Definition and History of Cytogenetics

Cytogenetics is a science concerned with the structure, number, function, and movement of chromosomes and the numerous variations of these properties as they relate to the transmission, recombination and expression of the genes.

History of Cytogenetics

Cytogenetics was developed from two originally separate sciences – cytology and genetics. To fully understand the development of cytogenetics as a discipline, one has to look into its history.

The scientists chosen to be featured in this historical consideration made significant contributions to these sciences, and in this respect represent milestones. Many other significant contributions were made by other men all of whom could not be mentioned in this account.

Johannes Sachariassen and Zacharias (1588-1631). Jan ssen – Two Dutch eyeglass makers, father and son, between the years 1591 and 1608 produced the first operational compound microscope.

They combined two double convex lenses in a tube. The magnification was not more than ten times, but it nevertheless caused great excitement.

Robert Hooke (1635-1703)

An architect as well as a microscopist and the first curator of the Royal Society of London, in 1665 described cork and other cells and introduced the term cell. H is was the first drawing ever made of cells.

Microscopes at that time magnified 100 to 200 times with a distortion of shape and color that increased with magnification. Nevertheless, these microscopes revealed many new things.

Still, it was necessary to wait for better lenses to see anything more. Scientists waited for 160 years, and during this period they, naturally, argued about what they had seen.

Joseph Gottlieb Kolreuter (1733-1806)

German information about hybrids between plant varieties that might resemble one parent or the other or present a combination of their features. Camerarius was the first to experiment in this field.

For a number of years Kolreuter crossed different types of tobacco with one another. Later, he crossed other plant genera such as pinks, Aquilegia, Verbascum, and others.

One of his most valuable observations on reciprocal crosses showed the equality of contributions from the two parents. Thus, he provided clear evident that in reciprocal crosses, the hereditary contribution of the two parents to their offspring was equal.

Robert Brown (1773-1858)

A Scottish botanist, in 1828 discovered the cell nucleus in the flowering plant Tradescantia. Although he practiced medicine as a surgeon for five years, he later abandoned this and turned his efforts toward botanical sciences.

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He was liberation to the Linnaean Society and curator at the British Museum. His remarkable account (1828) of the properties and behavior of the nucleus stand unmodified and without correction. He was a very skillful and careful observer. He also observed the random thermal motion of small particles, still known as Brownian movement.

Wilhelm August Oscar Hertwig (1849-1922)

A Professor of anatomy, in 1876 and 1877 studied reproduction in the sea urchin, Paracentrotus lividus, and concluded that fertilization involves the union of sperm and egg. This study initiated the period of experimental cytology.

Walter Flemming (1843-1915)

An Austrian cytologist, in 1882 proposed the term mitosis. He showed that the chromosomes split longitudinally during nuclear division and the formation of daughter nuclei. He also applied the name chromatin to the stainable portion of the nucleus. He was a distinguished observer, technician, and teacher.

August Weismann (1834-1914)

A German biologist, in his essays of 1883 and 1885 put forth his germplasm theory, which was an alternative explanation to Lamarck’s theory of acquired characteristics.

Weismann speculated that the chromosomes of the sex cells were the carriers of his germsplasm, but he erred in assuming that each chromosome could contain all hereditary material.

He also postulated that a periodic reduction in chromosome number must occur in all sexual organisms and that during fertilization a new combination of chromosomes and hereditary factors takes place.

His theory was that the alternation of reduction and fertilization is necessary for maintaining constant chromosome numbers for sexual reproduction. At that time this process had not been observed under the microscope, and its mechanism was a matter of speculation.

Wilhelm Roux (1850-1924)

A German zoologist, in 1883 proposed that it was the chromosomes that contain the units of heredity. He speculated on the question of how the hereditary units could behave in such a way that each daughter cell receives all that is in the parent cell and becomes a complete cell and not half a cell or only part of a parent cell.

The most likely constituents of the nucleus to fill these requirements were the chromosomes. His hypothesis was that not only the chromosomes but individual parts of each chromosome were important in determining the individual’s development, physiology, and morphology.

Proof of this hypothesis was not given until later. This was in direct contrast to Weismann’s idea, that each chromosome could contain all hereditary material.

Edouard van Beneden (1845-1910)

He showed that in the round worm, Ascarismegalocephala, the number of chromosomes in the games is half the number that is in the body cells, and that in fertilization, the chromosome contributions of egg and sperm to the zygote are numerically equal. Through this observation he confirmed Weismann’s theory on reduction and fertilization.

Edward Strasburger (1844-1912)

Strasburger demonstrated that the principles of ferlization developed by Oscar Hertwig for animals held also for plants.

Strasburger made reciprocal crosses between different plant species and found that the results were similar. Since the egg and sperm were unequal with respect to size and amount of cytoplasm carried, he suggested that the cytoplasm was not responsible for hereditary differences between species.

