Compendium-Canaries, Volume 2

The traces of genes

The history of genetics

All the domestic animals we know today, and our useful plants have evolved from wild animal and plant species. Early humans hunted animals and gathered plants. Gradually, over the course of many millennia, these activities were replaced by the deliberate reproduction of animals and plants. People no longer had to go after the animals and plants.

The domestic dog is probably the oldest pet of mankind. Scientists estimate that the domestication of the wolf in Europe began about 25,000 years ago.1 A genetic calculation shows that the dog and the wolf separated as a species at least 135,000 years ago, so we must assume that the wolf was a companion of humans much longer than "dog".2

The diversity of today's dog breeds is due to the unconscious application of genetic laws. In many a litter there were puppies that differed slightly from their parents in shape or temperament. For example, individual dogs may have been more eager to hunt, better able to withstand cold or heat, or simply more beautiful in the eyes of humans. Depending on the value of these characteristics, these dogs were increasingly used for further breeding.

What began with the wolf was also attempted with other animals in later millennia. Slowly, step by step, individual breeds developed in this way from different animal and plant species. Without any science, based only on experience - which was certainly also passed on orally - desirable characteristics could be consolidated, and improved, and undesirable ones suppressed. This selection breeding has remained the predominant breeding method worldwide to this day.

Animal and plant breeding always causes artificial evolution, human-directed evolution!

Very often children resemble their parents in a striking way. For example, in their shape, in the colour of their eyes, in the shape of their nose or mouth. Who hasn't heard the saying when looking into the pram: "Just like your father". Perhaps the child also looks much more like its maternal grandfather than its natural father. In any case, the blueprint of these physical features seems to be passed on from ancestors to offspring.

Just as material goods are passed on - "inherited" - from parents to offspring, physical features and character traits seem to be passed on from one generation to the next.

First attempts at explanation

It has been handed down from antiquity that some scholars were already thinking about the laws of heredity at that time. The Greek philosopher ANAXAGORAS (499 to 428 B.C.) believed the daughter was already preformed in the sperm of the left testicle and the son in the sperm of the right testicle (preformation theory), and thus the paternal characteristics were passed on to the offspring. His compatriot ARISTOTELES (384 to 322 B.C.) thought similarly, but he already described those children resembled not only their parents but also their ancestors.

PLATON (428 to 348 B.C.) assumed that father and mother were equally involved in the transmission of characteristics.

The ancient views shaped natural philosophical considerations right up to modern times because the scientific instruments were lacking. Even after the first microscopes were constructed around 1600, the path to knowledge of the inheritance of physical characteristics was still long.

In 1677, ANTONI VAN LEEUWENHOEK (1632 to 1723) developed the microscope to new perfection and used it to discover unicellular organisms, bacteria, blood cells and sperm. But he too saw a complete organism already preformed in the sperm.

It was not until the embryological investigations in 1817 by CHRISTIAN HEINRICH PANDER (1794 to 1865) and the discovery of the egg cell in mammals in 1827 by KARL ERNST VON BAER (1792 to 1876) that the ancient views of heredity came to an end.

In 1831 ROBERT BROWN (1773 to 1858) discovered the nucleus in plant cells, which led THEODOR SCHWANN (1810 to 1882) and MATTHIAS JACOB SCHLEIDEN (1804 to 1881) to establish the cell theory of all living things. In 1857, the Swiss anatomist and physiologist RUDOLF ALBERT VON KÖLLIKER (1817 to 1905) described the mitochondria in muscle cells.

First milestones

JOHANN GREGOR MENDEL was born in 1822 in Heinzendorf near Odrau (Austrian Silesia - today the Czech Republic) as the son of a penniless farmer. Like many children of his time, he had to help his parents in the business from an early age. He thus learned how to graft fruit trees and breed bees. Due to his weak stature, he could not inherit his parents' farm. It was therefore decided that Johann Gregor should become a priest. In 1843 he entered the Abbey of St Thomas in Brünn (today Brno) and became a monk of the Augustinian Order.

