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BIO120H1 Lecture Notes - Meiosis, Dna Replication

Course Code
Benjamin Wright

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Lecture 3
Evolutionary Significance of Genetic Variation
Mendelian genetics: Scitable- Gregor Mendel and the principles of inheritance
Inheritance: Scitable- Mitosis, meiosis, and inheritance
Key Concepts/ Terms:
Genetic polymorphism
Heritable variation
Quantitative inheritance
Gene-environment interaction
Mendel’s Experiment:
Conclusion from pea experiment:
1)Inheritance determined by discrete particles (genes)
2)Each organism carries two copies (alleles) of each gene
3)Organisms form gametes (sperm/ pollen, eggs/ ovules) containing only one allele
4)Offspring inherit one allele from each parent at random
Genotype: The genetic constitution of an organism
Phenotype: The organism as observed
1)Evolutionary significance of genetic variation: Darwin, Mendel, Fisher
2)Origin of genetic variation: Mutation, Recombination
3)Kinds of genetic variation: Discrete traits, Quantitative traits
1)Number one:
a.Spermists and Ovists in the 1700s believed only one parent contributed to inheritance
b.Theory of blending inheritance postulated that genes from both parents mix together
irreversibly (1800s)
c.In the Origin of Species Darwin provided a causal explanation of the origin of biological
diversity: The Theory of Natural Selection
d.Starting point of theory, occurrence of heritable variation (differences among individuals
in morphology, behaviour, and reproductive performance that have a genetic basis.
e.Darwin thought this a fact but didnt understand the mechanism of inheritance or how
hereditary variation arises
f.He argued that certain variations were more advantageous than others for the survival and
reproduction of their possessors
g.Organisms having these variations are more likely to survive and reproduce then those
h.This process results in evolution by natural selection
i. Natural selection is the fundamental process responsible for organic evolution
j. Natural selection can only occur if there is hereditary (genetic) variation
k.The more genetic variation there is in a population, the greater opportunity for the
operation of natural selection
l. Mendels experiments demonstrating underlying mechanisms of inheritance was published
soon after the Origin of Species, Darwin was not aware (took several decades for them to
be widely known and more for them to be connected to Darwin)
m.His laws demonstrated the particular nature of inheritance, and this allows favourable
variants to persist and spread via natural selection
n.1930 R.A. Fisher showed amount of genetic variation with respect to fitness is directly
correlated with rate of evolutionary change by natural selection
o.Fitness is genetic contribution of an individual to succeeding generations, relative to the
contributions of other individuals in the population
p.Stated in Fishers Fundamental Theorem of Natural Selection: The rate of increase
in fitness of a population at any time is equal to its genetic variance in fitness at that time.
Genetic Analysis of Variation:
-Discontinuous variation: (mendelian genetic)
oGenes of major effect, dominance and recessiveness, spread of alleles
-Continuous variation: (quantitative genetics)
oMany genes of minor effect, important environmental effects, selection response
2)Number Two:
a.Mutation is ultimate genetic variation and makes evolution possible
b.Caused by random chemical mistakes, primarily during process of DNA replication
c.Many types of mutations:
i.Point mutations, causing the substitution of a single base pair of DNA
ii. Insertions or deletions, collectively called indels’
iii.Chromosomal (or structural) mutations such as inversions and translocations
d.Mutation rates per gene vary from less than 10-9 to more than 10-4
e.Even in the same organism mutation rates can vary for different genes by three orders of
f.Mutations can range from
g.Mast majority of newly arisen mutations are harmful, mutations may become beneficial if
organisms enter novel (new) environment
h.Discrete variation and quantitative variation are the types of variation that occur in
natural populations
i.Discrete variation:
occurs when two or more alleles at a given gene locus occur in the same
if the alleles have major effect on phenotype, this can result in
discontinuous patterns of variation
such patterns are known as morphological polymorphisms (eye colour and
industrial melanism
sometimes polymorphism involves biochemical characters (blood groups and
ii.Quantitative variation: (very important for understand population evolution)
relation between gene number and phenotypic variability
occurs when there are more continuous differences in phenotype in a
population (human height)
occurs when a trait is controlled by many genes, each of which alone has only
a very small influence on the character
usually also affected by environmental factors