Thomas Morgan expanded on Mendelís theories about 50 years later. Mendel proved that genes and alleles come in pairs. Morgan proved that these Mendel "factors" come on chromosomes. For his experiment, he used an organism called Drosophila Melanogaster, more commonly known as the fruit fly, instead of Mendelís pea plants. The fruit fly was a good organism to use because it breeds in about four days and Morgan was able to reproduce an entire community of fruit flies in about a week. Fruit flies lay hundreds of eggs every time they breed and Thomas Morgan could collect tons of data every couple days. Fruit flies have only 8 chromosomes, so if the chromosomes became important to Morganís experiments, the fruit flies would simplify things remarkably, and it would be easier to see mutations because of the multiple fruit flies in each generation.
In all generations of fruit flies, exactly half of them are male and half of them are female. The only genetic difference between the genders is the sex chromosomes. Fruit flies have three pairs (six) of homologous chromosomes (two of the same chromosome in each pair) which are the same between males and females. They also have one pair of sex chromosomes. A male fly has an X and a Y chromosome, and a female has two Xs. The gender of the offspring depends on the gamete of the male; if the gamete had an X in it, than the offspring will be female (because you will always get an X from a female), and if the gamete had a Y in it, than the offspring will be male. Letís assume that the maleís X chromosome codes for a trait in the phenotype. (The Y wouldnít have it because it is a totally different chromosome.) In the F1 generation (the generation after the parent's generation), only the daughters would get that trait, because the sons would get a Y from the father. But in the F2 generation (generation after F1), daughters and sons alike can inherit the trait, because the daughters from the F1 generation have the trait. But if the daughters have it on both of their X chromosomes, than every fruit fly will inherit it. This is important for when we discuss how Morgan conducted his experiments.
Morgan modeled all of his experiments after Mendel's experiments. He chose a few easily identifiable traits that appeared in fruit flies in pairs. The first set of alleles that Morgan chose to study was eye color. During his experiments, Morgan saw that some traits just "came out of nowhere," so to speak. He couldnít find a cause for these traits, so he referred to them with the term mutation. To Morgan, the most intriguing of the mutations was one that involved eye color. He had an entire strand of red-eyed fruit flies, and then suddenly, a white-eyed one showed up. He matched the white-eyed fly (genotype Xw Y) with a red-eyed one (genotype XW XW), where W is the red-eyed gene and w is the white-eyed gene, and X and Y represent the sex chromosomes. Morgan discovered that all offspring were red-eyed, so the red gene must be dominant. After this, Morgan mated the F1 generationís offspring. According to Mendel, there should be a 1:3 ratio for white- to red-eyed flies. Morganís experiment came out very close to that, but he noticed that none of the women had white eyes. He did more experiments to try to understand why this happened, and he finally reached a conclusion: the eye color gene was physically attached to the X chromosome. Because men only had one X, it is easier for the recessive trait, the white eyes, to appear in the phenotype. His experiment proved that one of Mendelís Laws, the Law of Independent Assortment, was wrong.
Morgan formed the Chromosome Theory of Heredity after his fruit fly experiment. He was proven correct in his assumptions that chromosomes contain many genes, and that some chromosomes are related to sex. He also proved that genes on one chromosome were inherited together, and genes on different chromosomes were not. He noticed that there were no exceptions to his general rule. He formulated three laws that go with his theory of heredity:
Morgan basically was the man who figured out what genes were, without knowing about DNA. He published many papers and books. Morgan was "the man who made it happen." Mendel had laid the groundwork for someone like Thomas Hunt Morgan to come in and set genetics in motion.Mendel
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