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Chapter 3
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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).
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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.