All living things pass on traits from one generation to the next according toa systematic set of "blueprints." These blueprints are contained in the long, thread-like chromosomes that lie inside the cell nucleus of all living things. On these chromosomes are genes that determine the hereditary traits of the offspring.

Egg and sperm cells, or sex cells, are specially formed to carry only one setof the 23 different chromosomes that are normally found in the human body. (Regular body cells have two sets of the 23 chromosomes.) When a mother's egg is fertilized by the father's sperm, the egg inherits one set of chromosomes from each parent, for a total of 46 chromosomes.

Some characteristics can only be inherited through genes and chromosomes: blood type, eye color, maleness or femaleness, etc. These are called hereditarytraits. Most characteristics, however, are a result of both heredity and environment. For instance, a person can inherit a general body type, but environmental factors such as diet and exercise may change that body type.

The study of heredity the science called genetics--started in the 1800s, whenscientists first began trying to explain the existence of different speciesand variations within the same species. At that time, French biologist Jean Baptiste de Lamarck strongly believed that acquired characteristics would improve when routinely used over time. Those characteristics that were not used simply faded away. Lamarck also maintained that acquired characteristics wereinherited from one generation to the next. In other words, Lamarck believed that if a giraffe continuously stretched its neck to reach for food, it woulddevelop a longer neck. And the longer neck would be passed on to the next giraffe generation. Although his belief that acquired characteristics were inherited was incorrect, Lamarck was on the right track. He implied that traits can be inherited from generation to generation--that species undergo long-termevolutionary changes.

In 1859, Charles Darwin published his landmark book The Origin of Species, in which he outlined his theory of evolution through natural selection.Darwin believed that members of a particular species have slightly differentcharacteristics. In the competition for space, food, and shelter, some of these characteristics would make a particular plant or animal better able to survive and produce offspring than others of its species. Therefore, these advantageous characteristics would persist in future generations, while those lessadvantageous ones would disappear as their carriers died out. After centuries or millennia of competition or natural selection, recent members of a species might be quite different from their ancestors. This theory gained advocates like the revered English physician Thomas Huxley (1825-1895), who, as "Darwin's Bulldog," did more than anyone else to overcome opposition to Darwiniantheory. But even with all the support, Darwin's theory still lacked an explanation for how the differences in species occurred.

Darwin, realizing that he needed to explain the mechanics of variation, asserted that tiny particles floating in an individual's bloodstream entered the eggs and sperm to determine hereditary characteristics. But Francis Galton proved him wrong with a simple blood transfusion experiment between two different types of rabbits. The transfusion didn't change the offspring of the rabbits as it should have if Darwin were correct.

In 1884, August Weissmann proposed that a special hereditary substance existed in the egg nucleus, which he termed "germ plasm." His theories concerning the behavior of this substance--later identified as chromosomes--were eventually proved correct. However, he mistakenly believed that the germ plasm passedintact from generation to generation, unchanged by any environmental factors. Weissmann's theory, therefore, could not adequately account for the changesthat occurred between generations and drove Darwin's theory of evolution.

It wasn't until 1900 that the second important theory concerning heredity wasdiscovered, although it had been formulated some forty-five years earlier. Gregor Mendel, an Austrian monk, had begun experimenting with pea plants at about the same time that Darwin set forth his ideas on natural selection. Through his efforts, Mendel demonstrated that actual physical "hereditary factors"could be transmitted independently. Mendel ultimately established the basiclaws of heredity--the missing key to Darwin's natural selection theory--and set the standard for the field of genetics. His revolutionary theories, however, were met with disinterest during his lifetime and remained largely unknownuntil 1900, when they were independently rediscovered by Hugo de Vries, KarlCorrens (1864-1933), and Erich Tschermak (1871-1962).

De Vries took Mendel's theories further. Unlike the Austrian monk, he believed that variations, rather than arising from gradual or transitional steps, occurred in jumps he called mutations. This formed the cornerstone of de Vries's mutation theory, which he proposed in 1901.

Despite these theories, no biological mechanism for heredity had yet been found. Walther Flemming had discovered chromosomes during the 1870s but, unawareof Mendel's work, did not understand their genetic significance. In 1903, ayoung graduate student, Walter S. Sutton, at last made the connection. He hadobserved that during cell division in regular cells, chromosomes were present in pairs. But in the cell division of reproductive cells, only one member of each pair entered a sperm or egg. The chromosomes became pairs again when the egg joined the sperm in the fertilization process. Sutton saw that this pairing, unpairing, and pairing again paralleled the movement of Mendel's "hereditary units." independently came to the same conclusion, and together theirhypothesis came to be known as the chromosomal theory of inheritance.

By 1909, when Wilhelm Johannsen coined the term gene to describe the "hereditary units" on the chromosome, Mendelian theory and chromosomal theoryhad been widely accepted by scientists. American geneticist Thomas Hunt Morgan, however, remained unconvinced and set out to empirically prove or disproveMendel's theory of inheritance. Following his many experiments with the fruit fly, Morgan was won over, convinced that genes were the trait-determiners and that they are arranged in a certain order on each chromosome. He also noticed that all the genes on the same chromosome were usually inherited together. Morgan referred to these as linked genes.

Further experiments showed that traits did not always follow Mendel's basic laws of heredity. Morgan showed that offspring don't always inherit all of thegenes on a chromosome. He called this occurrence crossing over. By 1915, Morgan, along with Hermann Muller, Calvin Bridges (1889-1938), and AlfredSturtevant (1891-1970), had fully developed the concepts of linkage and crossing over.

Yet some still refused to acknowledge the great strides made by biologists. The Ukrainian biologist Trofirm Denisovich Lysenko (1898-1976) gained controlof Soviet biological research between 1928 and 1965, and, with the backing ofJoseph Stalin (1879-1953), imposed his erroneous view that acquired characteristics could be inherited. Although his influence waned with the rise of Nikita Khrushchev (1894-1971), Lysenko severely damaged the Soviet Union's reputation in the international scientific community. His legacy would not be erased until the launching of Sputnik I in 1957.

By 1953, James Watson and Francis Crick had developed a model of deoxyribonucleic acid (DNA), the building blocks of genes, thus deciphering the genetic code and providing a key to the chemical basis of heredity. In recent decades,most research on heredity has focused on the function of DNA, its regulatoryprocesses, and its evolution.

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