314 CHAPTER 8
The Basic Building Blocks of Development
8.3 Summarize the role of chromosomes and genes in determining the
transmission of traits and the inheritance of disorders.
Any study of the human life span must begin with looking at the complex material con-
tained in the cells of the body that carries the instructions for life itself. After discussing
the basic building blocks of life, we will discuss how the processes of conception and the
development of the infant within the womb take place.
CHROMOSOMES, GENES, AND DNA Genetics is the science of heredity. Understanding
how genes transmit human characteristics and traits involves defining a few basic terms.
DNA (deoxyribonucleic acid) is a very special kind of molecule (the smallest par-
ticle of a substance that still has all the properties of that substance). DNA consists of two
very long sugar–phosphate strands, each linked together by certain chemical elements
called amines or bases arranged in a particular pattern. (See Figure 8. 1 for a representa-
tion of DNA.) The amines are organic structures that contain the genetic codes for build-
ing the proteins that make up organic life (hair coloring, muscle, and skin, for example)
and that control the life of each cell. Each section of DNA containing a certain sequence
(ordering) of these amines is called a gene. These genes are located on rod-shaped struc-
tures called chromosomes, which are found in the nucleus of a cell.
Humans have a total of 46 chromosomes in each cell of their bodies (with the excep-
tion of the egg and the sperm). Twenty-three of these chromosomes come from the moth-
er ’s egg and the other 23 from the father ’s sperm. Most characteristics are determined
by 22 such pairs, called the autosomes. The last pair determines the sex of the person.
The two chromosomes of this pair are called the sex chromosomes. Two X-shaped chromo-
somes indicate a female, while an X and a Y indicate a male.
The 46 chromosomes can be arranged in pairs, with one member of each pair com-
ing from the mother and the other member from the father. Let’s consider just one of
these pairs for the moment.
In this particular pair of chromosomes, assume that there is a gene influencing hair
color on each chromosome. The observable color of the person’s hair will be determinedgenetics
the science of inherited traits.
chromosomes
tightly wound strand of genetic mate-
rial or &0#.
gene
section of &0# having the same
arrangement of chemical elements.
DNA (deoxyribonucleic acid)
sRecial molecule that contains the
genetic material of the organism.
Figure 8.1 DNA Molecule
In this model of a DNA molecule, the two
strands making up the sides of the “twisted
ladder” are composed of sugars and phos-
phates. The “rungs” of the ladder that link
the two strands are amines. Amines contain
the genetic codes for building the proteins
that make up organic life.
CCWatch the Video Family and Twin Studies