BIO120H1 Lecture Notes - Lecture 15: Mutation Rate, Neo-Darwinism, Dna Replication

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12 Nov 2011
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Lecture 15: NeoDarwinism & the evolutionary significance of genetic variation
Recall: Requirement for Darwins theory
Ÿ Variation: variation among individuals in a population
Ÿ Heredity: progeny resemble their parents more than related individuals
Ÿ Selection: some forms between at surviving and breeding in a given environment
Basic terms used in genetics
Ÿ Genotypes: genetic constitution of an organism- used in relation to a particular gene or gene
combination (ex. AA. aA)
Ÿ Phenotype: the organism as observed- used when discussing a trait of a feature of an
organism that varies
Ÿ Genome: the entire organisms DNA including both genes and non-coding regions
What is a gene?
Ÿ The functional unit of inheritance
Ÿ A unit of hereditary information located on the chromosomes consisting of DNA
Ÿ A DNA sequence composed of codons essential for a specific biological function
Ÿ DNAàRNAàprotein synthesis
Where does genetic variation come from?
Ÿ Mutation
Ÿ Recombination
Ÿ Gene flowànew members coming into the population
Ÿ Hybridizationàmixing between species
Independent assortment & Recombination
Ÿ Independent assortment & recombination during meiosis generated enormous diversity (in
humans with n=23 chromosomes, 223 possible gamete combination)
Ÿ Most genetic variability in a population results from sexual reproduction; in any given
generation input from mutation very small.
Mutation in fruit fly
Single recessiveàwhite eye color
Sickle-cell anemia
Jumping genes or mobile (transposable) genetic elements in corn. Arise by mutation and can
move around the genome.
Petal color mutant in daisy (3 scenarios)
Ÿ Pollinators not visit it, this mutation will have no fitness
Ÿ Maybe there is another mutant, replace white by yellow.
Ÿ The two forms coexisting, both the yellow and white.
Mutation-ultimate source of genetic variation
Ÿ Stable change in DNA sequence resulting in a change of genotype
Ÿ Occurs at a very low but variable rate in all organisms
Ÿ Effects: neutral (no effect), deleterious (although it does kills you, but will cause 0 fitness),
lethal (YOU ARE DEAD), beneficial; in many cases their fitness effects depend on
Ÿ To be important for evolution, must occur in germ cells (sperm/egg)-somatic mutations NOT
Characteristics of mutation
Ÿ Mutation is an unstoppable phenomenon
Despite cellular mechanisms to correct errors during DNA replication
Ÿ Mutation is not directed by the organism or the environment
Random with respect to effects on fitness genes
Ÿ Rates depends on the type of mutation
Also varies among genes
Ÿ Environmental insults can affect mutation rate
Mutagens, high temperature
Types of mutation
Ÿ Point mutations
Ÿ Insertions/deletion (including jumping genes)
Ÿ Changes in repeat number
Ÿ Chromosomal rearrangements (mostly lethal)
The distribution of fitness effects of the new mutations?
Fitness distribution for new mutants: experiments with yeast
Ÿ Many deleterious- some lethal
Ÿ Many have no detectable effect-neutral or nearly
Ÿ Very few are beneficial
Motoo Kimura
Ÿ Theoretical population geneticist, first to recognize the importance of neutral mutations.
Mutation rate in eukaryotes (no need to memorize)
Ÿ Each human carries 3-5 recessive lethal alleles
Ÿ Alleles causing death when homozygous
Ÿ Mating among relatives cause a higher incidence of offspring mortality
Inheritance and the transmission of genes among generation
Ÿ How is genetic information transmitted from parents to offspring- mechanism of
Ÿ How are traits expressed in parents and offspring is this influenced by how many genes
control a trait?
Gregor Mendel
Ÿ Priest and father of modern genetics
Ÿ Through controlled crosses with peas established the laws of inheritance
Ÿ Mendels Laws re-discovered at the beginning of the 20th century by Hugo de Vries & Carl