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Lecture 5

BIOC15Fall2013 Lecture 5 and Lecture 6 Notes.docx

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Biological Sciences
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Karen Williams

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BIOC15Fall2013 Lecture 5 and Lecture 6 Notes Lecture 5: Mendel and genetics Critical questions about selective breeding before Mendel’s studies o Concluding remarks by Abbot Cyril Napp at 1837 annual meeting of the Moravian Sheep Breeders Society: o 3 basic questions  what is inherited? How is it inherited? What is the role of chance in heredity? o Napp presided over the monastery where Mendel began his seminal genetic experiments in 1864 How does a single gene segregate in a genetic cross o Tyrosinase-related-protein I is a gene in the coat colour pathway of SOay sheep o The light colour coat phenotype is explained by homozygosity of the recessive TyrpI gene with a G to T transversion mutation at position 869 o GGT  GTT Mendel studied the inheritance of alternative traits in pea plants o Mendel inferred laws of genetics that allowed prediction about which trait would appear, disappear or reappear o He worked in the garden at the monastery o Mendel’s paper ‘Experiments in plant hybrids’ was published in 1866 and became the cornerstone of modern genetics Mendel’s law of segregation o Two alleles for each trait separate during gamete formation o Two gametes, one from each parent unite at random during fertilization o Mendel’s laws (principles) o law of segregation: alternative trait factors that come together in the offspring separate again when the offspring produce gametes (gametes segregate) o law of dominance: hybrids between two alternate forms of a trait resemble one of the parental types o law of independent assortment: differences for one trait are inherited independently of differences for another trait Garden peas (Pisum sativum) o organisms have the same genotype when they have the same set of genes o organisms have the same phenotype if they resemble each other in some way o phenotype in greek = the form that is shown o Mendel looked at flower colour, seed colour, seed shape, pod colour, pod shape, stem height, flower position Mendel’s experiments: Design o Mendel chose plants with different phenotypes o Selectively bred the platns for 2 years to generate pure lines  populations that consistently produce offspring with the same phenotype of the trait being studied o Mendel chose 7 pea traits  each with 2 contrasting phenotypes Mendel’s experiments: Crosses o Parental generation = plants from pure lines (pure breeding lines) o Pure white flower x pure purple flower o Progeny of this cross = Filial generat1on (F ) o Reciprocal corss  when you cross a male and a female from different strains o Purple flower female x white flower male and purple flower male x white flower female o Mendel observed that 1 was identical to the purple flowered parent  did not matter if the female or the male Mendel’s experiments: The organism o The garden pea  good because within a short amount of time he could obtain a lot of individuals 1 o o self – fertilization  the egg and pollen come from the same plant o cross-fertilization  brushing pollen from one plant onto a female organ of another plant o the ovary becomes the pea plant Seed shape experiment o crosses round seed with wrinkled seed (P generation) o F 1ere all round  round is dominant o Then self-crossed 1 to ge2 F o 25% RR, ½ Rr and 25% rr  75% were round and 25% were wrinkled o so the offspring has a 50% chance of inheriting either gamete from each parent o R allele allows for making of a protein that catalyzes the conversion of amylose (unbranched) to amylopectin (branched starch)  r allele does not have functional SBE1, therefore in ss individuals, the starch is unbranched and gives the pea a wrinkled outer surface Mendel’s explanation o the 1:2:1 genotypic ration from the self-cross could be explained by: o there are hereditary determinants of a particulate nature (now called genes) o each plant has a pair of factors (now called 2 alleles of the same gene) o each allele segregated independently into the gametes o each gamete carries only 1 allele per gene o fertilization between pollen and ovum is random with regard to which allele is being carried Monohybrid crosses o monohybrid crosses reveal units of inheritance and law of segregation o mendel repeated the same experiment with the other 6 plant characteristics and got the same 3:1 phenotypic ratio o YY x yy  all Yy o Yy x Yy  YY, 2xYy, yy  75% yellow, 25% green o He called the yellow trait dominant and the green trait recessive Dihybrid cross o Dihybrid crosses reveal the law of independent assortment o Crosses of plants differeing in two traits  for example pea shape and colour o Produces parental types and recombinant types o Each F1 dyhybrid produces 4 possibl egametes in a 1:1:1:1 ratio o Example: Yy Rr (yellow seed and round seed)  Y, y, R, r are the gametes o Recombinant type = new phenotypic combination (not like the parents) o Parental type = phenotypic combination like one of the parents o 4 phenotypic classes occurred in t2e F progeny  yellow round (like parent), yellow wrinkled (recombinant), green round (recombinant), green wrinkled (like parent) Independent assortment in crosses of F1 dihybrids produces a 9:3:3:1 phenotype ratio o Y_R_ =
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