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A new era in cytogenetics,
the field of investigation concerned with studies of the chromosomes,
began in 1956 with the discovery by Jo Hin Tjio and Albert Levan
that human somatic cells contain 23 pairs of chromosomes. Since
that time the field has advanced with amazing rapidity and has
demonstrated that human 
chromosome aberrations rank as major causes of fetal death and
of tragic human diseases, many of which are accompanied by mental
retardation.
Human genetics
is but one small piece of the much larger field of classical and
molecular genetics, which often is said to have begun with the
work of the Austrian monk Gregor Mendel in the mid-1800s. Mendel
studied the garden pea, exploring in quantitative terms the transmission
of sharply defined traits such as plant height, seed colour, and
seed texture from one generation to the next. Although Mendel
knew nothing about the modern concepts of genes and chromosomes,
he deduced from observations that each parent plant carries a
pair of determining units for each trait studied, that one trait
unit can sometimes dominate the other, and that the units are
transmitted as some kind of physical entities from parent to offspring
during reproduction. (The pairs of trait units are now recognized
to be corresponding genes on paired chromosomes.) The major conclusion
of Mendel's studies represented a dramatic break with the mainstream
thought of the time and are often summarized as Mendel's laws
Mendel's
Laws.
- Paired
trait units separate, or segregate, during the formation of
gametes (sex cells)-that is, an offspring inherits from a parent
either one trait unit or the other, but not both.
- Derived
from Mendal's experiments, in which he studied the simultaneous
inheritance of different traits, is that the units for the traits
assort independently-that is, the unit an offspring inherits
for one trait is independent of the unit it inherits for another
trait.
It is now
recognized that Mendel's laws have many exceptions and that, in
fact, they represent only a subset of the whole process of genetic
inheritance. Nevertheless, in both peas and humans, they still
explain the pattern and frequency of transmission for a large
number of genetic traits,
including
many common human diseases such as cystic fibrosis and sickle-cell
anemia. Subsequent work in the 1900s by numerous researchers,
using model organisms ranging from fruit flies to corn to viruses
that infect bacteria, provided a more comprehensive view of the
complexities of genetic transmission. In addition, their studies
took the first steps toward a molecular explanation of genetic
observations, including the discovery that deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA)-long strands built of molecular
subunits called nucleotides
chained end to end-constitute the genetic material in all living
things. In 1953 James Watson and Francis Crick proposed a structure
for DNA-a double helix of intertwined nucleotide strands. This
event marks what many consider the birth of modern molecular genetics.
Gina
Kolata
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