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Lecture 9

BIOL 207 (09/30/13) Lecture 9: Mendel Invents Genetics | Lecture 10: Classical Genetics

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Biology (Biological Sciences)
Mike Harrington

BIOL 207 (September 30,2013) Lecture 9: Mendel Invents Genetics | Lecture 9: Classical Genetics Mendel’s Model  Genes are in pairs o And gametes only have one out of the pair  Random fertilization occurs because of this  Mendel’s First Law: the Law of Segregation o If the two alleles in a gene were to be separated during gamete formation, they end up on different gametes  Punnet Square: table for determining the possible outcomes and probabilities of genetic crossing Homozygote – cell/organism with identical alleles on the gene we’re interested in  As an adjective  homozygous Heterozygote – cell/organism with different alleles on the gene we’re interested in  As an adjective  heterozygous  If an organism displays a dominant phenotype, its genotype is denoted A/_ because we do not know if the second gene is dominant or recessive and we know that at least one of the 2 is dominant. o Way to figure it out is to do a test cross  Cross the organism in question with an organism with the genotype a/a  If approximately ½ the offspring display the dominant phenotype and the other ½ display the recessive phenotype, then the organism in question has the genotype: A/a  If ALL the offspring display dominant phenotypes, then we know that the organism in question has a genotype of A/A  A good model is one that is: a. Simple b. Explains most or all data c. Testable (Therefore, the model testing the existence of aliens in the universe is NOT a good one because it is not simple nor is it testable) Mendel’s Third Experiment  Mendel predicted that the F 1eneration (parents = A/A and a/a) have a genotype of A/a a. What did he do about it? i. Cross the offspring from the F1generation with pure-breeding green plants (a/a) ii. Result was a one to one ratio between green and yellow pea plants 1. Experiment was successful in proving his model I-Clicker Question: Q: If you cross a white-petalled and purple-petalled pea plant. What phenotype will the offspring exhibit? A: A or C  A  100% purple (if the purple-petalled plant had a genotype of A/A  C  50% purple and 50% white (if the purple-petalled plant had a genotype of A/a) Monohybrid – organism that is heterozygous for only 1 gene of interest  A/a Dihybrid – organism is heterozygous on 2 genes of interest  A/a•B/b Mendel’s 4 Experiment (9:3:3:1 ratio)  Dihybrid pea plants created by crossing pure breeding yellow & round (A/A•B/B) and pure breeding green & wrinkled (a/a•b/b) pea plants o Resulting phenotypic ratio:  9:3:3:1 (yellow&round:green&round:yellow&wrinkled:green&wrinkled) o What is this ratio really?  Two super imposed 3:1 ratios (You can think about it like FOIL from math)  (3:1)(3:1)  (3*3):(3*1):(1*3):(1*1)  9:3:3:1  The round:wrinkled ratio = 3:1  The yellow:green ratio = 3:1 Mendel’s 2 Law: Law of Independent Assortment  Gene pairs will segregate independently of each other in the creation of gametes Questioning the Validity of Mendel’s Results  What if he just made up the numbers? o His experiments have been re-tested and yielded the same ratios o He tested his model experimentally  There are exceptions to his model but he only conducted his experiments on peas so it is only natural that his la
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