Clicker Question 16-1 • The F1 offspring of Mendel's classic pea cross always looked like one of the two parental varieties because A) one phenotype was completely dominant over another. B) each allele affected phenotypic expression. C) the traits blended together during fertilization. D) no genes interacted to produce the parental phenotype. E) different genes interacted to produce the parental phenotype.Mendel’s Model for The Law of Segregation Mendel developed a hypothesis based on four related concepts: Alternative versions of genes (=alleles) account for variations in inherited characters. For each character, an organism inherits two alleles, one from each parent. If two alleles differ, the dominant allele determines character appearance. Two alleles for a heritable character segregate into different gametes. 1"2"3"4"Alternative versions of genes account for variations in inherited characters Alleles is the term for alternative version of genes Figure'14.4'Allele'for'purple'flowers"Locus'for'flower5color'gene"Homologous"pair"of"chromosomes"Allele'for'white'flowers"1"For'each'character,'an'individual'inherits'2'alleles'(1'from'each'parental'gamete)'2"Phenotype vs. Genotype Figure'14.6'3 1121Phenotype Purple Purple Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1 Note:"genotype"can"differ,"yet"give"rise"to"same"phenotype"The Law of Segregation “During gamete formation, different alleles end up in different gametes” This segregation corresponds to the distribution of homologous chromosomes to different gametes in meiosis. Two'alleles'for'a'heritable'character'segregate'into'different'gametes'4"MEIOSIS"gametes'Clicker Question Q16-2 • When crossing an organism that is homozygous recessive for a single trait with a heterozygote, what is the chance of producing an offspring with the homozygous recessive phenotype? • A) 0% • B) 25% • C) 50% • D) 75% • E) 100%Fig."14C7"EXPERIMENT"RESULTS"Dominant"phenotype,""unknown"genotype:"PP"or"Pp?"RECESSIVE"PHENOTYPE,"""""KNOWN"GENOTYPE:" PP"×"If"PP" If"Pp"or"Sperm" Sperm"p" p" p"p"P"P"P"p"Eggs" Eggs"Pp"Pp"Pp"Pp"Pp"Pp"pp"pp"or"All"offspring"purple"1/2"offspring"purple"and"1/2"offspring"white"PREDICTIONS"Testcross: figuring out if an organism with a dominant phenotype is a homozygote or a heterozygoteMendel’s Laws 1.'Law'of'segregaGon:'two'alleles'of'a'character'segregate'into'different'gametes'2.'Law'of'Independent'Assortment:'When"two"or"more"characters"are"inherited,"each"pair"of"alleles"segregates"into"gametes"independently"" ""YYRR'P'GeneraGon!Gametes' YR' yr'×"yyrr'YyRr'Hypothesis"of"dependent"assortment'Hypothesis"of"independent"assortment'F2'GeneraGon"(predicted"offspring)'1⁄2"YR'YR'yr'1"⁄2"1"⁄2"1⁄2" yr'YYRR' YyRr'yyrr'YyRr'3"⁄4"1"⁄4"Sperm"Eggs"Phenotypic"ra\o"3:1"YR'1"⁄4"Yr'1"⁄4"yR'1"⁄4"yr'1"⁄4"9"⁄16"3"⁄16"3"⁄16"1"⁄16"YYRR'YYRr'YyRR'YyRr'Yyrr'YyRr'YYrr'YYrr'YyRR'YyRr'yyRR' yyRr'yyrr'yyRr'Yyrr'YyRr'Phenotypic"ra\o"9:3:3:1"315" 108"101"32"F1'GeneraGon!Eggs"YR'Yr'yR' yr'1"⁄4"1"⁄4"1"⁄4"1"⁄4"Sperm"Figure'14.8'Independent Assortment: Inheritance of more than one trait Mendel’s"results:"Clicker question Q16-3 Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the progeny of the cross BbTt × BBtt will have black fur and long tails? • A) 1/16 • B) 3/16 • C) 3/8 • D) 1/2 • E) 9/16Midterm exam • Last opportunity to view your exam and submit a regrade: next Monday, between 1:30-3PM, York Hall 2300 • Next Wed, all exams will be returned in class • At that point, only exams in which points were added incorrectly can be submitted for regradeSolving Genetics Problems • Read “Tips for genetics problems”, p.283 in your textbook • Solve problems in Problem set 5 • Come to discussion sections and office hours • Graded homework with genetics problems next Friday, due Monday, 10PM • Start working on the homework early, so that you can get help from the TA’s on Monday, if you need itIA"IB"i"A"B"none"(a)"The"three"alleles"for"the"ABO"blood"groups"""""""and"their"associated"carbohydrates"Allele"Carbohydrate"Genotype"Red"blood"cell"appearance"Phenotype"(blood"group)"IAIA""or"IA i"A"B"IBIB""or"IB i"IAIB"AB"ii" O"(b)"Blood"group"genotypes"and"phenotypes"Codominance""two'alleles'are'equally'dominant'and'are'co5expressed'Mul\ple"Alleles " ""more'than'two'alleles'for'one'gene'Human"ABO"Blood"Groups:"4"blood"groups"(phenotypes)"from"combina\ons"of"3"alleles"for"an"enzyme"(genotype)"Fig. 5-22c Normal red blood cells are full of individual hemoglobin molecules, each carrying oxygen."Fibers of abnormal hemoglobin deform red blood cell into sickle shape."Human genetic disease: Sickle-Cell AnemiaFig. 5-22 Primary structure'Secondary and tertiary structures'Quaternary structure'Normal hemoglobin (top view)'Primary structure'Secondary and tertiary structures'Quaternary structure'Function'Function'β subunit'Molecules do not associate with one another; each carries oxygen.'Red blood cell shape'Normal red blood cells are full of individual hemoglobin moledules, each carrying oxygen.'10 µm'Normal hemoglobin'β'β'α'α'1' 2' 3' 4' 5' 6' 7'Val'His'Leu'Thr'Pro'Glu'Glu'Red blood cell shape'β subunit'Exposed hydrophobic region'Sickle-cell hemoglobin'β'α'Molecules interact with one another and crystallize into a fiber; capacity to carry oxygen is greatly reduced.'β'α'Fibers of abnormal hemoglobin deform red blood cell into sickle shape.'10 µm'Sickle-cell hemoglobin'Glu'Pro'Thr'Leu'His'Val'Val'1' 2' 3' 4' 5' 6'
View Full Document