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Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino Chapter 3 Mendelian Genetics Copyright © 2010 Pearson Education, Inc. Outline • Mendel’s Experimental Approach • Monohybrid cross • Test cross • Dihybrid cross • Independent assortment • Trihybrid cross Mendel’s Experimental Approach • Mendel chose garden pea (Pisum sativum) as his model system because: 1. it is easy to grow 2. it can be crossbred artificially 3. it grows to maturity in one season • Mendel used: 1. seven visible features, each with two contrasting traits 2. true-breeding parental strains http://www.xtec.es/~jllort1/biolegseu ropa/mendel.jpg Results of Mendel’s Research (1866) • Mendel determined discrete units of inheritance exist and predicted their behavior during gamete formation http://sps.k12.ar.us/massenga le/images/mendel2.jpg • Mendel’s postulates eventually accepted as basis for Mendelian or transmission genetics • Mendel’s findings rediscovered at the turn of the century by three geneticists The Monohybrid Cross (one-factor cross) • Monohybrid crosses involve a single pair of contrasting traits • Original parents are P1 generation, and their offspring are F1 generation • Offspring arising from selfing (selffertilizing) the F1 generation are F2 generation The Monohybrid Cross • In F1 generation of a monohybrid cross, all plants have just one of two contrasting traits • This trait is dominant over the other recessive trait that disappeared in F1 • In F2 generation, 3/4 plants exhibit same trait dominant trait as F1 and 1/4 exhibit contrasting recessive trait that disappeared in F1 generation Traits used by Mendel Figure 3-1 Copyright © 2006 Pearson Prentice Hall, Inc. Mendel’s Discoveries • To explain these results, Mendel proposed existence of "particulate unit factors" for each trait • He suggested these factors: 1. serve as basic units of heredity (now called genes) 2. are passed unchanged from generation to generation 3. determine various traits expressed by each individual plant Mendel’s First Three Postulates • Mendel proposed three postulates of inheritance: 1. Unit factors exist in pairs 2. In a pair of unit factors for a single characteristic in an individual, one unit factor is dominant and other is recessive 3. Paired unit factors segregate (separate) independently during gamete formation Genetic Terminology • Genes: Units of inheritance • Alleles: Alternate forms of a single gene • Genotype: the genetic makeup of an individual • Phenotype: the physical expression of the genetic makeup Genetic Terminology • Homozygous: when the alleles for a trait in an individual are the same (eg: AA or aa) • Heterozygous: when the alleles for a trait in an individual differ (eg: Aa) The Monohybrid Cross Figure 3-2 Copyright © 2006 Pearson Prentice Hall, Inc. Punnett Square • A Punnett square allows genotypes and phenotypes resulting from a cross to be visualized easily Mendel’s Monohybrid ratio: 3:1 Figure 3-3 Copyright © 2006 Pearson Prentice Hall, Inc. The Testcross • A testcross is a test to determine whether an individual displaying the dominant phenotype is homozygous or heterozygous for that trait The Dihybrid Cross (two-factor cross) • A dihybrid cross involves two pairs of contrasting traits The Dihybrid Cross Mendel’s dihybrid ratio: (9:3:3:1) Mendel’s Fourth Postulate: Independent Assortment • Segregating pairs of unit factors assort independently during gamete formation • Each gamete receives one member of every pair of unit factors. For one pair, whichever unit factor received does not influence the segregation of the second pair. • Therefore, all possible combinations of gametes will form with equal frequency Laws of Probability and Genetics • Product Law: – Used to calculate probability of two independent events occurring at the same time – The probability of both events occurring is product of the probability of each individual event Predicted Results of a Dihybrid Cross Mendel’s dihybrid ratio: 9:3:3:1 The Testcross: Two Characters • Testcrosses can also be applied to individuals that express two dominant traits but whose genotypes are not known • Eg: The yellow round seed phenotype in the F2 generation can result from one of the four GGWW, GgWW, GGWw or GgWw genotypes. • Homework: Do a test cross of above four genotypes (cross each one separately with homozygous recessive ggww). Find ratios for each. The Trihybrid Cross (three-factor cross) • Trihybrid crosses involving 3 independent traits show Mendel's rules apply to any # of traits • The forked-line method is easier to use than a Punnett square for analysis of inheritance of larger # of traits • Product law can be used to predict frequency with which two independent events will occur simultaneously Figure 3-9 Copyright © 2006 Pearson Prentice Hall, Inc. Predicted results of a trihybrid cross using forked-line method Mendel’s Laws – to remember! • Independent segregation: Alleles of a gene segregate (separate) independently during gamete formation. Eg: Dd • Independent Assortment: Alleles of two genes assort independently during gamete formation. Eg: Gg and Ww