4 Apr


1880 A. Laveran observes the malaria parasite in the red blood cells of a soldier who is suffering from malaria. This is the first case of a disease shown to be caused by a single-celled eukaryote.

1881 E. G. Balbiani discovers “cross-striped threads” within the salivary gland cells of Chironomus larvae. However, he does not realize that these are polytene chromosomes.

L. Pasteur develops the first artificially produced vaccine, against anthrax, a deadly disease of cattle, sheep, and humans.

1882 W. Flemming discovers lampbrush chromosomes and coins the term mitosis. R. Koch describes the microscopic structure of Mycobacterium tuberculosis, the bacterium he proves to be the cause of human tuberculosis.

1883  E. van Beneden studies meiosis in a species of the round worm, Ascaris, which (fortunately) has a diploid chromosome number of only four. He shows that the gametes contain half as many chromosomes as the somatic tissues and that the characteristic somatic number is reestablished at fertilization. He also de- scribes fertilization in mammals. R. Koch isolates Vibrio cholerae, the bacterium that causes cholera, and de- scribes its microscopic anatomy. W. Roux suggests that the filaments within the nucleus that stain with basic dyes are the bearers of the hereditary factors. A. Weismann points out the distinction in animals between the somatic cell line and the germ cells, stressing that only changes in germ cells are transmitted to further generations. A. F. W. Schimper proposes that chloroplasts are capable of division and that green plants owe their origin to a symbiotic relationship between chlorophyll- containing and colorless organisms.

1884  A. Kossel isolates basic proteins from the nuclei of goose erythrocytes and names them histones.

H. C. Gram devises a diagnostic staining procedure that allows bacteria to be assigned to Gram-negative or Gram-positive categories.

1887 A. Weismann postulates that a periodic reduction in chromosome number must occur in all sexual organisms. T. Boveri discovers chromatin diminution during the embryogenesis of parasitic nematodes and shows that the process is linked to the differentiation between germ cells and somatic cells.

1888 W. Waldeyer coins the word chromosome for the filaments referred to by Roux (1883). T. Boveri describes the centriole. E. Roux and A. E. J. Yersin demonstrate that the diphtheria bacillus produces a toxin.

1889 F. Galton publishes Natural Inheritance. In it he describes the quantitative mea- surement of metric traits in populations. He thus founds biometry and the sta- tistical study of variation.

1890 R. Altmann reports the presence of “bioblasts” within cells and concludes that they are “elementary organisms” that live as intracellular symbionts and carry out processes vital to their hosts. Later (1898), C. Benda names these organ- elles mitochondria.

E. von Behring shows that blood serum from previously immunized animals contains factors that are specifically lethal to the organisms used for the immu- nization. These factors are now called antibodies.

1891 J. W. Tutt describes industrial melanism in the British peppered moth and suggests that the phenomenon results from natural selection of those moths which carry color variations that make them inconspicuous to predators.

1892 D. I. Ivanovski demonstrates that the tobacco mosaic disease is caused by an agent that can pass through filters with pores too small for bacteria and can neither be seen with the light microscope nor grown upon bacteriological media.

1896 E. B. Wilson publishes the first edition of the Cell in Development and Heredity. This treatise distills the information gained concerning cytology in the half cen- tury since Schleiden and Schwann put forth the cell theory. The influential third edition published 30 years later is three times the size of the first.

1898 R. C. Ross shows from experiments done in India that Anopheles mosquitoes can transmit malaria to birds. M. W. Beijerinck shows that the tobacco mosaic disease agent multiplies only in host cells. He proposes that the agent is a molecule dissolved in the juice of the infected plant and endowed with the power of replication, but only with the help of mechanisms provided by the host cell. C. Golgi develops histological procedures that selectively stain axons and den- drites. During observations of nervous tissues, he discovers the organelle we now call the Golgi apparatus in his honor. S. G. Navashin discovers the double fertilization that occurs in seed-bearing plants and concludes that endosperm cells are triploid.

