● 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".

### HISTORY OF SEX DETERMINATION

● 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.