Consequently, he came to the conclusion that the nucleus and its chromosomes are the material basis of hereditary and, at the same time, the material governing development.

Theodor Boveri (1862-1915)

By shaking sea urchin eggs at a critical time in their development, he produced some eggs without nuclei and some with nuclei as usual. Each of these kinds of eggs were fertilized by a normal sperm from another species of sea urchin.

Eggs lacking a nucleus produced larvae resembling the species from which the sperms were obtained, but those with nuclei developed into hybrids, showing the characteristics of both species.

The cytoplasm in the two kinds of eggs had not been altered and it was therefore presumed that the nucleus and not the cytoplasm was responsible for the transmission not hereditary traits.

With his experiments on the double fertilization of sea urchin eggs, Toxopneustes(1902, 1904, 1907), Boveri also contributed to the formulation of the chromosome theory of inheritance,which will be discussed later.

Edmund Beecher Wilson (1856-1939)

The beginning of cytogenetics and of the chromosome theory of inheritance were clearly outlined by Wilson’s statement that the visible chromomeres on the chromosomes were in all probability much larger than the ultimate dividing unitsand that these units must be capable of assimilation, growth, and division without loss of their specific characteristics.

Walter S. Sutton

He showed the significance of reduction division and proposed the chromosome theory of heredity. He independently recognized a parallelism between the behavior of chromosomes and the Mendelian segregation of genes.

The first paper (1902) contained the earliest detailed demonstration that the somatic chromosomes of the lubber grasshopper, Brachystola magna, occur in definite distinshably different pairs of like chromosomes.

He knew of Boveri’s first paper (1902) on dispermic eggs). His 1903 paper contains a full elaboration of his hypothesis, including the view that the different chromosome pairs orient at random on the meiotic spindles, thus accounting for the independent segregation of separate pairs of genes seen by Mendel.

This cytological basis for genetics theory is also often called the Sutton-Bovgeri theory of chromosomal inheritance. From then on cytology and genetics began to have strong effects on each other, and this is generally considered the birth of cytogenetics.

Thomas Hunt Morgan (1866-1945)

He discovered the mutant white eye and consequently sex linkage in Drosophila. With this discovery, Drosophilagenetics had its beginning.

Morgan was concerned about the exceptions to Mendel’s second law of independent assortment.

This law implies that an organism cannot possess more gene pairs than the number of chromosomes in a haploid set, if it is granted that the genes are borne on chromosomes.

Within the first decade after the rediscovery of Mendelism, this logical consequence of the theory was sharply contracted by experience.

Cyril Dean Darlington (b.1903)

In an attempt to explain meiosis, he advanced the precocity theory. He assumed that the chromosomes have a tendency to be in a paired state at all times.

In mitosis this condition is met in that the chromosomes entering prophase are already double. According to this theory meiotic prophase is assumed to start precociously with chromosomes that have not yet split, and this is held responsible for chromosome pairing.

Darlington said that the chromosomes are in an unsatisfied, or unsaturated, state electrostatically. To become saturated they must pair homologously.

When the chromosomes become double in late pachytene, the satisfied state is between system chromatids instead of homologus chromosomes.

The paired homologues consequently fall apart and diplotene is initiated. This theory was logically beautiful in superficially explaining the genetic implications of meiosis.

Emil Heitz (b.1892)

Together with Bauer discovered the importance of the giant chromosomesin the salivary gland cells of dipteran insect species as important objects in cytogenetic research.

These structures had been discovered prior to this in 1881, but had not been identified as chromosomes. They represent bundles of chromosome subunits or chromatids.

In 1928 and 1929 Heitz was the first to distinguish two types of chromatin, which he named euchromatin and heterochromatin. Euchromatin stains lightly or not at all in interphase and prophase, while heterochromatin stains darkly in these stages.

Heterochromatin is an extremely helpful marker for chromosome mapping in the pachytene stage of meiotic prophase. In 1931 Heitz showed a correlation between the number of nucleoli in the interphase nucleous and the number of a particular type of chromosome, now called the nucleolus organizer chromosome.

A study of these chromosomes indicated that the nucleolus is organized at a specific site on the chromosome.

Sol Spiegelman (b.1914)

In 1965 Spiegelman together with Ritossa showed that the genes producing the ribosomal RNA of Drosophila are located in the nucleolus organizer regions of the chromosomes.

It appears now that the precursor material or ribosomal RNA is manufactured by the nucleolus, or organizer, and is then transferred to the nucleolus for final assembly into ribosomes. These findings are in line with recent research that indicates that living organisms cannot exist without nucleolar organizer chromosomes.

In summary, several contributors play significant roles in the development of cytogenetics as a discipline. Cytogenetics is a union of cytology and genetics. Several contributors play significant roles in the development of cytogenetics as a discipline.

It is defined as a science concerned with the structure, number, function, and movement of chromosomes and the numerous variations of these properties as they relate to the transmission, recombination and expression of the genes.

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