1899 G. Grassi shows from experiments done in Italy that the malaria vector in hu- mans is the Anopheles mosquito.

1900 H. de Vries in Amsterdam, Holland, and C. Correns in Tu¨bingen, Germany publish the results of breeding experiments that paralleled Mendel’s studies. They used Pisum sativum (and other plant species as well) and got the same F1 and F2 ratios. When de Vries published his results, he never mentioned Men- del. However, in his paper published later in 1900, Correns gave Mendel credit for his analysis done 35 years earlier.

Correns was the first to observe the 9:3: 3:1 ratio when analyzing the results of dihybrid crosses. In Great Britain, W. Bateson became Mendel’s chief apostle.

Over the next decade he had Mendel’s paper translated into English, and he invented the terms allelomorph, epistasis, genetics, heterozygote, homozygote, and Mendelism. He also introduced the nota- tion used in diagramming the generations in breeding experiments (P1, P2, P3, F1, F2, F3, etc.). J. Loeb demonstrates that frog and sea urchin eggs can begin parthenogenic development after mechanical stimulation. K. Pearson develops the chi-square test. K. Landsteiner discovers the blood-agglutination phenomenon in humans. P.

Ehrlich proposes that antigens and antibodies bind together because they have structural complementarity.

H. de Vries adopts the term mutation to describe sudden, spontaneous, drastic alterations in the hereditary material of Oenothera. T. H. Montgomery studies spermatogenesis in various species of Hemiptera. He concludes that maternal chromosomes only pair with paternal chromo- somes during meiosis. K. Landsteiner demonstrates that humans can be divided into three blood groups: A, B, and C.

The designation of the third group was later changed to O. I. I. Mechnikov reports observing white blood cells engulfing bacteria and thus founds the study of cellular immunity. E. von Behring wins the Nobel Prize for his studies on antiserum therapy.

1902 C. E. McClung notes that in various insect species, equal numbers of two types of spermatozoa are formed; one type contains an “accessory chromosome” and the other does not. He suggests that the extra chromosome is a sex determi- nant, and he next argues that sex must be determined at the time of fertiliza- tion, not just in insects, but perhaps in other species (including humans). T.

Boveri studies the development of haploid, diploid, and aneuploid sea urchin embryos. He finds that in order to develop normally, the organism must have a full set of chromosomes, and he concludes that the individual chromosomes must carry different essential hereditary determinants. W. S. Sutton advances the chromosome theory of heredity, which proposes that the independent assortment of gene pairs stems from the behavior of the synapsed chromosomes during meiosis. Since the direction of segregation of the homologs in a given bivalent is independent of those belonging to any other bivalent, the genes they contain will also be distributed independently. F. Hofmeister and E. Fischer propose that all proteins are formed by the con- densation of amino acids bound through regularly recurrent peptide linkages. R. C. Ross receives the Nobel Prize in Medicine for his studies of the transmis- sion of the malaria parasite by mosquitoes.

1903 W. Waldeyer defines centromeres as chromosome regions with which the spin- dle fibers become attached during mitosis.

1904 A. F. Blakeslee discovers heteromixis in fungi. A. Laveran publishes Les Trypanosomes et Trypanosomiasis, a great monograph which summarizes the current status of knowledge concerning diseases caused by flagellated protozoa.

1905 L. Cue´not performs crosses between mice carrying a gene that gives them yel- low fur. Since they always produce yellow furred and agouti offspring in a 2 : 1 ratio, he concludes they are heterozygous. W. E. Castle and C. C. Little show in 1910 that yellow homozygotes die in utero. This dominant allele in the agouti series (Ay) is thus the first gene shown to behave as a homozygous lethal. R. Koch receives the Nobel Prize in Physiology and Medicine for his researches on the etiology of tuberculosis.

1906   W. Bateson and R. C. Punnett report the first case of linkage (in the sweet pea).

1907 R. G. Harrison cultures fragments of the central nervous systems of frogs in hemolymph and observes the outgrowth of nerve fibers. In so doing, he invents tissue culture. E. F. Smith and C. O. Townsend show that a specific bacterium, Agrobacterium tumefaciens, is responsible for crown gall disease.

A. Laveran receives the Nobel Prize in Medicine for his contributions to the understanding of protozoan parasites that cause diseases, especially the sporo- zoans and trypanosomes responsible for malaria and African sleeping sickness, respectively.

