Chapter 3 The images on this CD have been lifted directly, without change or modification, from textbooks and image libraries owned by the publisher, especially from publications intended for college majors in the discipline. Consequently, they are often more richly labeled than required for our purposes. Further, dates for geological intervals may vary between images, and between images and the textbook. Such dates are regularly revised as better corroborated times are established. Your best source for current geological times is a current edition of the textbook, whose dates should be used when differences arise.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mendel’s experimental results  The table summarizes results of Mendel’s experiments following seven characters during crosses of the garden pea. For example, Mendel observed that in the F1 generation, the character (or trait), seed color, occurred 6,022 yellow (dominant) : 2,001 green (recessive), a ratio of 3.01:1, very close to his predicted ratio of 3:1.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mendel’s experimental method  In this experiment of a cross between true breeding white- and purple-flowered plants, Mendel pried open the surrounding petals of the purple-flowered plant and removed the male part, thus preventing self-fertilization. Then he dusted the anther with pollen he had selected from the white-flowered plant. The resulting seeds were planted and grew, all producing purple flowers.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Independent segregation—single trait, flower color  Mendel’s cross of pea plants for flower color started with true breeding white-flowered (recessive) and purple-flowered (dominant) plants. All F1 offspring of this cross were purple-flowered, and genetically heterozygous (Pp). When these were crossed, the resulting F2 offspring averaged 3 purple- for every 1 white-flowered plant, a 3:1 phenotypic ratio. However, the ratio of genotypes is 1:2:1 (1PP: 2Pp : 1pp).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Testcross  By just looking at a dominant phenotype, for example, this plant with purple-flowers, you would not know if it was homozygous or heterozygous for the dominant allele. To determine its genotype, Mendel performed a testcross. In this illustration, the dominant phenotype (unknown genotype) was crossed with a plant known to be homozygous recessive, for example, the white-flowered plant. If all offspring are purple (Alternative 1), then the unknown flower is homozygous dominant; if offspring are half and half, purple and white (Alternative 2), then the unknown flower is heterozygous.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Independent assortment—multiple traits, seed shape and seed color  Mendel followed two traits together to see if they influenced each other. Vertically at left, the phenotypic outcomes into the F2 generation are followed for seed shape—round (dominant) and wrinkled (recessive); and seed color—yellow (dominant) and green (recessive). An underline in a genotype indicates that either a dominant or a recessive allele is possible. If the alleles assorted or moved into gametes without affecting each other, then the predicted ratio is 9:3:3:1, which is about what Mendel observed. Vertically at the right, the allele combinations resulting from each successive cross are mapped, showing the genotypes.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mitosis and meiosis  In meiosis, chromosomes replicate, homologous pairs align, and each duplicated homologue separates during cell division. Then, a second cell division separates replicated chromosomes, and four haploid daughter cells (gametes) are produced. In mitosis, chromosomes replicate, but one cell division separates replicated chromosomes into two diploid daughter cells.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gene linkage  Two fruit fly genes reside on the same chromosome—wing length and eye color. Because they are located on the same chromosome, they ride together into the gametes, thereby reducing the number of genotypes and phenotypes possible.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Multiple genes, one trait  Polygenetic inheritance is illustrated with three genes, A, B, and C; hence, six alleles, for wheat seed color. Alleles contributing to the color are indicated by a capital letter; others not contributing are in lower case. In each generation, the six alleles are shown by circles— solid if contributing and open if not adding to seed color. In the F2 generation, the phenotypes expressed are additive, producing a continuous range of seed color. If graphed by frequency, they form a bell-shaped curve.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 3.1 Gregor Johann Mendel (1822—1884)  Mendel discovered a mechanism of inheritance while conducting experiments on garden peas at a monastery in Brunn, Austria (now Brno, Czech Republic).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 3.3 Flower Anatomy of the Garden Pea  Pollen, containing sperm, is produced in the male anther; the ovules contain eggs in the female carpel. To prevent self-pollination, Mendel cut away the male anthers and dusted the female carpel with pollen from another plant of his choosing to control the cross. Seeds ripened in the pod, their characteristics were scored, and then these seeds were planted to grow and flower. The characteristics of these full plants were then scored.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 3.8 Homologous Pairs of Chromosomes from Humans  The 23 pairs are shown for a male, top; and a female, bottom. Note that the male sex chromosomes are XY; the female XX.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 3.11 Multiple Alleles, One Trait  In humans, the gene that controls blood type has three different alleles: A, B, and O, resulting in four different blood phenotypes: A, B, AB, O, specifically type A (A/A or A/O); type B (B/B or B/O); type AB (A/B); and type O (O/O).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 3.13 Polygenic Trait in Humans—Height  Aligned by height, the students show a range of continuous phenotypic variation, with most in the middle.