/kloc-in the 0/9th century, especially in the second half of the century, cytology has made unprecedented development: A. weismann first put forward the viewpoint of studying heredity at the cellular level, categorically denied acquired inheritance and put forward the theory of germplasm continuity; O. Hertwig studied the fertilization of sea urchins and confirmed that animal cells will undergo meiosis during gamete formation. W Roux1883 put forward the viewpoint of equal distribution of genetic material; Sutton and Montgomery proved the continuity of chromosomes. Sutton pointed out: "The chromosomes of male and female parents are connected in pairs and separated from each other during meiosis, which is the material basis of Mendel's inheritance"; Boveri studied the fertilization of ascaris lumbricoides and confirmed the individuality of chromosomes. He believes that half the cells in a fertilized egg come from the father and half from the mother. This is as predicted by E.B.Willson: "Cells continue to occur through division, and genetic phenomena are the result of this continuity."
The chromosome hypothesis of these cytological results was not recognized and supported during the period of 1903 ~ 19 10, and many geneticists opposed it. Batson is one of them. The linkage phenomenon was first discovered by batson in the cross of 1906 sweet pea. Although chromosome theory has been put forward for three years, in the face of linkage phenomenon, he not only put forward the terms of exclusion and citation, but also "cherish your exception!" Nothing has been done except this famous saying.
Later, Morgan made a decisive contribution to chromosome theory.
Chromosome theory mainly comes from the observational facts of cytology. Morgan thinks this is "speculative" speculation, demands more direct evidence, and claims that he will never accept this theory without experimental basis.
In Morgan's view, admitting that that trait is controlled by "particles" and that particles are located on a single different chromosome is rooted in his evolutionary thinking background, which means denying his original conceptual system to some extent. Now, it can be considered that before 19 10, evolutionists, as embryologists, held a negative attitude towards the preformed theory and the particle theory insisted by cytologists. Although the debate between preforming theory and evolution theory obviously subsided 100 years ago, and the latter ended in victory, embryologists are too sensitive to any trace of preforming thinking.
Of course, the original particle genetic theory did have the shortcoming of simplification. It was not until 1909 that W. Johannsen put forward the concepts of genotype and phenotype. Prior to this, particle theorists had to discuss heredity from the prefabricated thinking of one-to-one correspondence between genetic factors and biological traits. That is to say, although the villain theory has been ignored for a long time, the trace that genotype is a reduced phenotype has not been shaken.
Why did Morgan quickly become the main advocate of chromosome theory and the provider of a series of powerful evidence? 1904, American cytologist Wilson invited him to Columbia University. As friends and colleagues, their offices are separated by a wall, and there are some students who study the chromosome inheritance of fruit flies and insects as the medium. Wilson's work here has deeply influenced Morgan.
1908, Morgan introduced Drosophila melanogaster and established Drosophila Room as genetic experimental material. 1910 A white-eyed male fly was found in May, and he immediately mated with a normal red-eyed female fly. Results F 1 was all red-eye, which indicated that white eyes were recessive mutations. In F _ 2 produced by sibling mating of F 1, the phenomenon that white eyes are confined to male fruit flies appears, which indicates that blinking is genetically related to sex. He backcrossed the white-eyed male fly with the red-eyed female fly F 1, and the results showed that the female fly could also have white eyes. Then cross the white-eyed female fly with the red-eyed male fly, and the phenomenon that the male fly is all white-eyed and the female fly is all red-eyed appears. He called this phenomenon the cross inheritance of mother elephant and father elephant.
The inheritance of traits varies with sex, which is a new phenomenon that Mendel's monoecious pea can't express, but it reminds Morgan of the research on sex chromosomes at the turn of the century.
189 1 year later, H. Henking found and named X chromosome containing heterochromatin in the testis cells of Hemiptera. 1902 (C.E. McClung) links chromosomes with insect sex; Wilson (1905) studied Hemiptera and Orthoptera, and thought that sex was determined by sex chromosomes, especially N.M.Stevens discovered the sex determination mechanism of Drosophila.
Using these cytological results, it can be concluded that the above genes are located on the X chromosome, but Morgan is still hesitant. The main reason is that in the face of complex experimental results, the evolutionary view still affects Morgan's thinking on prefabrication view which is not based on "villain and vitality". It is hard for him to believe that chromosome is a kind of "particle" that can fundamentally control everything.
The hybrid results of Drosophila Morgan contradict the sex determination mechanism of moths and birds studied in Britain. It turned out that this parallel opposition was just another evidence that the gene was located on the chromosome.
Drosophila has only eight chromosomes, and its individual traits are diverse. He is worried that many traits will be included in the same chromosome and will be "Mendelian" with it. The number of chromosomes varies greatly among species, which makes him think that chromosomes may not be a matrix that can control all traits.
Careful scientific thinking, precious skepticism! When further research made him wake up to these problems, the famous paper "Sex Restriction Inheritance in Drosophila" was born, which was the first time in genetics to associate a specific gene with a specific chromosome.
However, it is Morgan's collaborator Briggs' classic research that directly proves the chromosome theory. When he repeated the mating between the white-eyed female fly and the red-eyed male fly, he found the primary exceptional offspring of about 1/2000, that is, the "direct transmission of traits" of the female partial mother and the son partial father; Then the white-eyed female flies in the primary abnormality were mated with the normal red-eyed male flies, and the secondary abnormality with higher frequency (4%) was found. Briggs postulates that the white-eyed female fly with primary abnormality has XXY chromosome (that is, the X chromosome did not separate in the meiosis I stage of the female parent and entered the fertilized egg with * * *), and the four gametes formed by it combine with the two gametes of the normal parent, that is, eight offspring with different eyes, sex, fertility and vitality are formed, of which 4% are secondary abnormalities directly transmitted. Briggs also examined the sex chromosomes of these flies one by one, and the results were completely confirmed. This study intuitively and convincingly confirmed that chromosomes are the carriers of genes.
The birth of genetic chromosome theory is the crystallization of cytology and genetics, which declares the maturity of cytogenetics. As Miller said, "it laid the foundation for the whole modern genetics". Morgan also won the title in 1933, the anniversary of Nobel's birth 100. The Nobel Committee believes that "without Morgan's research, there would be no human genetics, and there would be no human eugenics". The rapid development of genetics has shown that this evaluation at that time was far from enough. As the scientific historian E. Mayr said in 1984, "The acceptance of inherited chromosome theory is by no means the end of chromosome research, but a sign of entering a new era of chromosome research".
Model essay on self-introduction in job interview 1
During my college years, besides my professional study, I also participated in various