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ASB 101 1/18/17
Lecture 3: Genetics
Things Darwin didn’t know:
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Evolution die to factors other than natural selection
Gregor Mendel
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Discovered principles of discrete inheritance
Published research on plant hybridization in 1866; ignored until rediscovered
7 characteristics of peas were studied by Mendel through controlled hybridization
Found evidence for discrete units of heredity
Mendel’s Conclusion:
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Info coding for biological traits exists as discrete units (pairs) called particles
Traits coded by the same particles are always expressed over to the traits coded by other particles
(dominant)
Called smallest particle representing heredity characteristic – gene
YY – genotype of the pure yellow line
yy – genotype of pure green line
Y is dominant to y (y is recessive)
Different types of 1 gene = allele
Law of Segregation of Particles:
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Gametes carry 1 copy of each gene from mother and father
• Possibility of a gamete carrying either 1 of the 2 original parental copies = 50%
• Gametes are united in conception
Law of Independent Assortment of Particles:
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• Only when 2 copies of the recessive gene are inherited do we observe the recessive form
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• The allele of a gene contributed for one trait that does not influence which allele is
contributed for other genes located close together on a chromosome = loci
The inheritance of polygenic traits (height, skin color, IQ) looks like phenotypic blending
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Misled biologists
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Total of 378 genetic loci in determining skin color
Phenotypic inheritance of many discrete particles looks like blending
Successive Molecular Biologists Showed:
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Different sequences of base pair = different alleles
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Observed variation in
phenotype can be determined by the genotype
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Modern genetics shows that genetic change can produce variations in ALL morphological,
physiological, and behavioral traits
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IV
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5% of the genome doesn’t code for protein products
Things Darwin Didn’t Know About
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Small human populations in different parts of the world have becomes geographically isolated
• Through genetic drift and founder effect, some polymorphic traits become concentrated by chance
in one or another sub-population
• Genetic drift will lead to variation between isolated populations even when there is no selection
acting to make them diverge
o Random diversity
• Polymorphic traits that are adaptively neutral can become unequally distributed over the globe
through drift and founder effects
• Most traits are not selectively neutral – may take long periods of time for selection to reverse the
effects of a single event of genetic drift
• Natural selection often acts in opposition to drift and often eliminates its importance in
populations larger than a few hundred individuals or over any significant time period
• Whenever some gene flow between such populations occurs, genetic boundaries between
populations are “blurred” taking form of Clines
• Blood groups (A,B and O) are distributed unequally over the globe, may be the result of
reproductive isolation and drift
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Genetic drift leads to variation between isolated populations when there is no selection
acting to counter-act drift
o Random variation = diversity
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Neutral traits can become unequally distributed over the globe via drift and founder
effects
Most traits aren’t selectively neutral
Natural selection counteracts genetic drift – takes long periods of time for selection to
reverse a single event of genetic drift
Natural selection acts in opposition to drift; eliminates its importance in populations
larger than a few hundred populations
Genetic drift – stochastic diversity
Natural selection – adaptive diversity
Human diversity is the result of both natural selection and genetic drift
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Homo sapiens is:
o Polymorphic -different forms of particular traits in different places
o Polytypic – frequencies of polymorphic alleles vary between populations