Just like with any type of map, a genetic one should illustrate the exact positions of distinctive features. Well, when dealing with a geographic map, these markers appear to be recognizable components of the landscape, including roads, rivers as well as buildings. So, a question arises, what markers can we employ in a genetic landscape?
The first genetic maps, worked out in the early decades of the 20th century for such organisms as the fruit fly, employed genes as markers. It took a lot of time before it was realized that genes happen to be segments of DNA molecules. Instead, genes were considered to be abstract entities solely responsible for the transmission of heritable features from parent to offspring.
In order to be useful in genetic analysis, a heritable characteristic needs to exist in two or more alternative forms or phenotypes. Every phenotype is specified by a different allele of the definite gene. To start with, the only genes that could be explored were those, which specify phenotypes distinguishable by visual examination. For instance, the first fruit-fly maps demonstrated the positions of genes for eye color, body color, wing shape, etc, all of these phenotypes being visible by looking at the flies with a low-power microscope or even the naked eye.
The given approach turned to be fine in the early days, though geneticists soon understood that there were only a restricted number of visual phenotypes whose inheritance could be explored, and in many cases their analysis was quite complicated due to the fact that a single phenotype could be impacted by more than one gene. For instance, by 1922 more than 50 genes had been mapped onto the four fruit-fly chromosomes, though nine of these happened to be for eye color. Then in later research, geneticists exploring fruit flies had to learn to distinguish between fly eyes colored red, vermilion, garnet, light red, carnation, ruby, sepia, cinnabar, scarlet, cardinal, claret, pink, purple or brown. In order to make gene maps more comprehensive it would be necessary to spot characteristics which were more distinctive and also less complex than visual ones.
The solution was to utilize biochemistry to distinguish phenotypes. It has been particularly crucial with two types of organisms, which are humans and microbes. Microbes, such as bacteria and yeast, boast have v few visual characteristics, therefore gene mapping with these organisms is bound to rely on biochemical phenotypes.
Just like with any type of map, a genetic one should illustrate the exact positions of distinctive features. Well, when dealing with a geographic map, these markers appear to be recognizable components of the landscape, including roads, rivers as well as buildings.