A gene initiates synthesis of a protein by transcription of
the DNA into messenger ribonucleic acid (mRNA), which is a
single-stranded molecule complementary to the DNA. After
processing within the cell nucleus, the mRNA moves into the
cytoplasm, where it binds to ribosomes. The translation of
RNA into protein takes place on the ribosomes. In addition, a
typical gene is not a simple uninterrupted length of DNA. Most
genes are made up of coding sequences (exons) separated by
noncoding regions (intervening sequences, or introns). Tran-
scription of the gene into RNA begins at an initiation site in
advance of the first coding sequence and terminates beyond
the end of the last sequence. After the entire DNA segment has
been transcribed into mRNA, the intervening sequences are re-
moved, and the coding sequences are spliced together before
the mRNA is translated into a polypeptide (Fig. 4).
Any gene occupies a specific chromosomal position, or locus.
The alternative genes at a particular locus are said to be alleles.
If a pair of alleles are identical, the individual is homozygous; if
they are different, the individual is heterozygous.
Mutation
Genetic variation has its origin in mutation. Although any
change in DNA is a mutation (whether it is a microscopically
detectable change in the structure of a chromosome or a single
base change in the genetic code), the term is usually applied to
a stable change in DNA that alters the genetic code and thus
leads to synthesis of an altered protein. Mutation can occur in
reproductive cells or somatic cells, but the genetically signifi-
cant ones are those that occur in reproductive cells and can be
transmitted to future generations. Natural selection acts upon
the genetic diversity generated by mutation to preserve benefi-
cial mutations and eliminate deleterious ones.
A very large amount of genetic variation exists in the hu-
man population. Everyone carries many mutations; some are
newly acquired, but others are inherited through innumerable
generations. Although the exact number is unknown, it is likely
that everyone is heterozygous at numerous loci, perhaps as
many as 20%.
Single-gene inheritance
The patterns of inheritance of characteristics determined by
single genes or gene pairs depend on two conditions:
(1) whether the gene in question is on an autosome (autosomal
inheritance) or on the X chromosome (X-linked inheritance);
and (2) whether the gene is dominant (expressed in heterozy-
gotes, when it is present on only one member of a chromosom-
Human Genetics (continued)
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Fig. 3 The double-helix structure of a DNA molecule.
(Credit: Darryl Leja/National Human Genome Research Institute)
Fig. 4 Simplified diagram showing (a) the structure of a gene, (b) the mature messenger
RNA (mRNA) after the two intervening sequences have been spliced out and the "cap"
and "tail" added, and (c) the polypeptide translated from the mRNA.
(Copyright © McGraw Hill)
