Gene Mutations

Gene Mutations
Many congenital formations in humans are inherited, and some show a clear
mendelian pattern of inheritance. Many birth defects are directly attributable
to a change in the structure or function of a single gene, hence the name single
gene mutation. This type of defect is estimated to account for approximately
8% of all human malformations.
18 Part One: General Embryology
With the exception of the X and Y chromosomes in the male, genes exist
as pairs, or alleles, so that there are two doses for each genetic determinant,
one from the mother and one from the father. If a mutant gene produces an
abnormality in a single dose, despite the presence of a normal allele, it is a
dominant mutation. If both alleles must be abnormal (double dose) or if the
mutation is X-linked in the male, it is a recessive mutation. Gradations in the
effects of mutant genes may be a result of modifying factors.
The application of molecular biological techniques has increased our
knowledge of genes responsible for normal development. In turn, genetic
analysis of human syndromes has shown that mutations in many of these
same genes are responsible for some congenital abnormalities and childhood
diseases. Thus, the link between key genes in development and their role in
clinical syndromes is becoming clearer.
In addition to causing congenital malformations, mutations can result in
inborn errors of metabolism. These diseases, among which phenylketonuria,
homocystinuria, and galactosemia are the best known, are frequently accompanied
by or cause various degrees of mental retardation.
Diagnostic Techniques for Identifying Genetic Abnormalities
Cytogenetic analysis is used to assess chromosome number and integrity.
The technique requires dividing cells, which usually means establishing cell
cultures that are arrested in metaphase by chemical treatment. Chromosomes
are stained with Giemsa stain to reveal light and dark banding patterns
(G-bands; Fig. 1.6) unique for each chromosome. Each band represents 5 to
10×106 base pairs of DNA, whichmay include a fewto several hundred genes.
Recently, high resolution metaphase banding techniques have been developed
that demonstrate greater numbers of bands representing even smaller
pieces of DNA, thereby facilitating diagnosis of small deletions.
New molecular techniques, such as fluorescence in situ hybridization
(FISH), use specific DNA probes to identify ploidy for a few selected chromosomes.
Fluorescent probes are hybridized to chromosomes or genetic
loci using cells on a slide, and the results are visualized with a fluorescence
microscope (Fig.1.15). Spectral karyotype analysis is a technique in which
every chromosome is hybridized to a unique fluorescent probe of a different
color. Results are then analyzed by a computer.

C L I N I C A L C O R R E L A T E S - Birth Defects and Spontaneous Abortions:

C L I N I C A L C O R R E L A T E S
Birth Defects and Spontaneous Abortions:
Chromosomal and Genetic Factors
Chromosomal abnormalities, which may be numerical or structural, are
important causes of birth defects and spontaneous abortions. It is estimated
that 50% of conceptions end in spontaneous abortion and that 50% of these


Figure 1.4 Events occurring during the first and second maturation divisions. A. The
primitive female germ cell (primary oocyte) produces only one mature gamete, the mature
oocyte. B. The primitive male germ cell (primary spermatocyte) produces four spermatids,
all of which develop into spermatozoa.
abortuses have major chromosomal abnormalities. Thus approximately 25%
of conceptuses have a major chromosomal defect. The most common chromosomal
abnormalities in abortuses are 45,X (Turner syndrome), triploidy,
and trisomy 16. Chromosomal abnormalities account for 7% of major birth
defects, and gene mutations account for an additional 8%.
Numerical Abnormalities
The normal human somatic cell contains 46 chromosomes; the normal gamete
contains 23. Normal somatic cells are diploid, or 2n; normal gametes
are haploid, or n. Euploid refers to any exact multiple of n, e.g., diploid or
triploid. Aneuploid refers to any chromosome number that is not euploid; it is
usually applied when an extra chromosome is present (trisomy) or when one
is missing (monosomy). Abnormalities in chromosome number may originate
during meiotic or mitotic divisions. In meiosis, two members of a pair
of homologous chromosomes normally separate during the first meiotic division
so that each daughter cell receives one member of each pair (Fig. 1.5A).
Sometimes, however, separation does not occur (nondisjunction), and both
members of a pair move into one cell (Fig. 1.5, B and C ). As a result of
nondisjunction of the chromosomes, one cell receives 24 chromosomes,
and the other receives 22 instead of the normal 23. When, at fertilization,
a gamete having 23 chromosomes fuses with a gamete having 24 or 22 chromosomes, the result is an individual with either 47 chromosomes
(trisomy) or 45 chromosomes (monosomy). Nondisjunction, which occurs
during either the first or the second meiotic division of the germ cells, may
involve the autosomes or sex chromosomes. In women, the incidence of
chromosomal abnormalities, including nondisjunction, increases with age,
especially at 35 years and older.

