02-ch9-Mendel1 - Chapter 9 Patterns of Inheritance of...

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Unformatted text preview: Chapter 9 Patterns of Inheritance of Inheritance Gregor Mendel 1822-1884 “The Father of Modern Genetics” heredity artificial selection – selection by plant and animal tifi breeders of individuals with certain desirable traits Before Mendelian genetics, plant breeders operated under the following assumptions: th • Each parent contributes equally to offspring = mostly correct mostly correct • “Hereditary determinants” blend in offspring = mostly incorrect character – observable feature - e.g., flower color, pea shape trait – particular form of a character - e.g., white vs. purple, round vs. wrinkled heritable trait – passed on from parent to offspring (via DNA) Mendel chose for his experiments: • characters with well-defined contrasting traits with well contrasting traits • true-breeding traits (e.g., two white flower parents produce 100% white flower offspring) produce 100% white flower offspring) self-pollination – one plant reproduces itself by lf it fertilizing its own ova with its own sperm carpel stamen(s) Mendel followed inheritance for several generations parental generation (P) – plants providing & receiving pollen (Mendel performed reciprocal crosses) first filial generation (F1) – offspring of parental cross (Mendel allowed this generation to self-pollinate) second filial generation (F2) – offspring resulting from self-pollination of F1 generation Mendel’s first experiments involved monohybrid crosses hybrid – offspring of two different varieties monohybrid cross cross • focus on only one character • two true-breeding parental lineages differing in this trait are crossed in the P generation • resulting F1 generation is allowed to self-pollinate (producing (producing F2 generation) P1 F1 100% purple offspring F2 ~75% purple offspring P1 F1 F2 From these results, Mendel called the traits: dominant vs. recessive S s dominant allele – determines the organism’s appearance allele the organism appearance recessive allele – no noticeable effect, but can be passed on “blending” hypothesis was refuted by these experiments Which pea flower color is dominant? How would we abbreviate these alleles? would we abbreviate these alleles? P p eukaryotic chromosome (DNA wrapped around proteins) gene – unit of hereditary information (segment of DNA) allele – alternative form of a gene; codes for a specific trait AA aa organism has two copies (one on each homologous chromosome) recall that meiosis will send only one allele to each gamete homozygous – (diploid) individual contains two copies of same allele S S s s heterozygous – (diploid) individual contains two different alleles for same character gene different alleles for same character / gene S s genotype – genetic constitution SS ss Ss phenotype – observable appearance law of segregation – the two alleles for a given gene separate during gamete production, so each gamete receives only one copy; paired condition restored in receives only one copy; paired condition restored in offspring (by fertilization) What is the F1 genotype for this cross? is the for this cross? P1 round vs. wrinkled SS F1 ss 100% offspring were round Ss Ss Ss Ss What are the F2 genotypes for this cross? F1 100% offspring were round Ss F2 Ss Ss Ss ~75% offspring were round offspring ro SS Ss Ss ss Punnet Square Monohybrid cross (start with P1) Note the parental gametes are shown outside the square; diploid offspring are shown inside the square. What are the F1 phenotypes for this cross? What are the F1 genotypes for this cross? p p P Pp Pp P Pp Pp Monohybrid cross (self-pollination of F1 generation) What are the F2 phenotypes for this cross? P P p p What are the F2 genotypes for this cross? P p P PP Pp p pP pp F2 phenotype ratio, monohybrid cross: 3:1 B = black b = brown how about mine? what’s my phenotype? and my genotype? Note: if you missed class today, you missed some important practice exercises. It will be your responsibility to communicate with your fellow students to fill in the details. If a black lab of unknown genotype is crossed with a brown lab, explain what each litter has revealed about the puppies’ parentage: 1) genotype of unknown black parent ___________ are we 100% sure? _____ 2) genotype of unknown black parent ___________ are we 100% sure? _____ 3) genotype of unknown black parent ___________ are we 100% sure? _____ dihybrid cross • focus on two characters... P generation = SSYY x ssyy • How many genotypes occur in the F1 generation? • Describe phenotype(s) in the F1 generation. • F1 generation is allowed to self-pollinate (cross individuals heterozygous for both traits)... are the genotypes of the haploid gametes • What are the genotypes of the haploid gametes produced by this cross? Do the two characters stick together or not? sY Practice Exercise Sy What are the F2 genotypes for this cross? SY Sy sY sy SY Sy sY sy SY sy Practice Exercise, Part II How many distinct How many distinct F2 genotypes are there? are there? Hint, re-arrange the letters with capitals first: sSyY = sSYy = SsyY = SsYy 1) To find the phenotype ratio... first, complete the chart: genotype SSYY SSYy SSyy SsYY SsYy Ssyy ssYY ssYy ssyy # in chart phenotype 2) total for each phenotype: round yellow yellow =___ round green =___ wrinkled yellow =___ wrinkled green =___ F2 phenotype ratio for a dihybrid cross: 9:3:3:1 ...
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This note was uploaded on 12/07/2011 for the course BIO 2 taught by Professor Poenie during the Fall '08 term at University of Texas.

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