Download Overview Discontinuous variation Genetic methodology Continuous

Overview
Discontinuous variation
Hereditary material is DNA, a double helix of
complementary polynucleotides.
Genes are segments of DNA encoding the amino acid
sequence of a polypeptide.
Hereditary variation is caused by variant forms of genes
known as alleles.
Alleles can be studied at many levels.
Each species has its own distinctive pool of genes.
Evolution is a consequence of genetic changes in a
population over time.
Relatively rare compared to continuous
variation
Variation falls into distinct categories
Usually due to differences between two alleles
Š designated by letters, e.g., A and a
Š A dominant to a
Polymorphism: multiple alleles of single gene
Mutants: rare, exceptional variants
Š normal phenotype called wild-type
Alleles arise through mutation, DNA change
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Genetic methodology
Continuous variation
Isolation of mutations (natural or induced)
Analysis of progeny of controlled matings
(crosses)
Biochemical analysis of underlying cellular
processes
Microscopic analysis of chromosomes
(cytogenetics) and phenotypes
Direct analysis of DNA
Unbroken range of phenotypes; no discrete
classes
Intermediate phenotypes more common
than extreme ones
A result of a combination of genetic and
environmental variation
Underlying genetic basis often difficult to
study
Important in plant and animal breeding
Š genomics: sequencing genome
Š bioinformatics: extraction of information from
DNA
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Genes, environment, organism
Model 1: genetic determination
Š characters specified by genes
Model 2: environmental determination
Often, there are
multiple inputs into
phenotype.
Š characters determined by environment
Model 3: genotype-environment interaction
Š gene expression is conditioned by environment
Norm of reaction: set of environment-genotype relations
for given genotype
Š single genotype may have several phenotypes, depending upon the
environment
Developmental noise: random events in development
leading to phenotypic variation
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Genes and evolution
Darwin recognized role of hereditary
variation in evolution (but was unaware of
true mechanism of heredity)
Genetic variation (product of mutation) is
raw material for evolutionary change
Natural selection: differential reproduction
of individuals with different alleles
Random genetic drift: change in frequencies
of genetic variants resulting from random,
non-selective processes
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
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Genetics and human affairs
>1000 inherited genetic diseases in humans
Cancer is caused by mutation in somatic
cells
Genetics pertains to social policy
Š debate over role of genetics and IQ
Š genetics of sexual orientation
Biotechnology and genetic engineering
Š new pharmaceuticals
Š new varieties of plants and animals
Š concerns over ethics and safety
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Genetics
Experimental science of heredity
Grew out of need of plant and animal
breeders for greater understanding of
inheritance of economically important
characters
Gregor Mendel: discovered principles of
heredity
Today, genes are explained in molecular
terms
Chapter 1: Genetics and the organism
From gene to protein
Physical and chemical basis
Genome: basic complement of DNA of an organism
Š haploid: one copy of genome (fungi, algae, bacteria)
Š diploid: two copies of genome (plants, animals)
Genes: regions of chromosomal DNA encoding
polypeptides
Š different genes on each chromosome
Š homologs: chromosomes with same genes
Š in diploids, one homolog inherited from each parent
DNA: complementary polynucleotide chains
Š A-T and G-C base pairs
Š double helix
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
© 2002 by W. H. Freeman and Company
Protein
Š linear chain of amino acids (polypeptide)
encoded by gene
subject to variation
Š folds into 3 dimensional structure
Š may associate with other proteins
DNA → mRNA → polypeptide
Š DNA → mRNA : transcription
Š mRNA → polypeptide : translation
genetic code
ribosomes
tRNA
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
Genetic variation
Alleles
Š alternative forms of a gene encoding proteins
with altered amino acid sequence
Š located at same position (locus) on chromosome
Phenotype: appearance or physiological
expression of gene
Genotype: alleles present in individual
Discontinuous variation (qualitative)
Continuous variation (quantitative)
Chapter 1: Genetics and the organism
© 2002 by W. H. Freeman and Company
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