Question

Give an example of a cross, showing complete linkage.

Answer 1

An example of complete linkage is the inheritance of the genes for grey body colour and long wings in male Drosophila melanogaster (fruit flies). Morgan (1920) demonstrated that when a male fly with grey body and long wings is crossed with a female having black body and vestigial wings, the F₁ hybrids resemble the male parent. If an F₁ male is test crossed with a double recessive female (black body, vestigial wings), instead of expecting four types of offspring in equal numbers (as would be the case with independent assortment), only two types of offspring appear, resembling the two grandparental phenotypes. This indicates that the genes for grey body and long wings are completely linked and inherited together without recombination.