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Mendel and the Gene Idea
Chapter 14 Mendel and the Gene Idea
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Figure 14.1 Gregor Mendel and his garden peas
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Figure 14.2 Research Method Crossing Pea Plants
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 offspring (F1) 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. RESULTS
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Figure 14.3 When F1 pea plants with purple flowers are allowed to self-pollinate, what flower color appears in the F2 generation P Generation (true-breeding parents) Purple flowers White F1 Generation (hybrids) All plants had purple flowers 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 cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. RESULTS Both purple-flowered plants and white- flowered 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.
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Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
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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
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Figure 14.5 Mendel’s law of segregation (layer 1)
P Generation F1 Generation P p Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp 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 F1 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.
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Figure 14.5 Mendel’s law of segregation (layer 2)
P Generation F1 Generation F2 Generation P p Pp PP pp Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Gametes: F1 sperm F1 eggs 1/2 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 F1 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 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. Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 3 : 1
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Figure 14.6 Phenotype versus genotype
3 1 2 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) pp Ratio 3:1 Ratio 1:2:1
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then 1⁄2 offspring purple
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 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 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. RESULTS
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Phenotypic ratio approximately 9:3:3:1
Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently? YYRR P Generation Gametes YR yr yyrr YyRr Hypothesis of dependent assortment independent F2 Generation (predicted offspring) 1 ⁄2 3 ⁄4 1 ⁄4 Sperm Eggs Phenotypic ratio 3:1 Yr yR 9 ⁄16 3 ⁄16 1 ⁄16 YYRr YyRR Yyrr YYrr yyRR yyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F1 Generation 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 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.
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Figure 14.9 Segregation of alleles and fertilization as chance events
Rr Segregation of alleles into eggs alleles into sperm R r 1⁄2 1⁄4 Sperm Eggs
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Figure 14.10 Incomplete dominance in snapdragon color
P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW White CWCW Pink CRCW Sperm Cw 1⁄2 Eggs CR CR CR CW CW CW
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Table 14.2 Determination of ABO Blood Group by Multiple Alleles
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Figure 14.11 An example of epistasis
BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16 Sperm Eggs
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Figure 14.12 A simplified model for polygenic inheritance of skin color
AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny
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Figure 14.13 The effect of environment on phenotype
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Figure 14.14 Pedigree analysis
Ww ww WW or First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ff FF or Ff FF Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)
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Figure 14.15 Achondroplasia
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Figure 14.16 Large families as excellent case studies of human genetics
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Figure 14.17 Testing a fetus for genetic disorders
(a) Amniocentesis Amniotic fluid withdrawn Fetus Placenta Uterus 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 hours 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 Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping
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Unnumbered Figure p.273 George Arlene Sandra Tom Sam Wilma Ann Michael
Daniel Alan Tina Christopher Carla
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