1908 G. H. Hardy and W. Weinberg, working independently, formulate the so-called Hardy-Weinberg law of population genetics. I. I. Mechnikov and P. Ehrlich share the Nobel Prize in Physiology and Medi- cine for their contributions to immunology.

1909 G. H. Shull advocates the use of self-fertilized lines in production of commer- cial seed corn. The hybrid corn program that resulted created an abundance of foodstuffs worth billions of dollars. A. E. Garrod publishes Inborn Errors of Metabolism, the earliest discussion of the biochemical genetics of humans (or any other species).

He concludes that the metabolic disease alkaptonuria is a “rare recessive character in the Mende- lian sense.” F. A. Janssens suggests that exchanges between nonsister chromatids produce chiasmata. C. C. Little initiates a breeding program that produces the first inbred strain of mice (the strain now called DBA). W. Johannsen’s studies of the inheritance of seed size in self-fertilized lines of beans leads him to realize the necessity of distinguishing between the appear- ance of an organism and its genetic constitution. He invents the terms phenotype and genotype to serve this purpose, and he also coins the word gene. C.

Correns and E. Bauer study the inheritance of chloroplast defects in varie- gated plants, such as Mirabilis jalapa and Pelargonium zonale. They find that the inability to form healthy chloroplasts is in some cases inherited in a non- Mendelian fashion. H. Nilsson Ehle puts forward the multiple-factor hypothesis to explain the quantitative inheritance of seed-coat color in wheat. P.

Ehrlich leads a team of organic chemists who synthesize Salvarsan, the first drug to successfully treat spirochaete infections.

1910 T. H. Morgan discovers white eye and consequently sex linkage in Drosophila. Drosophila genetics begins. W. Weinberg develops the methods used for correcting expectations for Men- delian segregation from human pedigree data under different kinds of ascertain- ment applied to data from small families. C. Mereschkowsky proposes that certain of the organelles found in cells began as symbionts, and he coins the term symbiogenesis to describe this process. P. Rous shows that injection of a cell-free filtrate from chicken sarcomas in- duces new sarcomas in recipient chickens.

1911 T. H. Morgan proposes that the genes for white eyes, yellow body, and minia- ture wings in Drosophila are linked together on the X chromosome. W. R. B. Robertson points out that a metacentric chromosome in one orthop- teran species may correspond to two acrocentrics in another and concludes that during evolution metacentrics may arise by the fusion of acrocentrics. Whole- arm fusions are called Robertsonian translocations in his honor.

1912   A. Wegener proposes the continental drift concept.

F. Rambousek suggests that the “cross-striped threads” within the salivary gland cells of fly maggots are chromosomes. T. H. Morgan demonstrates that crossing over does not take place in the male of Drosophila melanogaster. He also discovers the first sex-linked lethal.

1913 Y. Tanaka reports that crossing over does not take place in the female of Bom- byx mori. In this species, the female is the heterogametic sex. E. M. East and R. A. Emerson use a multiple factor hypothesis to explain the inheritance of ear length in Zea mays. W. H. Bragg and W. L. Bragg demonstrate that the analysis of x-ray diffraction patterns can be used to determine the three-dimensional atomic structure of crystals. A. H. Sturtevant provides the experimental basis for the linkage concept in Drosophila and produces the first genetic map.

1914 C. B. Bridges discovers meiotic nondisjunction in Drosophila. C. C. Little postulates that the acceptance or rejection of transplanted tumors in mice has a genetic basis. T. Boveri suggests that cancers arise as the result of the proliferation of a single cell that has a genetic imbalance due to errors in the number of chromosomes it received during mitosis.

1915 F. W. Twort isolates the first filterable bacterial virus. R. B. Goldschmidt coins the term intersex to describe the aberrant sexual types arising from crosses between certain different races of the gypsy moth, Lyman- tria dispar. J. B. S. Haldane, A. D. Sprunt, and N. M. Haldane describe the first example of linkage in vertebrates (mice). T. H. Morgan, A. H. Sturtevant, H. J. Muller, and C. B. Bridges publish The Mechanism of Mendelian Heredity, which summarizes the early work on Dro- sophila. C. B. Bridges discovers bithorax, the first homeotic mutation of Drosophila. W. H. Bragg (father) and W. L. Bragg (son) share the Nobel Prize in physics for initiating the study of x-ray crystallography.