The Chromosome Theory of Inheritance

The Chromosome Theory of Inheritance
Traits of a new individual are determined by specific genes on chromosomes
inherited from the father and the mother. Humans have approximately 35,000
genes on 46 chromosomes. Genes on the same chromosome tend to be inherited
together and so are known as linked genes. In somatic cells, chromosomes
appear as 23 homologous pairs to form the diploid number of 46. There are
22 pairs of matching chromosomes, the autosomes, and one pair of sex chromosomes.
If the sex pair is XX, the individual is genetically female; if the pair is
XY, the individual is genetically male. One chromosome of each pair is derived
from the maternal gamete, the oocyte, and one from the paternal gamete, the
Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 5
sperm. Thus each gamete contains a haploid number of 23 chromosomes, and
the union of the gametes at fertilization restores the diploid number of 46.
MITOSIS
Mitosis is the process whereby one cell divides, giving rise to two daughter
cells that are genetically identical to the parent cell (Fig. 1.2). Each daughter
cell receives the complete complement of 46 chromosomes. Before a cell enters
mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During
this replication phase the chromosomes are extremely long, they are spread
diffusely through the nucleus, and they cannot be recognized with the light microscope.
With the onset of mitosis the chromosomes begin to coil, contract,
and condense; these events mark the beginning of prophase. Each chromosome
now consists of two parallel subunits, chromatids, that are joined at a
narrow region common to both called the centromere. Throughout prophase
the chromosomes continue to condense, shorten, and thicken (Fig. 1.2A),
but only at prometaphase do the chromatids become distinguishable
(Fig. 1.2B). During metaphase the chromosomes line up in the equatorial plane,


Figure 1.2 Various stages of mitosis. In prophase, chromosomes are visible as slender
threads. Doubled chromatids become clearly visible as individual units during
metaphase. At no time during division do members of a chromosome pair unite. Blue,
paternal chromosomes; red, maternal chromosomes.
6 Part One: General Embryology
and their doubled structure is clearly visible (Fig. 1.2C ). Each is attached by
microtubules extending from the centromere to the centriole, forming the mitotic
spindle. Soon the centromere of each chromosome divides, marking the
beginning of anaphase, followed by migration of chromatids to opposite poles
of the spindle. Finally, during telophase, chromosomes uncoil and lengthen,
the nuclear envelope reforms, and the cytoplasm divides (Fig. 1.2, D and E ).
Each daughter cell receives half of all doubled chromosome material and thus
maintains the same number of chromosomes as the mother cell.
MEIOSIS
Meiosis is the cell division that takes place in the germ cells to generate male
and female gametes, sperm and egg cells, respectively. Meiosis requires two cell
divisions, meiosis I and meiosis II, to reduce the number of chromosomes to
the haploid number of 23 (Fig. 1.3). As in mitosis, male and female germ cells
(spermatocytes and primary oocytes) at the beginning of meiosis I replicate
their DNA so that each of the 46 chromosomes is duplicated into sister chromatids.
In contrast to mitosis, however, homologous chromosomes then align
themselves in pairs, a process called synapsis. The pairing is exact and point
for point except for the XY combination. Homologous pairs then separate into
two daughter cells. Shortly thereafter meiosis II separates sister chromatids.
Each gamete then contains 23 chromosomes.
Crossover
Crossovers, critical events in meiosis I, are the interchange of chromatid segments
between paired homologous chromosomes (Fig. 1.3C ). Segments of
chromatids break and are exchanged as homologous chromosomes separate.
As separation occurs, points of interchange are temporarily united and form an
X-like structure, a chiasma (Fig. 1.3C ). The approximately 30 to 40 crossovers
(one or two per chromosome) with each meiotic I division are most frequent
between genes that are far apart on a chromosome.
As a result of meiotic divisions, (a) genetic variability is enhanced through
crossover, which redistributes genetic material, and through random distribution
of homologous chromosomes to the daughter cells; and (b) each germ cell
contains a haploid number of chromosomes, so that at fertilization the diploid
number of 46 is restored.
Polar Bodies
Also during meiosis one primary oocyte gives rise to four daughter cells, each
with 22 plus 1 X chromosomes (Fig. 1.4A). However, only one of these develops
into a mature gamete, the oocyte; the other three, the polar bodies, receive
little cytoplasm and degenerate during subsequent development. Similarly, one
primary spermatocyte gives rise to four daughter cells, two with 22 plus 1


Figure 1.3 First and second meiotic divisions. A. Homologous chromosomes approach
each other. B. Homologous chromosomes pair, and each member of the pair consists of
two chromatids. C. Intimately paired homologous chromosomes interchange chromatid
fragments (crossover). Note the chiasma. D. Double-structured chromosomes pull apart.
E. Anaphase of the first meiotic division. F and G. During the second meiotic division,
the double-structured chromosomes split at the centromere. At completion of division,
chromosomes in each of the four daughter cells are different from each other.
X chromosomes and two with 22 plus 1 Y chromosomes (Fig. 1.4B). However,
in contrast to oocyte formation, all four develop into mature gametes.