This animation represents an experiment of monohybrid crossing carried out by the geneticist Thomas H. Morgan.
In this experiment, Morgan looks at the characteristic of eye colour in a fly, the fruit fly. It is composed of two lines "pure", which means all crosses in the same line give individuals with the same parental phenotype. One of these lines presents the "wildtype" phenotype with red eyes and the other with the "mutant" phenotype with white eyes.
Because of a rigorous statistical method over a large population of flies, Morgan studied the crossing of wildtype females with red eyes with mutant males with white eyes (parental generation P).
It states that 100% of F1 descendants present the wildtype phenotype. One of two parental characteristcs has disappeared. It deduces that the mutant allele (w) of the gene responsible for eye colour is recessive, and the wildtype allele (W⁺) is dominant.
In a second round, he crosses the individuals of the F1 generation and notices that in the F2 generation, the recessive characteristic reappears in 25% of the descendants. These results are confirmed with the laws established by Mendel, but there is a difference: The individuals of the F2 generation that present the recessive characteristic are all males.
Morgan deduced that the gene for eye colour is carried by the sex chromosomes in the drosophila and, more particularily, by the X chromosome. It is not present on the Y chromosome. Also in males, only hte X chromosome carried by the maternal gamete, determines eye colout. The mode of transmission of the gene for eye colour is linked to the sex.
In drosophila as in humans, the nuclei of cells contain a certain number of chromosomal pairs called autosomes (3 pairs in the drosophila and 22 in the human) and one pair of sex chromosomes. In the female, the pair of sex chromosomes is comprised of two X chromosomes whereas in the male, it is comprised of one X chromosome and one Y chromosome.