From 1844 to 1848 he studied theology at the Brno Theological School and from 1851 to 1853 at the University of Vienna. There he worked with Professor CHRISTIAN DOPPLER (1803 to 1853), the discoverer of the Doppler effect, among others, and also occupied himself with mathematics, chemistry, zoology, botany, and palaeontology.

From 1854 onwards, Mendel began to study the different variants in plants in the monastery garden of Altbrünn Abbey. Over the next eight years he experimented mainly with peas, because the varieties of these pure-bred plants and their seeds could be clearly distinguished in seven different characteristics.

Systematic heredity experiments soon determined his work. He kept meticulous records of the number of plants bred and their appearance. Based on his records and numerical results, he recognised mathematical regularities that occurred from one generation to the next.

In 1865, he summarised his findings on crossbreeding experiments in three basic rules (Mendel's rules), which are still valid

Picture 1 The Augustinian Monk Johann Gregor Mendel

today. In doing so, Johann Gregor Mendel made himself a pioneer of modern hereditary science, which we now call "classical genetics".

In 1868, Mendel became abbot at his monastery and largely discontinued his experiments. Mendel's work and his rules of heredity were not recognised in his time. Even after his death in 1884, his findings did not receive any attention for the time being. His successor on the abbot's chair certainly contributed to this, as he had Mendel's entire estate burnt in the monastery courtyard. Apart from his published writings (including "Versuche über Pflanzenhybriden", published in the Verhandlungen des Naturwissenschaftlichen Vereins von Brünn)3 and a few letters to the botanist CARL WILHELM VON NÄGELI (1817 to 1891), no other documents have survived.

A student of von Nägeli, the German botanist and plant geneticist CARL CORRENS (1864 to 1933), received among other things Mendel's letters from Nägeli's estate around 1900 and recognised their importance. The botanist HUGO DE VRIES (1848 to 1935) and the botanist and geneticist ERICH TSCHERMAK-SEYSENEGG (1871 to 1962) are also regarded today as the rediscoverers of Mendel's rules. Through their own experiments, these three researchers were able to experimentally confirm Mendel's findings.

Since then, hardly a year has passed without researchers and scientists discovering new insights into the structure of plant and animal cells or into mechanisms of heredity. Many of these discoveries can be considered milestones in the history of genetics. Some important milestones are listed here.

Chromosomes are discovered

Shortly after the publication of Mendel's main work, JOHANNES FRIEDRICH MIESCHER (1844 to 1895) discovered nucleic acid in fish sperm and other biological material in 1869 and named it "nuclein" - derived from the Latin nucleus (kernel).

In 1888, the anatomist HEINRICH WILHELM WALDEYER (1836 to 1921) introduced the name "chromosomes" for the stainable nuclei.

EDUARD STRASBURGER (1844 to 1912) discovered the division of the plant nucleus and, together with THEODOR BOVERI (1862 to 1915), described the constancy of the number of chromosomes in different species. Boveri coined the term "centrosome" for the central body of the cell.

In 1902/04, Boveri and WALTER STANBOROUGH SUTTON (1877 to 1916) discovered that chromosomes occurring in pairs behave in exactly the same way as the hereditary factors described by Gregor Mendel and thus founded the chromosome theory of heredity.

The British geneticist WILLIAM BATESON (1861 to 1926) contributed significantly to the dissemination of Mendel's ideas and coined the term "genetics" in 1906, which was soon officially applied to the entire new branch of science.

The term "gene" (from Greek geneá = descent) was introduced in 1909 by the Danish botanist WILHELM LUDVIG JOHANNSEN (1857 to 1927). He used it to refer to the hypothetical "hereditary factors" discovered by Gregor Mendel. He also coined the terms "genome" (hereditary material) and "phenotype" (appearance) that are common today.

The US zoologist and geneticist THOMAS HUNT MORGAN (1866 to 1945) used the fruit fly (Drosophila melanogaster) for the first time in his crossbreeding experiments, and it has been the most frequently used experimental animal by geneticists ever since.

Morgan provided evidence that it is indeed in the chromosomes that hereditary traits (genes) are located and present there in a specific order and at specific intervals. He also found out that there are traits that are usually inherited together (linked genes) and are then located on the same...