Page 26 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

18 Chapter 2 Mendel’s Principles of Heredity

Figure 2.6 The homunculus: A misconception. Well into the self-fertilization (or selfing), both egg and pollen come
nineteenth century, many prominent microscopists believed they saw from the same plant. The particular anatomy of pea flow-
a fully formed, miniature fetus crouched within the head of a sperm. ers, however, makes it easy to prevent self-fertilization and
© Klaus Guldbrandsen/SPL/Science Source instead to cross-fertilize (or cross) two individuals by
brushing pollen from one plant onto a female organ of
another plant, as illustrated in Fig. 2.7c. Peas offered yet
another advantage. For each successive generation, Mendel
could obtain large numbers of individuals within a rela-
tively short growing season. By comparison, if he had
worked with sheep, each mating would have generated only
a few offspring and the time between generations would
have been several years.
Second, Mendel examined the inheritance of clear-cut
alternative forms of particular traits—purple versus white
flowers, yellow versus green peas. Using such either-or
traits, he could distinguish and trace unambiguously the
transmission of one or the other observed characteristic,
because no intermediate forms existed. (The opposite of
these so-called discrete traits are continuous traits, such
as height and skin color in humans. Continuous traits show
many intermediate forms.)
Third, Mendel collected and perpetuated lines of peas
that bred true. Matings within such pure-breeding (or
true-breeding) lines produce offspring carrying specific
parental traits that remain constant from generation to gen-
role of chance in heredity? A key component of Mendel’s eration. These lines are also called inbred because they
breakthrough was the way he set up his experiments. have been mated only to each other for many generations.
What did Mendel do differently from those who Mendel observed his pure-breeding lines for up to eight
preceded him? First, he chose the garden pea (Pisum sa- generations. Plants with white flowers always produced
tivum) as his experimental organism (Figs. 2.7a and b). offspring with white flowers; plants with purple flowers
Peas grew well in Brünn, and with male and female organs produced only offspring with purple flowers. Mendel
in the same flower, they were normally self-fertilizing. In called constant but mutually exclusive, alternative traits,

Figure 2.7 Mendel’s experimental organism: The garden pea. (a) Pea plants with white flowers. (b) Pollen is produced in the
anthers. Mature pollen lands on the stigma, which is connected to the ovary (which becomes the pea pod). After landing, the pollen grows a
tube that extends through the stigma to one of the ovules (immature seeds), allowing fertilization to take place. (c) To prevent self-fertilization,
breeders remove the anthers from the female parents (here, the white flower) before the plant produces mature pollen. Pollen is then
transferred with a paintbrush from the anthers of the male parent (here, the purple flower) to the stigma of the female parent. Each fertilized
ovule becomes an individual pea (mature seed) that can grow into a new pea plant. All of the peas produced from one flower are encased in
the same pea pod, but these peas form from different pollen grains and ovules.
(a): © Andrea Jones Images/Alamy
Cross-
fertilization:
pollen
transferred, Anthers
dusted onto removed
Stigma stigma of previously
Anthers recipient
( )
Seed
Ovules formation
( ) within
ovary
Seed
germination
(a) Pisum sativum (b) Pea flower anatomy (c) Cross-pollination