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Each
human individual is made up of several hundred million million
microscopic cells (plus considerable noncellular material
such as bone and water). Cells
come in a variety of shapes and sizes. Some are  rounded,
some flat, some angular, some irregular, and some (e.g.,
nerve cells) have long projections (See Figure 1, the nucleus,
containing the DNA is shown in blue). A typical cell,
such as a white blood cell, is about 1/2,000 inch in diameter. The
part of the cell of greatest genetic interest, the inner
part or nucleus, is usually roughly spherical. All
the cells in the body are descended from a single fertilized
egg, which by successive divisions has produced the vast
number and various cell types that comprise the human body. The
nucleus contains a number of worm like or threadlike microscopic
bodies, called chromosomes. Every species has a characteristic
number of chromosomes-a typical human cell has 46. |
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The nucleus
of a fertilized human egg starts out with 23 chromosomes
from the mother's egg and a corresponding set of 23 from
the father's sperm. A sperm or egg cell, containing a single set of
chromosomes, is said to be haploid. A cell with two sets,
a total of 23 pairs or 46 chromosomes, is diploid. The
fertilized egg divides into two, these two into four, and so
on throughout embryonic development, and for many kinds of
cells, throughout life. This process of cell division,
called mitosis, distributes these chromosomes precisely. Before
the cell divides, each chromosome has split longitudinally
in half, and each daughter cell receives on of the halves.
Thus, after cell division, each of the daughter cells has identical
chromosomes, the same as in the parent cell. This precise process
assures that every cell in the body has an identical 46-chromosome
makeup in its nucleus. Yet, as always in biology, there are
exceptions. A red blood cell has no nucleus and therefore
no chromosomes. Sometimes the chromosomes divide without
a cell division, doubling the number. For example, liver
cells, are usually polyploid, that is, having four or more
sets of chromosomes. The great bulk of cells, however,
play by the rules; they are nucleated and have 46 chromosomes. Sometimes,
two embryos will develop from the same fertilized egg, either
because the two cells separate after the first division or
each develops separately or, more often, by the multicellular
embryo dividing into two parts at a later stage. This leads
to identical twins. They necessarily have identical
chromosomes and resemble each other closely; the differences
they possess are due to environmental factors and the vagaries
of development. |
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In the
formation of a sperm or egg, the chromosome number is halved
(fortunately, or otherwise each generation would have twice
as many chromosomes as the previous one). During the
process of meiosis, the chromosomes are allocated so that
each gamete (sperm or egg) has one representative of each
pair, for a total of 23. The members of different pairs
behave independently in meiosis. If the chromosome from
the number 1 pair in a sperm is maternal, that is, derived
from the mother, the chromosome from the number 2 pair is
equally likely to be either maternal or paternal, and so
on. For convenience, the human chromosomes are identified
by a number, starting with the largest. Most chromosomes
have a short and a long arm, designated by p and q respectively. Hence,
7p designates the short arm of the 7th largest chromosome.
