Mobile genetic elements make up nearly half of the human genome and are inserted more or randomly

Question:

Mobile genetic elements make up nearly half of the human genome and are inserted more or randomly throughout it. However, in some spots these elements are rare, as illustrated for a cluster genes called HoxD, which lies on Chromosome 2 (Figure Q9–10). This cluster is about 100 kb in length and contains nine genes whose differential expression along the length of the developing embryo helps establish the basic body plan for humans (and other animals). In Figure Q9–10, lines that project upward indicate exons of known genes. Lines that project downward indicate mobile genetic elements; they are so numerous they merge into nearly a solid block outside the HoxD cluster. For comparison, an equivalent region of Chromosome 22 is shown.