Gregor Mendel (1822-1844) & the Foundations of Genetics.

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    19-Dec-2015

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<ul><li> Slide 1 </li> <li> Gregor Mendel (1822-1844) &amp; the Foundations of Genetics </li> <li> Slide 2 </li> <li> Early Views of Inheritance The Humunculus - in the egg or sperm? Pangenesis - the mechanism of acquired inheritance each tissue has its own genes, which migrate to the egg &amp; sperm Blended Inheritance - characters take on characteristics of both parents </li> <li> Slide 3 </li> <li> Why Mendel Liked Peas Several variable characters with two discrete traits easy to score (yellow or green) Can control fertilization, including self- fertilization can produce pure lines Offspring always have one of the parental traits </li> <li> Slide 4 </li> <li> Slide 5 </li> <li> X 100% F1F1 P </li> <li> Slide 6 </li> <li> X F1F1 75% F2F2 + 25% </li> <li> Slide 7 </li> <li> X F1 75% F2 + 25% X aa AA Aa aa 25% AA 50%Aa P </li> <li> Slide 8 </li> <li> X aabb AABB P X F1 AaBb F2 aaBb aaBB aabb AABB AABb AaBb AaBB AAbb Aabb 6% 1 19% 3 56% 9 19% 3 </li> <li> Slide 9 </li> <li> Mendels Inferences Alternate traits caused by alternate forms of genes, the unit of heredity An organism has two genes, one from each parent, for each character can produce pure lines Offspring always have one of the parental traits Sperm &amp; eggs always have just one allele (gene variant), because they segregate When two alleles are different, one is fully expressed and one is masked (dominant or recessive) </li> <li> Slide 10 </li> <li> Mendels First Conclusion: Law of Segregation All allele pairs randomly segregate during gamete formation Paired condition restored with fusion (fertilization) Aa a A 1:1 </li> <li> Slide 11 </li> <li> Mendels Second Conclusion: Law of Independent Assortment Each allele pair segregates independently of all others AaBb aBABabAb 1:11:1: </li> <li> Slide 12 </li> <li> Chromosomes are the location of genes Chromosomes: long, threadlike associations of genes found in the nucleus consisting of protein &amp; DNA Mendels Laws hold for chromosomes, within chromosomes there is some shuffling, called crossing-over Humans: 46 chromosomes - 22 pairs of autosomes plus 2 sex chromosomes (X and Y) </li> <li> Slide 13 </li> <li> Slide 14 </li> <li> Mendels Laws are a powerful source of variation 2 possible combinations of chromosomes to form gametes &gt; 8,000,000 different gametes 23 When two gametes combine (fertilization), there are approximately (8 million) combinations 2 Actual # of possible combinations of zygotes (fertilized eggs) in humans = 70, 368, 744, 177, 664 </li> <li> Slide 15 </li> <li> Somatic vs Germ Cells 2n 4n Somatic (body) vs Germ (reproductive) Cells 2n 4n 2n 4n 2n nn nn Mitosis (no change) Meiosis (change) No shuffling Shuffling </li> <li> Slide 16 </li> <li> Somatic vs Germ Cells 46 92 Somatic (body) vs Germ (reproductive) Cells in Humans 2n 4n 2n 46 92 2n 23 2n 23 Mitosis (no change) Meiosis (change) No shuffling Shuffling </li> <li> Slide 17 </li> <li> Crossing Over in Meiosis Another Way to Generate Variation Genes on the same chromosome are linked - independence of segregation depends on distance and frequency of crossing-over </li> <li> Slide 18 </li> <li> From DNA to Protein DNA Base Pairs A-C-G-T Triplet Codons for (20) Amino Acids AAA - CAT etc. RNA Intermediaries Protein (polymer of Amino Acids) </li> <li> Slide 19 </li> <li> What Proteins Do. Provide structure Catalyze reactions Recognize molecules Transport molecules Regulate gene expression </li> <li> Slide 20 </li> <li> Change in one base pair - may or may not change amino acid, changed amino acid may or may not change protein conformation Spontaneous, but also increased by radiation, heat, chemical mutagens Rate Infrequent: one in a billion bases Mutation Point Mutations AATAAGAAAATATGAA </li> <li> Slide 21 </li> <li> Detectable Genetic Mutations Many amino acid substitutions do not effect a proteins function - they are silent Non-silent substitutions affect the proteins conformation (shape) or expression (promote or stop) Sometimes silent substitutions become revealed when the environment is changed Many important genetic diseases (e.g. PKU, Sickle- Cell) Frequency: about one in a million amino acids </li> <li> Slide 22 </li> <li> Three Genetic Mutations Substitution Insertion Deletion AATAAGAAAATAGAA AATAAGAAAATATGAA AATAAGAAAATAAAGAA </li> <li> Slide 23 </li> <li> Chromosomal Mutations Chromosomes can be duplicated, portions can be translocated to a different chromosome or inverted on the same, or deleted Usually has profound consequences - sterility or worse Common, e.g. Downs syndrome 1:700 births Major mode of instantaneous speciation in self- fertilizing or inbreeding species, especially plants </li> <li> Slide 24 </li> <li> Only 1/3 more genes than a worm - Genes like components in assembly lines? Many more harmful mutations per generation Much less coding DNA ( rest junk or spacer or ?? ) Human genomes are complex, but . </li> <li> Slide 25 </li> <li> Genetic Load For humans, estimated by reduced fertility and increase in birth defects associated with conceptions between relatives 4 recessive lethals per individual, more than one new lethal per generation In womens eggs, chromosomal defects in eggs increase with age In mens sperm, DNA sequence changes increase with age In outbred human conceptions 70% of conceptions never come to term 2 per 1000 live births have genetic defects </li> <li> Slide 26 </li> <li> What Changes Gene Frequencies? Mutation Genetic drift (random change in small pops) Non-random Mating Migration = Gene Flow Natural Selection </li> <li> Slide 27 </li> <li> Purifying Selection Dominant or Sex-linked (X or Y) deleterious mutant alleles eliminated rapidly by natural selection Recessive autosomal deleterious mutant alleles reduced slowly by selection heterozygotes protect recessive deleterious mutant alleles never eliminated: a mutation - selection equilibrium is reached </li> <li> Slide 28 </li> <li> Stabilizing Selection decreases variation, doesnt shift mean Trait value Frequency Mean Trait value Frequency Old Mean ParentsOffspring </li> <li> Slide 29 </li> <li> Directional Selection may reduce variation, shifts mean Trait value Frequency Mean Trait value Frequency Old Mean ParentsOffspring </li> <li> Slide 30 </li> <li> Disruptive Selection increases variation, may shift mean Trait value Frequency Mean ParentsOffspring Trait value Frequency Old Mean </li> <li> Slide 31 </li> <li> Sexual Selection </li> <li> Slide 32 </li> <li> Forms of Sexual Selection Intrasexual (usually male-male competition) Weapons for within-sex competition Intersexual (usually females choosing males) Ornaments or signals to attract choosy mates Why are animals choosy: aesthetic preferences (Darwins hyp.) or signals indicate mate quality? </li> <li> Slide 33 </li> <li> Consequences of Sexual Selection Drives species away from the ecological optimum Major cause of sexual dimorphism via disruptive selection: since ornaments are an advantage in only one sex, there is selection for modifiers that lead to expression in one sex only </li> </ul>

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