Chapter 14 Mendel and the Gene Idea PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Gregor Mendel documented a particulate mechanism of inheritance through his experiments with garden peas Crossing pea plants 1 APPLICATION By crossing mating two true breeding varieties of an organism scientists can study patterns of inheritance In this example Mendel crossed pea plants that varied in flower color TECHNIQUE Removed stamens from purple flower 2 Transferred sperm bearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation P 3 Pollinated carpel Stamens Carpel male female matured into pod 4 Planted seeds from pod TECHNIQUE RESULTS When pollen from a white flower fertilizes eggs of a purple flower the first generation hybrids all have purple flowers The result is the same for the reciprocal cross the transfer of pollen from purple flowers to white flowers Figure 14 2 5 Examined First generation offspring F1 offspring all purple flowers Mendel discovered A ratio of about three to one purple to white flowers in the F2 generation EXPERIMENT True breeding purple flowered pea plants and white flowered pea plants were crossed symbolized by The resulting F1 hybrids were allowed to self pollinate or were crosspollinated with other F1 hybrids Flower color was then observed in the F2 generation P Generation true breeding parents Purple flowers White flowers F1 Generation hybrids All plants had purple flowers RESULTS Both purple flowered plants and whiteflowered plants appeared in the F2 generation In Mendel s experiment 705 plants had purple flowers and 224 had white flowers a ratio of about 3 purple 1 white Figure 14 3 F2 Generation Mendel observed the same pattern In many other pea plant characters Mendel s Model 1 Alternative versions of genes Account for variations in inherited characters which are now called alleles Allele for purple flowers Locus for flower color gene Figure 14 4 Allele for white flowers Homologous pair of chromosomes Using a Punnett square allows us to see the possible offspring outcomes from a particular cross Possible Alleles in gametes from Parent 1 A a A AA Aa a Aa aa Possible Alleles in Gametes from Parent 2 Probability in a monohybrid cross Rr Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm 2 R 1 R R 1 1 4 1 Eggs r 2 1 r R R 2 r 2 1 R r 4 1 4 r r 4 1 Mendel s law of segregation probability and the Punnett square Each true breeding plant of the parental generation has identical alleles PP or pp Gametes circles each contain only one allele for the flower color gene In this case every gamete produced by one parent has the same allele Union of the parental gametes produces F1 hybrids having a Pp combination Because the purpleflower allele is dominant all these hybrids have purple flowers When the hybrid plants produce gametes the two alleles segregate half the gametes receiving the P allele and the other half the p allele This box a Punnett square shows all possible combinations of alleles in offspring that result from an F1 F1 Pp Pp cross Each square represents an equally probable product of fertilization For example the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm Figure 14 5 Random combination of the gametes results in the 3 1 ratio that Mendel observed in the F2 generation P Generation Appearance Purple flowers White flowers Genetic makeup PP pp Gametes p P F1 Generation Appearance Genetic makeup Purple flowers Pp 2 Gametes 1 2 1 P p F1 sperm P p PP Pp F2 Generation P F1 eggs p pp Pp 3 1 Phenotype versus genotype Phenotype Purple 3 Purple Genotype PP homozygous 1 Pp heterozygous 2 Pp heterozygous Purple 1 Figure 14 6 White pp homozygous Ratio 3 1 Ratio 1 2 1 1 The testcross APPLICATION An organism that exhibits a dominant trait such as purple flowers in pea plants can be either homozygous for the dominant allele or heterozygous To determine the organism s genotype geneticists can perform a testcross TECHNIQUE In a testcross the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait white flowers in this example By observing the phenotypes of the offspring resulting from this cross we can deduce the genotype of the purple flowered parent Dominant phenotype unknown genotype PP or Pp Recessive phenotype known genotype pp If PP then all offspring purple If Pp then 1 2 offspring purple and 1 2 offspring white p p p p Pp Pp pp pp RESULTS P Pp P Pp P p Pp Figure 14 7 Pp Dihybrid Cross A dihybrid cross illustrates the inheritance of two characters Produces four phenotypes in the F2 generation EXPERIMENT Two true breeding pea plants one with yellow round seeds and the other with green wrinkled seeds were crossed producing dihybrid F1 plants Self pollination of the F1 dihybrids which are heterozygous for both characters produced the F2 generation The two hypotheses predict different phenotypic ratios Note that yellow color Y and round shape R are dominant P Generation YYRR yyrr Gametes F1 Generation YR Hypothesis of dependent assortment yr YyRr Hypothesis of independent assortment Sperm 1 RESULTS 1 CONCLUSION The results support the hypothesis of independent assortment The alleles for seed color and seed shape sort into gametes independently of each other 2 YR 1 Sperm 2 yr Eggs 1 F2 Generation 2 YR YYRR YyRr predicted 1 offspring 2 yr YyRr yyrr 3 4 1 1 4 Yr 1 4 yR 1 4 yr Eggs 1 1 1 4 YR 4 Yr 4 yR 4 Phenotypic ratio 3 1 4 YR 1 4 yr 9 16 YYRR YYRr YyRR YyRr YYrr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr 3 16 Yyrr yyRr 3 16 yyrr 1 16 Phenotypic ratio 9 3 3 1 Figure 14 8 315 108 101 32 Phenotypic ratio approximately 9 3 3 1 Incomplete Dominance In incomplete dominance the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation 2 1 Gametes Eggs F2 Generation 2 CR 2 CR 1 2 1 CR CR 2 CR Sperm 1 1 2 Cw CR CR CR CW 1 Figure 14 10 CR CW CW CW The ABO blood group in humans Is determined by multiple alleles Table 14 2 Epistasis An example of epistasis BbCc BbCc Sperm 4 BC 1 4 bC 1 4 Bc 1 4 bc 1 Eggs 4 BC BBCC BbCC BBCc BbCc 4 bC BbCC bbCC BbCc bbCc 1 1 4 Bc BBCc BbCc BBcc 4 bc BbCc bbCc Bbcc 1 1 16 9 Figure 14 11 16 3 Bbcc bbcc 16 4 Polygenic Inheritance AaBbCc aabbcc 64 20 64 Fraction of progeny 15 64 6 64 1 Figure 14 12 Aabbcc AaBbcc AaBbCc AaBbCc AABbCc AABBCc AABBCC Multifactorial
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