(d): Linkage is the phenomenon ofcertain genes staying together during inheritance through generations without any change or separation due to their being present on the same chromosome. Linked genes occur in the same chromosome. Strength of the linkage between two genes is inversely proportional to the distance between the two $i.e.$, two linked genes show higher frequency of crossing over (recombination) if the distance between them is higher and lower frequency if the distance is small.