BIO120H1 Lecture Notes - Lecture 4: Blending Inheritance, Neo-Darwinism, Wild Type
BIO120 – Lecture 4 – NeoDarwinism and the Evolutionary Significance of Genetic
Variation
Darwin:
•Darwin at the time didn’t know anything about genetics and inheritance mechanisms
oHis theory of evolution depended on 3 requirements
Variation (genetics)
•There must be variation among individuals in a population
Heredity (genetics)
•Children resemble their parents, passed on genes
Selection (ecology)
•Certain forms are better at surviving and mating than others in a
particular environment
•Darwin believed in blending inheritance (which was false)
McClintock:
•Jumping genes, transposable genetic elements of Maize
oArise by mutation, can move around the genome
oAka transposable genetic elements
Mendel:
•Father of modern genetics, was a priest
•Crossed peas, established laws of inheritance
•Mendel’s experiment
oP (cross 2 pure-breeding lines) (AA and aa)
oF1 – self the first gen (all Aa)
oF2 – should be the same but differ in some phenotypes (AA, Aa, Aa, aa)
3:1 phenotypic ratio
1:2:1 genotypic ratio
•Mendelian genetics = phenotypes controlled by 2 alleles, one dominant and one
recessive, and only a single gene locus
Fisher:
•Mathematical geneticist – brought genetics into evolution
•Had his theorem of natural selection
oLow genetic variance populations will not respond to environmental changes;
these are likely to go extinct
oHigher genetic variance lets the fitness of the population increase faster
This is important to conservational biology
Genotype:
•Genetic constitution of an organism
oE.g. Aa, AaBB
Phenotype:
•Observed
oTraits of an organisms
oOr a varying feature of the organism
•Phenotype = genes x environment
oGenes combined with environmental influences determines what phenotype is
expressed
Genome:
•Entire organisms’s DNA
oIncludes genes and non-coding regions
•Important in medical sciences, evolution & cell biology, etc.
DNA:
•Carries the genetic information of organisms
•DNA is replicated during meiosis, which is when mutations may occur in the DNA
Gene:
•Functional unit of inheritance
•Sequence of 3 nucleotides – codon
•Unit of hereditary information on the chromosomes, consists of DNA
•Evolution needs genetic variation (which comes from):
oMutation
oRecombination
oGene flow
oHybridization
•Genes code for proteins, proteins are put together to make an organism’s structure
oGenes also help to regulate bodily activities
Allele:
•Most organisms carry 2 copies of each gene (2 alleles)
oThese are diploid
Mutation:
•Ultimate source of genetic variation
•Stable change in DNA sequence -> change of genotype
•Very low but variable rate of occurrence in all organisms
oNot all genes have the same mutation rate
oSome areas of a genome might have higher relative rates of mutation compared
to other spots on the genome
oThere is also variation between mutation rates in different species
•Mutations are context dependent
oNeutral
Neither improves nor detracts from an organism’s fitness
oDeleterious & Lethal
Most mutations are this type; harms the organism’s fitness
oBeneficial
Improve the organism’s fitness
•Context-dependent aspect
oDepending on what environment the organism is in, the same mutation can be
beneficial in one environment and deleterious in another
oE.g. experiments with flies
Same mutation is neutral when flies grown at 20 degrees
Lethal when grown at 25 degrees
oThere are also some extremely lethal mutations (e.g. affecting organ function)
that will be lethal no matter what environment the organism is in
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