[5]
(a) Write the scientific name of the organism Thomas Hunt Morgan and his colleagues worked with for their experiments. Explain the correlation between linkage and recombination with respect to genes as studied by them.

(b) How did Sturtevant explain gene mapping while working with Morgan ?

(a) Drosophila melanogaster was the organism on which Thomas Hunt Morgan and his colleagues worked with for their experiments. [1]
[3]
(i) Thomas Hunt Morgan and his colleagues observed - that two genes located closely on a chromosome did not segregate independently of each other. The $F_2$ ratio deviates significantly from 9 : 3 : 3 : 1.
(ii) Tightly linked genes lesser recombination frequency of parental traits and higher frequency of parental types.
(iii)Loosely linked genes show higher percentage of recombinant frequency of parental traits and lower frequency percentage of parental type.
(iv) They concluded that the genes present on the same chromosome are said to be linked and the recombinant frequency depends on the relative distance on the chromosome.

[1]
(b) Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and mapped their positions on the chromosome while working with Morgan.