Page 98 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

90 Chapter 4 The Chromosome Theory of Inheritance

the machinery does not function properly, errors in chromosome
distribution can have dire repercussions on the individual’s health
and survival. Down syndrome, for example, is the result of a failure
of chromosome segregation during meiosis. The meiotic error gives
rise to an egg or sperm carrying an extra chromosome 21 which, if
incorporated in the zygote at fertilization, is passed on via mitosis to
every cell of the developing embryo. Trisomy—three copies of a
chromosome instead of two—can occur with other chromosomes as
well, but in nearly all of these cases, the condition is prenatally
lethal and results in a miscarriage.
Two themes emerge in our discussion of meiosis and mitosis.
First, direct microscopic observations of chromosomes during gam-
ete formation led early twentieth-century investigators to recognize
that chromosome movements parallel the behavior of Mendel’s
genes, so chromosomes are likely to carry the genetic material. This
Figure 4.1 Down syndrome: One extra chromosome chromosome theory of inheritance was proposed in 1902 and was
21 has widespread phenotypic consequences. confirmed in the following 15 years through elegant experiments
Trisomy 21 usually causes changes in physical appearance as performed mainly using the fruit fly Drosophila melanogaster.
well as in the potential for learning. Many children with Down
syndrome, such as the fifth grader at the center of the Second, the chromosome theory transformed the concept of a gene
photograph, can participate fully in regular activities. from an abstract particle to a physical reality—part of a chromo-
© Richard Hutchings/Science Source some that could be seen and manipulated.

4.1 Chromosomes: generally observed 1:1 ratio of males to females. These
two features of sex determination were among the earliest
The Carriers of Genes clues to the cellular basis of heredity.

learning objectives Genes Reside in the Nucleus

1. Differentiate among somatic cells, gametes, and The nature of the specific link between sex and repro-
zygotes with regard to the number and origin of their duction remained a mystery until Anton van Leeuwen-
chromosomes. hoek, one of the earliest and most astute of microscopists,
2. Distinguish between homologous and nonhomologous discovered in 1667 that semen contains spermatozoa
chromosomes. (literally sperm animals). He imagined that these mi-
3. List the differences between sister chromatids and croscopic creatures might enter the egg and somehow
nonsister chromatids. achieve fertilization, but it was not possible to confirm
this hypothesis for another 200 years.
Then, during a 20-year period starting in 1854
One of the first questions asked at the birth of an infant— (about the same time Gregor Mendel was beginning his
is it a boy or a girl?—acknowledges that male and female pea experiments), microscopists studying fertilization in
normally are mutually exclusive characteristics like the frogs and sea urchins observed the union of male and
yellow versus green of Mendel’s peas. What’s more, female gametes and recorded the details of the process
among humans and most other sexually reproducing spe- in a series of drawings. These drawings, as well as later
cies, a roughly 1:1 ratio exists between the two sexes. micrographs (photographs taken through a microscope),
Both males and females produce cells specialized for clearly show that egg and sperm nuclei are the only ele-
reproduction—sperm or eggs—that serve as a physical ments contributed equally by maternal and paternal
link to the next generation. In bridging the gap between gametes. This observation implies that something in the
generations, these gametes must each contribute half of nucleus contains the hereditary material. In humans, the
the genetic material for making a normal, healthy son or nuclei of the gametes are less than 2 millionths of a meter
daughter. Whatever part of the gamete carries this mate- in diameter. It is indeed remarkable that the genetic
rial, its structure and function must be able to account for link between generations is packaged within such an
the either-or aspect of sex determination as well as the exceedingly small space.