 The
two members of a chromosome pair, as viewed through a
microscope, are identical in shape and size, with an important
exception - the
sex chromosomes. In the human cell, the Y chromosome
is much smaller than the X chromosome. A body cell
from a female has two X chromosomes; a cell from a male
has an X and a Y. Through the process of meiosis,
an egg ends up with a single X chromosome (in addition
to 22 other chromosomes, called autosomes, for a total
of 23). A sperm has either an X or a Y plus 22 autosomes. At
fertilization, chance determines, whether the successful
sperm carries an X or Y chromosome, that is, determines
whether the developing embryo will be female or male. The
X and Y chromosomes are not numbered, so the chromosomes
of a gamete are numbered 1 through 22, plus X or Y. |
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Each chromosome
appears in the microscope as a three-dimensional object,
a condensed sausage-shaped blob, at some stages of the cell
division cycle, and as a long, often invisible thread at
others. The
core of the chromosome is a very long, extremely thin thread
of deoxyribonucleic acid; henceforth we shall use its more
familiar nickname, DNA. The DNA molecule in a chromosome
is surprisingly long. A single human chromosome, as seen
with an ordinary microscope, is about 1/5,000 inch long, yet
the DNA molecule in this chromosome is an inch or more in length,
compacted into the chromosome by successive coiling and supercoiling. The
total DNA in a human cell, if the DNA molecules of each chromosome
were lined up end to end, would be some 6 feet in length. The
DNA molecule is a double thread, coiled into a helix. The
genetically important constituents of DNA are four nucleotides
(or bases), abbreviated A, T, G, and C. The double thread
consists of two phosphate-sugar strands bridged by many pairs
of nucleotides, AT, TA, GC, or CG. A always pairs with
T and G with C. The DNA molecule can be thought of as
a twisted rope ladder with four kinds of stair steps. Each
chromosome has the base pairs in a specific order. The
genetic difference between one gene and another, or one person
and another, is not in the kinds of base pairs; always the
same four are used. It is the sequence in which these
occur that determines genetic individuality. With an
enormous number of base pairs (stair steps), the number of
orders (permutations) is astronomical. No wonder we
are all different; yet at the DNA level we are remarkably
alike, as the next paragraph will explain. |
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The chromosomes
of a sperm or egg contain about 3 billion base pairs, so
a body cell has 6 billion. The whole set of base pairs
in a gamete is called the genome. The precise processes
of DNA duplication and cell division ensure that each cell
(with few exceptions, to be discussed later) contains the same
sequence of DNA bases. Any two human genomes are alike
for the overwhelming majority of their bases; DNA samples from
two unrelated persons differ on the average at only about one
base per thousand. Yet 1/1,000 of 6 billion is 6 million. These
6 million base differences are sufficient to produce all the
genetic differences between those two persons. Although
any two genomes differ at some 1/1,000 of their bases, these
are not necessarily the same bases as those that are different
in another pair of genomes. So the great diversity of
shapes, sizes, color, behavior, disease susceptibility, and
so on that characterize humanity is no surprise. Even
though two persons share an overwhelming proportion of their
DNA, there are still enough differences that no two are genetically
alike, unless they are identical twins. If we had the
complete sequence of the DNA from two persons, or even 1 percent
of the DNA, we could (except for identical twins) be certain
whether they came from one person or two. In practice,
as will be dis cussed later, a much smaller fraction is analyzed,
so that identification becomes probabilistic rather than certain. |
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In addition
to differences in individual nucleotides, there are also
variations in their number. There are some DNA regions
in which a small number of bases is repeated a variable number
of times, so the total amount of DNA in different individuals
is not exactly the same. Some of the regions
that are of the greatest use forensically are such repeated
sequences in which the number of repeats varies from person
to person.
At present,
the Human Genome Project is nearing completion. In
June 2000 it was about 90 percent complete. The object
is to determine the complete sequence of base pairs in a
representative person or a composite of several persons. Soon
we shall know the complete encoded genetic information in
a genome. This contains the totality of the genetic
instructions in an egg or sperm, which together with all
of the environmental influences determine the developmental
outcome. The chromosomal DNA is not quite the totality
of the biological inheritance, for a tiny fraction of the
genetic information transmitted from one generation to
the next is in the maternally transmitted mitochondria.A
gene is a stretch of DNA from 1,000 to 100,000 or more
base pairs in length that has a specific function; usually
a gene is responsible for a particular protein. Alternative forms
of the gene are called alleles. For example, a specific
