Page 22 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

PART I Basic Principles: How Traits Are Transmitted
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chapter

Mendel’s Principles

of Heredity

Although Mendel’s laws can predict the probability that an indi-
vidual will have a certain genetic makeup, the chance meeting of
particular male and female gametes determines an individual’s
actual genetic fate.
© Lawrence Manning/Corbis RF

chapter outline

• 2.1 The Puzzle of Inheritance
• 2.2 Genetic Analysis According to Mendel
• 2.3 Mendelian Inheritance in Humans
A QUICK GLANCE at an extended family portrait is
likely to reveal children who resemble one parent or the
other, or who look like a combination of the two (Fig. 2.1). Some children, however,
look unlike any of the assembled relatives and more like a great, great grandparent.
What causes the similarities and differences of appearance and the skipping of
generations?
The answers lie in our genes, the basic units of biological information, and in
heredity, the way genes transmit physiological, anatomical, and behavioral traits
from parents to offspring. Each of us starts out as a single fertilized egg cell that
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develops, by division and differentiation, into a mature adult made up of 10 (a hun-
dred trillion) specialized cells capable of carrying out all the body’s functions and
controlling our outward appearance. Genes, passed from one generation to the next,
underlie the formation of every heritable trait. Such traits are as diverse as the pres-
ence of a cleft in your chin, the tendency to lose hair as you age, your hair, eye, and
skin color, and even your susceptibility to certain cancers. All such traits run in fam-
ilies in predictable patterns that impose some possibilities and exclude others.
Genetics, the science of heredity, pursues a precise explanation of the biologi-
cal structures and mechanisms that determine inheritance. In some instances, the
relationship between gene and trait is remarkably simple. A single change in a sin-
gle gene, for example, results in sickle-cell anemia, a disease in which the hemoglo-
bin molecule found in red blood cells is defective. In other instances, the correlations
between genes and traits are bewilderingly complex. An example is the genetic basis
of facial features, in which many genes determine a large number of molecules that
interact to generate the combination we recognize as a friend’s face.
Gregor Mendel (1822–1884; Fig. 2.2), a stocky, bespectacled Augustinian monk
and expert plant breeder, discovered the basic principles of genetics in the mid- nineteenth
century. He published his findings in 1866, just seven years after Darwin’s On the Ori-
gin of Species appeared in print. Mendel lived and worked in Brünn, Austria (now Brno
in the Czech Republic), where he examined the inheritance of clear-cut alternative traits
in pea plants, such as purple versus white flowers or yellow versus green seeds. In so

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