## Mutations can be deliberately induced

The work of Hermann Joseph Muller in 1927 represented an important milestone in the study of mutations. Muller was a member of the laboratory of the well-known geneticist Thomas Hunt Morgan, and his experiments showed that the frequency of mutations could be increased by the application of an external factor, in this case ionizing radiation. His experiments were of ground-breaking importance, because thanks to them it was possible to obtain new mutants necessary for studying the inheritance of various traits. The organism of choice was the fruit fly Drosophila melanogaster.

In the fruit fly, sex is determined by the presence of the sex chromosomes X and Y, with the female carrying two X chromosomes and the male carrying one X chromosome and one Y chromosome (XY). It should be noted that the Y chromosome in flies is not involved in sex determination, but in sperm formation, and is only found in male flies. Due to the balance of X chromosomes in sex determination, it is ensured that after crossing a female (XX) with a male (XY), there will be a 1:1 sex ratio in the offspring (Figure 5.1). Muller developed a simple method to determine the frequency of recessive lethal (incompatible with life) mutations associated with the sex chromosome X. A recessive mutation occurs only if there is no standard allele of the same gene (A) in the genotype, i.e., in males who have only one X chromosome with a recessive allele (a), because this gene is not present on the Y chromosome. If there is only one recessive lethal mutation in the genotype, the individual in question will not develop, and so the gender ratio will be different. The principle of Muller's experiment was that a recessive lethal mutation (a) on the X chromosome leads to a reduction in the proportion of viable males, i.e., there is a change in the sex ratio (Figure 5.1). The genius of this experiment is in its simplicity: to determine the frequency of recessive lethal mutations, it is sufficient to count the resulting offspring from a cross and determine if males are missing.

With his experiments, Muller drew attention to a fact that may seem trivial to us today, but was revolutionary at the time: in evolution, heredity and variability do not play a role separately, but their combination is essential - inherited variability. It is also important to note that Mueller's work described the basic properties of the gene in great detail, at a time when the chemical basis of genes and DNA were unknown.

Although the material that carried genetic information was not known at the time, scientists assumed that the macromolecule in which the genetic information is written must be extraordinarily stable in order to fulfil its function - to carry the information for the creation of an individual and to be passed on undamaged to the next generations. At the time when Muller conducted his experiments, it was already clear that genetic information was somehow linked to the chromosomes. However, it was not yet known which chemical compound in the chromosome - proteins or nucleic acid - would be the physical carrier of the genetic information. To scientists, nucleic acids appeared to be too chemically simple and not variable enough to store complex genetic information. It was therefore assumed that genetic information was found primarily in the structure of proteins, for which DNA forms only a scaffold. Until the 1940s, most scientists considered the protein portion of chromosomes to be the basis of heredity. This is because in contrast to the four nucleotides of DNA, proteins consist of up to 20 different amino acids, the combination of which offers far greater possibilities for storing genetic information. Convincing the scientific community of the importance of DNA as a carrier of hereditary information was difficult and lengthy for this very reason, and required much experimental evidence. However, the discovery of the DNA structure in the mid-20th century by Watson and Crick was of crucial importance for mutation research. Watson and Crick described the DNA molecule as a double helix and proposed a semiconservative method of replication based on specific base pairing that ensured the accurate transmission of genetic information from generation to generation. As already mentioned in Chapter 3, this discovery represented an important milestone in genetics and the scientists were jointly awarded the Nobel Prize in 1962.