Thomas Hunt Morgan (1866-1945) establishes the chromosomal theory of heredity
Thomas Hunt Morgan, an embryologist who had turned to research in heredity, in 1907 began to extensively breed the common fruit fly, Drosophila melanogaster. He hoped to discover large-scale mutations that would represent the emergence of new species. As it turned out, Morgan confirmed Mendelian laws of inheritance and the hypothesis that genes are located on chromosomes. He thereby inaugurated classical experimental
After breeding millions of Drosophila in his laboratory at Columbia University, in 1910 Morgan noticed one fruit fly with a distinctive characteristic: white eyes instead of red. He isolated this specimen and mated it to an ordinary red-eyed fly. Although the first generation of 1,237 offspring was all red-eyed but for three, white-eyed flies appeared in larger numbers in the second generation. Surprisingly, all white-eyed flies were male.
These results were suggestive for hypotheses of which Morgan himself was skeptical. He was at the time critical of the Mendelian theory of inheritance, mistrusted aspects of chromosomal theory, and did not believe that Darwin's concept of natural selection could account for the emergence of new species. But Morgan's discoveries with white- and red-eyed flies led him to reconsider each of these hypotheses.
In particular, Morgan began to entertain the possibility that association of eye color and sex in fruit flies had a physical and mechanistic basis in the chromosomes. The shape of one ofDrosophila's four chromosome pairs was thought to be distinctive for sex determination. Males invariably possess the XY chromosome pair (Morgan used a more cumbersome notation) while flies with the XX chromosome are female. If the factor for eye color was located exclusively on the X chromosome, Morgan realized, Mendelian rules for inheritance of dominant and recessive traits could apply.
In brief, Morgan had discovered that eye color in Drosophilaexpressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait. But half the females from this union would now possess a copy of the white-eyed male's recessive X chromosome. This chromosome would be transmitted, on average, to one-half of second-generation offspring—one-half of which would be male. Thus, second-generation offspring would include one-quarter with white eyes—and all of these would be male.
Intensive work led Morgan to discover more mutant traits—some two dozen between 1911 and 1914. With evidence drawn from cytology he was able to refine Mendelian laws and combine them with the theory—first suggested by Theodor Boveri and Walter Sutton—that the chromosomes carry hereditary information. In 1915, Morgan and his colleagues published The Mechanism
• Discrete pairs of factors located on chromosomes like beads on a string bear hereditary information. These factors—Morgan would soon call them genes—segregate in germ cells and combine during reproduction, essentially as predicted by Mendelian laws. However:
• Certain characteristics are sex-linked—that is, occur together because they arise on the same chromosome that determines gender. More generally:
• Other characteristics are also sometimes associated because, as paired chromosomes separate during germ cell development, genes proximate to one another tend to remain together. But sometimes, as a mechanistic consequence of reproduction, this linkage between genes is broken, allowing for new combinations of traits.
Morgan's experimental and theoretical work inaugurated research in genetics and promoted a revolution in biology. Evidence he adduced from embryology and cell theory pointed the way toward a synthesis of genetics with evolutionary theory. Morgan himself explored aspects of these developments in later work, includingEvolution and Genetics published in 1925, and The Theory of the Gene in 1926. He received the Nobel Prize in Physiology or Medicine in 1933.
Mitosis or meiosis. I'm not sure of the answer. but if I had to guess I would say mitosis because from my understanding meiosis is only for reproductive organs...
You can live without a functional liver, but then you have to go through dialysis. And you can live even if you have an entire lung removed. The spleen can be removed if it's damaged and you'd still live. However, people without a spleen are more prone to infections. Also, women can have their uterus removed in a hysterectomy as a treatment for cancer, or other reasons. But of course, you'd need your heart, brain, at least one lung and one kidney to survive.
The K-Pg boundary is the boundary between the Cretaceous period and the Paleogene period, but also marking the end of the Mesozoic era and the beginning of the Cenozoic era. This boundary is dated to roughly 66 million years ago. The K-Pg boundary is characterized with a mass extinction. The dominant organisms on Earth all died out, including the dinosaurs, both terrestrial and aquatic, as well as the flying reptilians, with only the birds surviving from that super-family. The reason for the mass extinction has been an impact with an asteroid or a comet, that was very large and hit the Earth where the Yucatan Peninsula in modern day Mexico is located. The impact was devastating, with heat waves, tsunamis, earthquakes, triggering of the majority of the volcanoes, wildfires, all combined managed to devastate the environment, thus big portion of the species living in it as well. The lucky survivors were smaller organisms, and the mammals turned out to be the most successful in occupying the empty niches and have became the dominant organisms on the planet.
