Where Are the Genes Located?

The term gene was coined by Wilhelm Johannsen (1857–1927), a Danish botanist, plant physiologist, and geneticist.

Darwin used the name pangenesis, in his book Origin of Species. In this hypothesis Darwin proposed that the body continuously emits particles, gemmules that accumulated in an organism’s reproductive glands and provide heritable information to the reproductive cells.

Twenty years later Hugo De Vries, a Dutch botanist shortened pangenesis to pangene. Seemingly, Johannsen liked the short version, gene. Gene in Danish is gen, which is even shorter.

Johannsen also coined the terms phenotype and genotype in his paper Om arvelighed i samfund og i rene linier (On heredity in society and in pure lines), published in 1903 and in his book entitled Arvelighedslærens Elementer, published in 1905.

An organim’s phenotype
is all the observable characteristics that result from the interaction of its genotype with the environment.
An organism's genotype

is the genetic makeup of an organism, which determines a part of an organism’s phenotype.

Are the Genes in the Chromosomes?

An experimentalist, Thomas Hunt Morgan (1866 – 1945) investigated thousands upon thousands of common fruit flies (Drosophila melanogaster) trying to understand inheritance. After breeding many generations of fruit flies, he noticed that some flies had white eyes. Subsequent breeding of these flies with ordinary red-eyed flies resulted in all having red eyes. None of them had white eyes, why?

Fruit fly Drosophila Melanogaster
Photo by Mr. Checker CC

Mendel had observed hidden characteristics earlier; Therefore, Morgan proceeded to breed a second generation. As Mendel had predicted, this second generation consisted of a mixture of white- and red-eyed flies. After careful examination, Morgan realized that all white-eyed flies were male.

What was the mechanism? Let’s step back in time a few years and see some of the major earlier discoveries, so we get a glimpse what Morgan knew at the time.

The connection between Mendel’s laws and the chromosomes

Among the first scientist in the middle of 19th century to observe structures in the cell nucleus that we today call chromosomes, were Matthias Jakob Schleiden, Rudolf Virchow, and Johann Adam Otto Bütschli.

As scientists rediscovered Mendel’s work in the early 20th century, German biologist Theodor Heinrich Boveri (1862 – 1915), who worked with sea urchins (Echinoidea), highlighted the connection between Mendel’s rules of inheritance and the segregation of chromosomes. Also Walter Stanborough Sutton (1877 – 1916), an American geneticist, independently developed a theory that it is possible to apply Mendelian laws of inheritance to chromosome segregation

Edmund Beecher Wilson combined the independent works of Heinrich Boveri and Walter Sutton in his famous book and named it Boveri–Sutton chromosome theory. The first edition was published 1896 and entitled The Cell in Development and Inheritance. The final edition was published in 1925 and dedicated to Theodor Boveri.

Thomas Morgan and others argued that there is not enough evidence to support the idea that chromosomes were the sole carriers of heritable traits. They strongly questioned this theory. This theory was later proved to be correct, in Morgan’s own lab (see below).

Two types of chromosomes, X and Y were already known to determine the gender of species. Females have two X chromosomes and no Y chromosomes, while males have one X and one Y chromosome.

Knowing all this, Morgan suspected the X chromosomes somehow to cause this phenomenon, since the male flies only have one X chromosome and females have two X chromosomes. Perhaps the gene present in one of the female X chromosomes, responsible for red eyes shadowed a white-eye producing gene, located in the other chromosome; thus, preventing females to develop white eyes. Further, he reasoned that the white-eyed males got a single X chromosome containing the gene, which produced white eyes. Morgan wanted to prove his theory; therefore, he bred thousands of fruit flies. A work that he did over a 17-year period at Columbia University. Importantly, in his further work Morgan also showed that the locations of genes are in linear order along the chromosomes.

The explanation is that the gene producing red eyes is dominant and the gene producing white eyes is recessive. Two recessive genes in females are required to produce white eyes. In Morgan's experiments, the females had one dominant and one recessive gene. Since males have only a single X chromosome containing a single eye color gene, this one gene produces an outcome of red or white eyes. Regardless, the gene being recessive or dominant one. In the generation, which had none of the white-eyed flies, all the male flies just happened to get the X chromosome containing the red-eye producing gene.

Thomas Morgan was awarded the Nobel Prize in Medicine or Physiology in 1933 “for his discoveries concerning the role played by the chromosome in heredity".

Morgan’s student, Alfred Sturtevant created the first map of locations of the fruit fly’s (Drosophila melanogaster) genes.

Today we know that the fruit fly X chromosome has around 2,300 protein-coding genes. In comparison, the human X chromosome has 3,000 to 5,000 genes. Many of these genes associated with diseases, such as hemophilia and red-green color blindness. In 2018, the database FlyBase (FB2018_02 release) contains 31,933 fruit fly gene records. Almost half of them, 17,753 are mapped in the genome.