Page 24 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

16 Chapter 2 Mendel’s Principles of Heredity

corn or tomatoes; the rigorous analysis of systematically Figure 2.4 The earliest known record of applied
recorded information gleaned from these observations; and genetics. In this 2800-year-old Assyrian relief from the Northwest
the development of a theoretical framework that can ex- Palace of Assurnasirpal II (883–859 b.c.), priests wearing bird masks
plain the origins and relationships of these phenomena. In artificially pollinate flowers of female date palms.
the mid-nineteenth century, Gregor Mendel became the Image copyright © The Metropolitan Museum of Art. Image source: Art
Resource, NY
first person to combine data collection, analysis, and the-
ory in a successful pursuit of the true basis of heredity. For
many thousands of years before that, the only genetic prac-
tice was the selective breeding of domesticated plants and
animals, with no guarantee of what a particular mating
would produce.

Artificial Selection Was the First
Applied Genetic Technique

A rudimentary use of genetics was the driving force behind
a key transition in human civilization, allowing hunters and
gatherers to settle in villages and survive as shepherds and
farmers. Even before recorded history, people practiced ap-
plied genetics as they domesticated plants and animals for
their own uses. From a large litter of semitamed wolves, for
example, they sent the savage and the misbehaving to the
stew pot while sparing the alert sentries and friendly com-
panions for longer life and eventual mating. As a result of
this artificial selection—purposeful control over mating
by choice of parents for the next generation—the domestic
dog (Canis lupus familiaris) slowly arose from ancestral
wolves (Canis lupus). The oldest bones identified indisput-
ably as dog (and not wolf) are a skull excavated from a color, or taste. A 1929 botanical survey of three oases in
20,000-year-old Alaskan settlement. Many millennia of Egypt turned up 400 varieties of date-bearing palms,
evolution guided by artificial selection have produced mas- twentieth-century evidence of the natural and artificially
sive Great Danes and minuscule Chihuahuas as well as generated variation among these trees.
hundreds of other modern breeds of dog. By 10,000 years
ago, people had begun to use this same kind of genetic
manipulation to develop economically valuable herds of Desirable Traits Sometimes Disappear
reindeer, sheep, goats, pigs, and cattle that produced life- and Reappear
sustaining meat, milk, hides, and wools.
Farmers also carried out artificial selection of plants, In 1822, the year of Mendel’s birth, what people in Austria
storing seed from the hardiest and tastiest individuals for understood about the basic principles of heredity was not
the next planting, eventually obtaining strains that grew much different from what the people of ancient Assyria
better, produced more, and were easier to cultivate and har- had known. By the nineteenth century, plant and animal
vest. In this way, scrawny, weed-like plants gradually, with breeders had created many strains in which offspring often
human guidance, turned into rice, wheat, barley, lentils, carried a prized parental trait. Using such strains, breeders
and dates in Asia; corn, squash, tomatoes, potatoes, and could produce plants or animals with desired characteris-
peppers in the Americas; yams, peanuts, and gourds in Af- tics for food and fiber, but they could not always predict
rica. Later, plant breeders recognized male and female or- why a valued trait would sometimes disappear and then
gans in plants and carried out artificial pollination. An reappear in only some offspring.
Assyrian frieze carved in the ninth century b.c., pictured in For example, selective breeding practices had resulted
Fig. 2.4, is the oldest known visual record of this kind of in valuable flocks of merino sheep producing large quanti-
genetic experiment. It depicts priests brushing the flowers ties of soft, fine wool, but at the 1837 annual meeting of the
of female date palms with selected male pollen. By this Moravian Sheep Breeders Society, one breeder’s dilemma
method of artificial selection, early practical geneticists epitomized the state of the art. He possessed an outstanding
produced several hundred varieties of dates, each differing ram that would be priceless “if its advantages are inherited
in specific observable qualities, such as the fruit’s size, by its offspring,” but “if they are not inherited, then it is