided the foundation for the science of genetics

• He described the basic principles of inheritance

• Inheritance is explained by the behavior of chromosomes during cell division

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• Sexually-reproducing species are diploido Two copies of each chromosome, one from each parento Homologs – the two copies of each chromosome

• Thus two copies of most geneso But the two copies may be different alleleso Ex: PKU gene is on chromosome 12o Only X and Y have some different genes

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1 2 3 4 5 6 7

XX

8

9 10 11 12 13 14 15

17 18 19 20 21 22

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• Most cells of the human body are somatic cells with 46 chromosomes

• Gametes – sperm and egg cells – are haploid, with 23 chromosomeso Combining sperm and egg during fertilization

restores the diploid number of chromosomes

• Sexual reproduction enhances genetic variationo Creates new combinations of traits not found

in either parent

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1 2 3 4 5 6 7

X

8

9 10 11 12 13 14 15

17 18 19 20 21 22

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Somatic Cell

Reproductive Cell (gamete)

The Genetic Composition of a Species Evolves Over the Course of Many

Generations• The genetic makeup of a

population can change over many generationso This is called biological evolution

• Evolution is possible because of natural selectiono The process of differential survival and

reproduction based on genes and traitso Genetic changes accumulate over time

• Example: Evolution of the modern horse, Equus

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EquusHippidium and other genera

Nannippus

Pliohippus

Calippus

Merychippus

Parahippus

Stylohipparion

HipparionNeohipparion

Megahippus

Archaeohippus

Sinohippus

Anchitherium Hypohippus

Miohippus

Mesohippus

Paleotherium

Propalaeotherium

Pachynolophus

Epihippus

Orohippus

Hyracotherium

0

5

10

20

40

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• Important changes:– Larger size– Fewer toes– Modified jaw for

grazing

1.3 Fields of Genetics

Transmission Genetics

Molecular Genetics

Population Genetics

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Geneticists Focus on Model Organisms

• Model organisms – species studied by many researcherso Can compare resultso Determine principles that could apply to

other specieso Model species are easy to grow in the lab

• Examples:o Escherichia coli – a bacteriumo Saccharomyces cerevisiae – yeasto Drosophila melanogaster – fruit flyo Caenorhabditis elegans –

nematodeo Mus musculus – mouseo Arabidopsis thaliana – a plant

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Transmission Genetics Explores Inheritance Patterns

• Examines how traits are passed from parents to offspring

• The conceptual framework was provided by Gregor Mendel in the 1860so Genetic determinants pass from parent to offspring as discrete unitso Now we know these are genes

• Mendel used genetic crosseso Breed individuals and analyze traits of offspring

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Transmission Genetics – Example

Questions

• How are chromosomes transmitted?

• What are common patterns of gene inheritance?

• What are unusual patterns of inheritance?

• How is inheritance affected when genes lie on the same chromosome?

• How do variations in chromosome structure and number occur?

• How are genes transmitted by bacteria?

• How do viruses proliferate?

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Molecular Genetics Focuses on Biochemical Understanding

• The most modern field of genetics

• Deals with molecular features of DNA and how these underlie gene expressiono Organization, control and function of geneso Analysis of DNA, RNA and proteins

• Molecular geneticists often use a genetic approacho Study mutant genes with abnormal function to infer the normal function

of the gene• Example: Loss-of-function mutation

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Molecular Genetics – Example Questions• What are the molecular structures of DNA and RNA?

• What is the composition of chromosomes?

• How is the genetic material copied?

• How are genes expressed at the molecular level?

• How is gene expression regulated during development?

• What is the molecular nature of mutations?

• How do chromosomes exchange material?

• How have genetic technologies advanced understanding?

• What are the composition and function of genomes?

• What is the relationship between genes and disease?

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Population Genetics is Concerned with Genetic Variation and Evolution

• Population genetics deals with the genetic composition of populations and how it changes over time and space

• Connects genetic variation to the organism’s environmento Allele frequencies are an important type of data

• It connects the work of Mendel on inheritance to that of Darwin on evolution

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Population Genetics – Example Questions

• Why are multiple alleles of a gene maintained in a population?

• What factors affect prevalence of alleles in a population?

• What are the contributions of genetics and the environment to the outcome of a trait?

• How do genetics and the environment influence quantitative traits, such as size and weight?

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1.4 The Science of Genetics

Genetics is an experimental science

Genetic TIPS – problem solving strategies

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Genetics is an Experimental Science

• Geneticists use two basic scientific approaches – Hypothesis testing and Discovery-based science

• Hypothesis Testingo Also called the scientific methodo Allows scientists to validate or invalidate a hypothesis

• Discovery-Based Scienceo Collecting data without a preconceived hypothesiso Ex: Analyzing genes in cancer cells to identify mutations

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Scientific Process• The textbook “dissects” experiments into five

components to explain the scientific process:1. Background information2. Hypothesis3. Experimental Steps4. Raw Data5. Interpretation of the data

• While reading about experiments, consider alternatives!Develop your own skills to

• Formulate hypotheses• Design experiments• Interpret data

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Problem-Solving Skills

• In addition to learning foundational knowledge, develop your problem-solving skills

• The textbook includes Genetic TIPSo Topic, Information, and Problem-solving Strategy

• Ten strategies are used repeatedly:

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• Define key terms• Make a drawing• Predict the outcome• Compare and contrast• Relate structure and

function

• Describe the steps• Propose a hypothesis• Design an experiment• Use statistics• Make a calculation