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GENETIC DISEASES
15.1 Introduction
Genes are the fundamental units of heredity and encode specific functional
products, such as RNA molecules and polypeptides. They are encoded by
sequences of bases in DNA molecules and are found at particular positions
in chromosomes in the nucleus and also in the relatively small circular DNA
molecules in the mitochondria. Only 37 of the approximately 22 000 human
genes occur in mitochondria although mutations of these may become
clinically significant as described in Chapter 16. The genes constitute the
blueprint or the set of instructions which affects hereditary characteristics, for
example hair and eye color, height and the susceptibility to certain diseases.
When a cell divides the genetic information needs to be replicated accurately
so that these instructions pass on to the daughter cells. When changes occur
in the base sequence of DNA, either as a result of incorrect replication or from
random changes caused by physical or chemical agents, then the instructions
become corrupted. This is a mutation, and may eventually lead to disease
because the cell is unable to make, for example, a particular enzyme, hormone,
transporter or structural protein.
Chromosomes have a complex structure (Figure 15.1). Each is comprised of
a single double-stranded DNA molecule associated with numerous proteins.
OBJECTIVES
After studying this chapter you should be able to:
N outline the major features of the cell cycle;
N explain how genetic traits, including genetic diseases, are inherited;
N describe the general effects of single gene mutations;
N list the ways in which single gene disorders are treated;
N list the types of chromosomal mutations or aberrations;
N outline how genetic diseases are screenedin uteroand in neonates.
Figure 15.1 Scanning electronmicrograph of a
chromosome.Courtesy of Dr C.J. Harrison, Christie
Hospital, Manchester, UK.