Page 97 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

PART I Basic Principles: How Traits Are Transmitted
4

chapter

The Chromosome

Theory of Inheritance

Each of these three human chromosomes carries hundreds of genes.
© Adrian T. Sumner/Stone/Getty Images

chapter outline
IN THE SPHERICAL, membrane-bound nuclei of plant and • 4.1 Chromosomes: The Carriers of Genes
animal cells prepared for viewing under the microscope, • 4.2 Sex Chromosomes and Sex Determination
chromosomes appear as brightly colored, threadlike bod- • 4.3 Mitosis: Cell Division That Preserves Chromosome
ies. The nuclei of normal human cells carry 23 pairs of Number
chromosomes for a total of 46. Noticeable differences in • 4.4 Meiosis: Cell Divisions That Halve Chromosome
size and shape exist among the 23 pairs, but within each Number
pair, the two chromosomes appear to match exactly. (The • 4.5 Gametogenesis
only exceptions are the male’s sex chromosomes, desig-
nated X and Y, which constitute an unmatched pair.) • 4.6 Validation of the Chromosome Theory
Down syndrome was the first human genetic disorder • 4.7 Sex-Linked and Sexually Dimorphic Traits
attributable not to a gene mutation but to an abnormal number in Humans
of chromosomes. Children born with Down syndrome have
47 chromosomes in each somatic cell nucleus because they
carry three, instead of the normal pair, of a very small chro-
mosome referred to as number 21. The aberrant genotype, called trisomy 21, gives rise to
an abnormal phenotype, including a wide skull that is flatter than normal at the back, an
unusually large tongue, learning disabilities caused by the abnormal development of the
hippocampus and other parts of the brain, and a propensity to respiratory infections as well
as heart disorders, rapid aging, and leukemia (Fig. 4.1).
How can one extra copy of a chromosome that is itself of normal size and shape
cause such wide-ranging phenotypic effects? The answer has two parts. First and fore-
most, chromosomes are the cellular structures responsible for transmitting genetic in-
formation. In this chapter, we describe how geneticists concluded that chromosomes
are the carriers of genes, an idea that became known as the chromosome theory of
inheritance. The second part of the answer is that proper development depends not
just on what type of genetic material is present, but also on how much of it there is.
Thus the mechanisms governing gene transmission during cell division must vigilantly
maintain each cell’s chromosome number.
Cell division proceeds through the precise chromosome-parceling mechanisms of
mitosis (for somatic, or body cells) and meiosis (for gametes—eggs and sperm). When

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