1916   H. J. Muller discovers interference in Drosophila.

1917 F. d’Herelle discovers viruses that attack Salmonella typhimurium. He coins the term bacteriophage and develops methods for assaying virus titre. O. Winge calls attention to the important role of polyploidy in the evolution of angiosperms. C. B. Bridges discovers the first chromosome deficiency in Drosophila.

1918    H. J. Muller discovers the balanced lethal phenomenon in Drosophila.

1919 T. H. Morgan calls attention to the equality in Drosophila melanogaster between the number of linkage groups and the haploid number of chromosomes. C. B. Bridges discovers chromosomal duplications in Drosophila.

1920  A. F. Blakeslee, J. Belling, and M. E. Farnham describe trisomics in the Jimson weed, Datura stramonium.

F. G. Banting and C. H. Best isolate insulin and study its physiological proper- ties. H. J. Muller calls attention to the similarities between bacterial viruses and genes, and he predicts that phage studies will provide insights into the molecu- lar nature of genes.

C. B. Bridges reports the first monosomic (haplo-4) in Drosophila. L. V. Morgan discovers attached-X chromosomes in Drosophila. J. B. S. Haldane points out that the members of the heterogametic sex are often absent, rare, or sterile among the offspring of species hybrids.

A. F. Blakeslee, and three colleagues discover a haploid Datura. C. B. Bridges discovers chromosomal translocations in Drosophila. T. Svedberg builds the first ultracentrifuge. A. E. Boycott and C. Diver describe “delayed” Mendelian inheritance that con- trols the direction of the coiling of the shell in the snail Limnaea peregra.

A. H. Sturtevant suggests that the direction of coiling of the Limnaea shell is deter- mined by the character of the ooplasm, which is in turn controlled by the mother’s genotype.

The XX-XY type of sex determination is demonstrated for certain dioecious plants: for Elodea by J. K. Santos, for Rumex by H. Kihara and T. Ono, and for Humulus by O. Winge. R. Feulgen and H.

Rossenbeck describe the cytochemical test that is used for DNA localization and the determination of C values. H. Spemann and H. Mangold demonstrate that a living part of an embryo can exert a morphogenetic stimulus upon another part, bringing about its morpho- logical differentiation (embryonic induction). They thus discover and name the organizer. C. B. Bridges completes his cytogenetic analysis of the aneuploid offspring of triploid Drosophila, and he defines the ratios between the sex chromosomes and autosomes that control sexual phenotype.

A. H. Sturtevant proves the high reversion rate at the Bar locus in Drosophila is due to unequal crossing over. His analysis also uncovers the position effect phenomenon. F. Bernstein suggests that the A B O blood groups are determined by a series of allelic genes. T. H. Goodspeed and R. E.

Clausen produce an amphidiploid in Nicotiana. E. G. Anderson establishes that the centromere of the X chromosome of Dro- sophila is at the end opposite the locus of yellow. S. S. Chetverikov initiates the genetic analysis of wild populations of Dro- sophila. J. B. Sumner isolates the first enzyme in crystalline form and shows it (urease) to be a protein.

A. H. Sturtevant finds the first inversion in Drosophila. R. E. Clausen and T. H. Goodspeed describe the first analysis of monosomics in a plant (Nicotiana).

N. I. Vavilov publishes Origin and Geography of Cultivated Plants in which the center of origin hypothesis is developed. K. M. Bauer reports that the rejection of skin grafts does not occur when skin is transplanted from one monozygotic twin to another.

A. L. Du Toit concludes from the patterns of geological similarities between the east coast of South America and the west coast of South Africa that the two continents were once juxtaposed. These results provide the earliest evidence of continental drift. J. Belling proposes that interchanges between nonhomologous chromosomes result in ring formations at meiosis. J. B. S. Haldane suggests that the genes known to control certain coat colors in various rodents and carnivores may be evolutionarily homologous. J. Belling introduces the acetocarmine technique for staining chromosome squashes.

