● Morgan carried out several `color{Violet}"dihybrid crosses in Drosophila"` to study genes that were sex-linked.

● The crosses were similar to the dihybrid crosses carried out by `color{Violet}"Mendel in peas"`. For example Morgan `color{Violet}"hybridised"` yellow-bodied, white-eyed females to brown-bodied, red-eyed males and `color{Violet}"intercrossed"` their F1 progeny.

● He observed that the `color{Violet}"two genes"` did not `color{Violet}"segregate independently"` of each other and the F2 ratio `color{Violet}"deviated"` very significantly from the `color{Violet}"9:3:3:1 ratio"` (expected when the two genes are independent).

● Morgan and his group knew that the genes were located on the `color{Violet}"X chromosome"` and saw quickly that when the `color{Violet}"two genes"` in a dihybrid cross were situated on the `color{Violet}"same chromosome"`, the proportion of parental gene combinations were much `color{Violet}"higher"` than the non-parental type.

● Morgan attributed this due to the `color{Violet}"physical association or linkage"` of the two genes and coined the term `color{Brown}"linkage"` to describe this physical association of genes on a chromosome and the term `color{Violet}"recombination"` to describe the generation of `color{Violet}"non-parental"` gene combinations.

● Morgan and his group also found that even when genes were grouped on `color{Violet}"the same chromosome"`, some genes were very `color{Violet}"tightly linked"` (showed very `color{Violet}"low recombination"`).

● For example he found that the genes `color{Violet}"white and yellow"` were very tightly linked and showed only `color{Violet}"1.3 per cent recombination"` while `color{Violet}"white and miniature wing"` showed `color{Violet}"37.2 per cent"` recombination.

● His student, `color{Violet}"Alfred Sturtevant"`, used the `color{Violet}"frequency of recombination"` between gene pairs on the same chromosome as a measure of the `color{Violet}"distance between genes"`, and `color{Violet}"mapped"` their position on the chromosome.

● Today `color{Violet}"genetic maps"` are extensively used as a starting point in the sequencing of `color{Violet}"whole genomes"` as was done in the case of the `color{Violet}"Human Genome"` `color{Violet}"Sequencing Project"`.


● The mechanism of `color{Violet}"sex determination"` has always been a puzzle before the geneticists.

● The initial clue about the `color{Violet}"genetic/ chromosomal mechanism"` of sex determination can be traced back to some of the experiments carried out in `color{Violet}"insects"`.

● In fact, the `color{Violet}"cytological observations"` made in a number of insects led to the development of the concept of `color{Violet}"genetic/chromosomal"` basis of sex-determination.

● `color{Violet}"Henking"` (1891) could trace a specific nuclear structure all through `color{Violet}"spermatogenesis"` in a few insects, and it was also observed by him that `color{Violet}"50 per cent"` of the sperm received this structure after spermatogenesis, whereas the other 50 percent sperm did not receive it.

● Henking gave a name to this structure as the `color{Violet}"X body"` but he could not explain its significance.