allele of a particular gene is responsible for the enzyme
that converts the amino acid phenylalanine into tyrosine. When
this enzyme is missing or abnormal, the child develops
the disease, phenylketonuria, or PKU.The result
is severe mental retardation unless the child is treated;
happily, with a specific diet the child develops normally. A
child will develop PKU only if both representatives of the
appropriate chromosome pair carry the abnormal allele. If
there is only one PKU allele and the other is normal, the
child will be normal; the amount of enzyme produced by a
single normal allele is enough. Alleles that express
their characteristic trait only when present in duplicate,
like the PKU allele, are recessive. Those, like the
normal allele, that are effective when present singly, are
dominant. It is customary to designate genes by letter
symbols, so we can designate the PKU allele by a and
the normal alternative by A. An individual with two
representatives of the same allele, aa or AA,
is homozygous (noun: homozygote). If the two are
different, Aa,
the individual is heterozygous (noun:heterozygote). Finally,
we need two more words. Genotype is the genetic makeup
of the individual, such as AA or Aa. The
genotypic designation may be extended to include several
gene loci. Phenotype is the trait, such as mental retardation
if observed externally or the metabolic defect if measured
chemically. It may include several traits or it may
be a quantitative measure such as heightThe rules
of inheritance can be deduced from the behavior of chromosomes
in meiosis and fertilization. However, before the mechanism
of inheritance was understood, the rules were inferred by
the Austrian monk, Gregor Mendel, from his experiments breeding
garden peas. Although his studies were reported in
1865, they remained unknown until the principles were rediscovered
in 1900. It was immediately obvious that Mendel's
hereditary factors followed the same rules as chromosomes;
hence the genes must be carried by chromosomes.As stated
earlier, the human chromosomes are numbered from 1 to 22,
starting with the largest, plus the X and Y. Each gene
occupies a specific position (or locus) on a specific chromosome.
The gene causing PKU is at a locus on 12q, meaning that it
is on the long arm of chromosome number 12. Typically,
there are more than two different alleles at a locus in a
population. There may be hundreds in some extreme cases,
but of course any fertilized egg has at most two kinds. A
locus with more than one allele in the population is said
to be polymorphic. Highly polymorphic loci are particularly
useful for forensic identification.In the
process of meiosis, one member of each chromosome pair
is included in the gamete. Early in meiosis, the two homologous
chromosomes pair up. While lined up side by side they
often break at corresponding sites and exchange partners. Thus,
two genes that were formerly on the same chromosome may end
up on different chromosomes, if there has been an exchange
between them. The tendency for two different genes
on the same chromosome to be inherited together is called
linkage. The closer together two genes are on the chromosome,
the less probable it is that a break will occur between them
and the more probable that they are to be inherited together. This
property has been used in classical genetics to "map" the
position of genes on the chromosome; the closer together
two genes are, the more tightly linked they are in inheritance. This
method, developed in experimental animals, also is used
to locate genes on human chromosomes, although in recent
times it is often supplemented by more direct physical
means.Genes are ordinarily transmitted from generation
to generation unchanged. Sometimes,
however, the gene is changed, a rare process called mutation. For
example, the normal allele may change to the one causing
PKU. When a gene mutates, the mutant form is as stable
and as regularly transmitted as the original. Mutations
come in all sizes. A mutation may be a substitution
of one base for another, or one or more bases may be gained
or lost, or the order of a group of bases may be changed,
inverted for example.Chromosomes
are sometimes broken and reattached in new ways. Or
a whole chromosome may be lost or duplicated. All
of these come under the general name of mutation, although
the term is more often restricted to those changes that
are transmitted as a Mendelian unit.The genes
make up only a tiny fraction of the DNA. The rest,
the great bulk-about 97 percent-has no known function. It
is sometimes referred to as "junk DNA." Nevertheless, these
nongenic regions show the same genetic variability that genes
do, in fact usually more. These differences are not
overt, but can be detected by laboratory tests. Regions
of DNA that are used for forensic analysis are usually not
genes, but rather are located in those parts of the chromosomes
without known functions, or if part of a gene, not in the
part that produces a detectable effect. (One reason
for this choice has been to protect individual privacy.)
Nevertheless, the words commonly used for describing genes
(e.g., allele, homozygous, polymorphic) are carried over
to DNA regions used for identification. It is customary
to call the genotype for the group of loci involved in
a forensic analysis a profile. |
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Basic
Genetics 
