In a cell, where do genes reside? You’ve probably already heard the punchline: genes are found on chromosomes. You might have heard the next punchline, which heralded the contemporary genetic era: genes are DNA sequences that dictate proteins. These weren’t always stuff you could find up on Khan Academy, though! Chromosomes had not yet been examined under a microscope when Gregor Mendel began investigating heredity in 1843. Cell scientists could only begin to stain & detect subcellular features during cell divisions in the late 1800s using improved microscopes and procedures (mitosis and meiosis). Some scientists eventually began to look at Mendel’s hard work & re-evaluate his concept in terms of chromosomal behaviour. The biology community began to make the first tentative linkages between chromosomes, meiosis, as well as gene inheritance around the turn of the twentieth century.
The chromosomal hypothesis of inheritance proposed by Boveri and Sutton claims that genes are found at specified positions on chromosomes inheritance and that chromosome activity during meiosis can explain Mendel’s rules of heredity. The first convincing proof of the chromosomal idea came from Thomas Hunt Morgan, who researched fruit flies. Morgan identified a mutation that changed the colour of the eyes of flies. He discovered that male and female flies inherited the mutation in distinct ways. Morgan deduced that the eye colour gene should be on the X chromosome based on the inheritance pattern.
The Chromosome Theory of Inheritance
Who discovered that genes are found on chromosomes? This insight is commonly credited to Walter Sutton and Theodor Boveri. Sutton, the American, examined grasshopper chromosomes & meiosis. Boveri, a German, looked into the same topics in sea urchins. Sutton and Boveri published separate studies in 1902 and 1903 suggesting what is now known as the chromosomal hypothesis of inheritance. Single genes are found at specific positions on certain chromosomes, according to this idea, and the action of chromosomes during meiosis helps explain why genes are passed down according to Mendel’s rules.
The following examples support the chromosomal hypothesis of inheritance:In an organism, chromosomes, like Mendel’s genes, are found in matched (homologous) pairs. One person in the relationship comes from either the mother and the other on the father for both genes and chromosomes. In meiosis, the components of a homologous pair split, and each sperm or egg gets just one of these. The separation of alleles into gametes in Mendel’s rule of segregation is mirrored in this process. Under meiosis, components of various chromosomal pairs are sorted towards gametes independently of each other, much like alleles of various genotypes are sorted into gametes separately of one another in Mendel’s rule of independent assortment. The chromosomal hypothesis of heredity was contentious at first because it was developed before there was any concrete proof that attributes were carried on chromosomes. In the end, scientist Thomas Hunt Morgan and his students, who researched the biology of fruit flies, were able to prove it.
- H. Morgan: Fruit Flies
Morgan focused his genetic research on the fruit fly Drosophila melanogaster. Fruit flies make up for their lack of appeal (depending on your taste in insects) with practicality: they’re inexpensive, easy, and quick to raise. Hundreds of them may be raised in a little container with sugary sludge just at bottom, as many evolutionary biologists still do today! Morgan’s essential chromosomal theory-validating investigations began when he discovered a mutation in a gene that affects the colour of a fly’s eye. The eyes of a fly were changed from red to white as a result of this mutation. Morgan discovered that male and female flies received the eye colour gene in distinct ways. Female flies have two X chromosomes, while male flies have an X and a Y chromosome (XY) (XX). Morgan quickly realised that the eye colour gene was passed in the same way as the X chromosome did. Morgan, a sceptic of the chromosomal idea, would have been surprised by this!
An Inheritance Pattern That Is “Sex Limited”
What led Morgan to believe that the X chromosome was home to the eye colour gene? Let’s take a look at some of his numbers. When he mated the first white-eyed fly he found with normal, red-eyed female flies, the F1 progeny were all red-eyed, indicating that the white gene was recessive. Thus far, all good; there have been no surprises. Whenever the F1 flies were mated, something unexpected happened: every one of the female F2 flies had red eyes, but around half of the male F2 flies developed white eyes. Male and female flies were clearly acquiring the characteristic in separate ways. In reality, they inherited it in the same way as a certain chromosome, the X, did. Let’s investigate whether X chromosome inheritance can explain what Morgan witnessed. Female flies do have XX genotype, whereas male flies have an XY genotype, as previously stated.
Important relevant question: In Morgan ‘s experiments on linkage, the percentage of white eyed miniature winged recompbinants in F(2) generation is?