Overview: What Is the Chromosome Theory?
The results of Mendel’s experiments on cross-breeding pea plants led to principles of heredity, but he wasn’t able to observe either chromosomes or genes in cells. Microscopes in his time weren’t strong enough to detect those features. In the late 1800’s and early 1900’s, scientists observed the processes of meiosis and mitosis within the cell, and chromosomes were discovered. A scientist named Sutton connected his observations of cellular reproduction with Mendel’s theory, and proposed that genetic traits were carried on chromosomes. The chromosomes separated during mitosis and meiosis, ensuring that the next generation would receive genes from each parent.
What Are Genes?
Organisms have many more traits that are inherited than they have chromosomes, however. Sutton observed grasshoppers to develop the chromosome theory, but they only have 12 pairs of chromosomes. Therefore, there must be something on the chromosomes themselves to carry traits. Later, genes were observed on chromosomes. Genetic mapping — the connection between which genes carry which traits — is now a constant part of scientific discovery.
How Do Fruit Flies Contribute to Genetics?
Experiments during most of the twentieth century were done using common fruit flies. They are so small that hundreds of them can live in one jar, so they are relatively inexpensive. Because one generation of fruit flies only lives a few days, they multiply quickly enough that several generations can be observed during a few weeks.
How Are Sex Chromosomes Different From Each Other?
A scientist named Morgan determined that the fruit fly only had 4 pairs of chromosomes, and observed that one pair of chromosomes differed in males and females. Female fruit flies had a pair of large chromosomes, but males had one large chromosome and one that was smaller and shaped like a hook. He called the rod-shaped chromosome the X chromosome and the hook-shaped chromosome the Y chromosome. During meiosis, the sex chromosomes separate, so that the egg cells have only X chromosomes, and the sperm cells have either an X chromosome or a Y chromosome. If the resulting zygote is XX type, with one X chromosome from each parent, it will be female. If it is XY, it will be male.
What Are Sex-Linked Traits?
Morgan also observed that if he crossed a red-eyed female fruit fly with a white-eyed male fruit fly, all fruit flies in the F1 generation had red eyes, which would be expected for a dominant trait. The F2 generation had three red-eyed flies to one white-eyed fly, which would also be expected for a recessive allele. However, all the white-eyed flies were male. He theorized that since white eyes were recessive, the gene must only be carried on the X chromosome. For the male fruit fly to have white eyes it would have to inherit one X chromosome with the recessive trait, and one Y chromosome with no trait. Since the female fruit fly inherited two X chromosomes, and red eyes were dominant, either an RR genotype or an Rr genotype would produce a red-eyed female.
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