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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the Gene Idea.

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Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the Gene Idea."— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the Gene Idea

2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.1 Gregor Mendel and his garden peas

3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.2 Research Method Crossing Pea Plants 1 5 4 3 2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 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 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. TECHNIQUERESULTS

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.3 When F 1 pea plants with purple flowers are allowed to self-pollinate, what flower color appears in the F 2 generation P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendels experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Table 14.1 The Results of Mendels F 1 Crosses for Seven Characters in Pea Plants

6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.4 Alleles, alternative versions of a gene Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.5 Mendels law of segregation (layer 1) P Generation F 1 Generation P p Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: 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 F 1 hybrids having a Pp combination. Because the purple- flower 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.

8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.5 Mendels law of segregation (layer 2) P Generation F 1 Generation F 2 Generation P p P p P p P p PpPp PP pp Pp Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: F 1 sperm F 1 eggs 1/21/2 1/21/2 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 F 1 hybrids having a Pp combination. Because the purple- flower 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 F 1 F 1 (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. Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F 2 generation. 3 : 1

9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.6 Phenotype versus genotype 3 1 1 2 1 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.7 The Testcross 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 P p pp 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 organisms 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. RESULTS

11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently? YYRR P Generation GametesYRyr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1 2 YR yr 1 2 yr YYRRYyRr yyrr YyRr 3 4 1 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 4 Yr 1 4 yR 1 4 yr 1 4 9 16 3 16 1 16 YYRR YYRr YyRR YyRr Yyrr YyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 1 4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants one with yellow-round seeds and the other with green-wrinkled seedswere crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.9 Segregation of alleles and fertilization as chance events Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1 2 1 4 1 2 r r R r r Sperm Eggs

13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.10 Incomplete dominance in snapdragon color P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1 2 Eggs 1 2 C R C R C W C W C R C W

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Table 14.2 Determination of ABO Blood Group by Multiple Alleles

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.11 An example of epistasis BC bCBc bc 1 4 BC bC Bc bc 1 4 BBCcBbCc BBcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCCBbCC BBCc BbCc 9 16 3 16 4 16 BbCc Sperm Eggs

16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.12 A simplified model for polygenic inheritance of skin color AaBbCc aabbcc Aabbcc AaBbccAaBbCcAABbCc AABBCcAABBCC 20 64 15 64 6 64 1 64 Fraction of progeny

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.13 The effect of environment on phenotype

18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.14 Pedigree analysis Ww ww Ww wwWw ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf ff Ff ff Ff FF or Ff ff FF or Ff Widows peak No Widows peak Attached earlobe Free earlobe (a) Dominant trait (widows peak) (b) Recessive trait (attached earlobe)

19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.15 Achondroplasia

20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.16 Large families as excellent case studies of human genetics

21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 14.17 Testing a fetus for genetic disorders (a) Amniocentesis Amniotic fluid withdrawn Fetus PlacentaUterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Unnumbered Figure p.273 George Arlene Sandra Tom Sam Wilma AnnMichael Daniel AlanTina Christopher Carla


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