BIO120H1 Lecture Notes - Lecture 15: Mutation Rate, Neo-Darwinism, Dna Replication
Lecture 15: NeoDarwinism & the evolutionary significance of genetic variation
Recall: Requirement for Darwin’s 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 organism’s 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 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
“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”)
○ ATGCAGTà ATGGCAGT
Ÿ Changes in repeat number
○ ATGATGATGATGà ATGATGATGATGATG
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
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?
Ÿ Priest and “father of modern genetics”
Ÿ Through controlled crosses with peas established the laws of inheritance
Ÿ Mendel’s Laws re-discovered at the beginning of the 20th century by Hugo de Vries & Carl