You have two cats. The male has a dominant (SS) gene for short hairs, while the female has resessive genes (ss) for short hairs and has long hairs. As we know, the resessive genes are only expressed in the presence of other recessive genes. As the kitten of these two cats will receive an allele from each other it is impossible for the kitten to have long hair (ss), because when given an allele from each parent, this kitten will be heterozygous (Ss) and as the dominant gene is expressed in heterozygosis , the kitten has a 100% chance of owning short hair.
The chromosome theory of inheritance, or the idea that genes are located on chromosomes, was proposed based on experiments by Thomas Hunt Morgan using Drosophila melanogaster, or fruit flies.