B. O. Dodge initiates genetic studies on Neurospora. G. D. Karpechenko generates Raphanobrassica, an allotetraploid hybrid of the radish, Raphanus sativa, and the cabbage, Brassica oleracea. H. J. Muller reports the artificial induction of mutations in Drosophila by x-rays. L. J. Stadler reports the artificial induction of mutations in maize, and demon- strates that the dose-frequency curve is linear. F.

Griffith shows that a transformation to virulence can be induced in pneumo- cocci by mixing heat-killed bacteria from virulent strains with living bacteria from non-virulent strains. This lays the foundation for the work of Avery, Mac- Leod, and McCarthy (1944). L. F. Randolph distinguishes supernumerary chromosomes from the normal chromosomes of the plant cell. He calls the normal ones “A chromosomes” and the supernumerary ones “B chromosomes.” E. Heitz coins the terms euchromatin and heterochromatin. A.

Fleming reports that a mold of genus Penicillium secretes a substance that prevents the growth of certain bacteria. He names this antibacterial substance penicillin. Karl Kohmann discovers ATP. P. A. Levene and E. S. London propose a tetranucleotide hypothesis for the structure of thymonucleic acid (DNA). C. D. Darlington suggests that chiasmata function to hold homologs together at meiotic metaphase I and so ensure that they pass to opposite poles at ana- phase I. R. C. Tryon demonstrates successful selection for rate of maze learning in the From 1930 to 1932, a group of books and papers is published that constitutes the mathematical foundation of population genetics (1930, by R. A. Fisher; 1931, S. Wright; 1932, J. B. S. Haldane). R. E. Cleland and A. F.

Blakeslee demonstrate that the peculiar patterns of the transmission of groups of genes in various Oenothera races result from a system of balanced lethal and reciprocal translocation complexes.

K. Landsteiner is awarded the Nobel Prize in physiology or medicine for his contributions to immunology. His elucidation of the ABO blood group system made successful blood transfusions possible. C. Stern and, independently, H. B. Creighton and B. McClintock provide the cytological proof of crossing over. C. D. Darlington suggests that chiasmata can move to the ends of bivalents without breakage of the chromosomes. This process of terminalization, as he called it, is now known to occur in some species but not in others. B. McClintock shows in maize that if a segment of a chromosome has become inverted, individuals heterozygous for such an inversion often show reversed pairing at pachynema.

A. L. Fox synthesizes phenylthiocarbamide and observes that it tastes bitter to some people and others cannot taste it at all. M. Knoll and E. Ruska invent the prototype of the modern electron micro- scope. T. S. Painter initiates cytogenetic studies on the salivary gland chromosomes of Drosophila. H. Hashimoto works out the chromosomal control of sex determination for Bombyx mori.

A. W. K. Tiselius reports the invention of an apparatus that permits the separa- tion of charged molecules by electrophoresis. B. McClintock demonstrates in maize that a single exchange within the inver- sion loop of a paracentric inversion heterozygote generates an acentric and a dicentric chromatid. T. H. Morgan receives a Nobel Prize for his development of the theory of the gene.

M. Schlesinger reports that certain bacteriophages are composed of DNA and protein. P. L’He´ritier and G. Teissier experimentally demonstrate the disappearance of a deleterious gene from populations of Drosophila melanogaster maintained in population cages for many generations. A. Følling discovers phenylketonuria, the first hereditary metabolic disorder shown to be responsible for mental retardation. H. Bauer postulates that the giant chromosomes of the salivary gland cells of fly larvae are polytene.

A. F. Blakeslee utilizes Datura aneuploids to expose the morphogenic effects of genes residing on specific chromosomes. B. McClintock shows that the nucleolus organizer of Zea mays can be split by a translocation and that each piece is capable of organizing a separate nucleolus. She thus sets the stage for the later demonstration (1965) that the genes for rRNA are present in multiple copies. J. B. S. Haldane is the first to calculate the spontaneous mutation frequency of a human gene. E. Klenk identifies the glycolipid that accumulates in the brain of patients with the Tay-Sachs disease as a ganglioside. F. Zernicke describes the principle of the phase microscope.

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