Essential Cell Biology 5th edition

664 CHAPTER 19 Sexual Reproduction and Genetics
Figure 19−18 One disproven theory
of inheritance suggested that genetic
traits are passed down solely from the
father. In support of this particular theory
of uniparental inheritance, some early
microscopists fancied they could see a
small, perfectly formed human crouched
inside the head of each sperm.
ECB5 e19.18/19.18
Figure 19−19 Some people taste it,
some people don’t. The ability to taste
the chemical phenylthiocarbamide (PTC)
is governed by a single gene. Although
geneticists have known since the 1930s that
the inability to taste PTC is inherited, it was
not until 2003 that researchers identified
the responsible gene, which encodes a
bitter-taste ECB5 receptor. e19.19/19.19 Nontasters produce
a PTC receptor protein with amino acid
substitutions that are thought to reduce the
receptor’s activity.
MENDEL AND THE LAWS OF INHERITANCE
In organisms that reproduce asexually, the genetic material of the parent
is transmitted faithfully to its progeny. The resulting offspring are thus
genetically identical to a single parent. Before Mendel started working
with peas, some biologists suspected that inheritance might work that
way in humans (Figure 19−18).
Although children resemble their parents, they are not carbon copies of
either the mother or the father. Thanks to the mechanisms of meiosis
just described, sex breaks up existing collections of genetic information,
shuffles alleles into new combinations, and produces offspring that tend
to exhibit a mixture of traits derived from both parents, as well as novel
ones. The ability to track characteristics that show some variation from
one generation to the next enabled geneticists to begin to decipher the
rules that govern heredity in sexually reproducing organisms.
The simplest traits to follow are those that are easy to see or to measure.
In humans, these include the tendency to sneeze when exposed to
bright sun, whether a person’s earlobes are attached or pendulous, or the
ability to detect certain odors or flavors (Figure 19−19). Of course, the
laws of inheritance were not worked out by studying people with pendulous
earlobes, but by following traits in organisms that are easy to breed
and that produce large numbers of offspring. Gregor Mendel, the father
of genetics, focused on peas. But similar breeding experiments can be
performed in fruit flies, worms, dogs, cats, or any other plant or animal
that possesses characteristics of interest, because the same basic laws
of inheritance apply to all sexually reproducing organisms, from peas to
people.
In this section, we describe the logic of genetic inheritance in sexually
reproducing organisms. We see how the behavior of chromosomes during
meiosis—their segregation into gametes that then unite at random to
form genetically unique offspring—explains the experimentally derived
laws of inheritance. But first, we discuss how Mendel, breeding peas in
his monastery garden, discovered these laws more than 150 years ago.
Mendel Studied Traits That Are Inherited in a Discrete
Fashion
Mendel chose to study pea plants because they are easy to cultivate in
large numbers and could be raised in a small space—such as an abbey
garden. He controlled which plants mated with which by removing sperm
(pollen) from one plant and brushing it onto the female structures of
another. This careful cross-pollination ensured that Mendel could be certain
of the parentage of every pea plant he examined.
Perhaps more important for Mendel’s purposes, pea plants were available
in many varieties. For example, one variety has purple flowers,
another has white. One variety produces seeds (peas) with smooth skin,
another produces peas that are wrinkled. Mendel chose to follow seven
traits—including flower color and pea shape—that were distinct, easily
observable, and, most importantly, inherited in a discrete fashion: for
example, the plants have either purple flowers or white flowers—nothing
in-between (Figure 19−20).
Mendel Disproved the Alternative Theories of
Inheritance
The breeding experiments that Mendel performed were straightforward.
He started with stocks of genetically pure, “true-breeding” plants—those
that produce offspring of the same variety when allowed to self-fertilize.