Final answer:
The deletion of the locus control region for the β-globin gene cluster leads to absent expression of all the hemoglobin genes within the cluster, causing beta-thalassemia, a condition marked by severe anemia.
Step-by-step explanation:
The effect of deletion of the locus control region for the β-globin gene cluster is C. Absent expression of all of the hemoglobin genes in the cluster. The locus control region is essential for the chromosome to open up from its compact structure, making the genes accessible for transcription. Deletion of this region would mean that the necessary hemoglobin subunits are not produced, as gene expression would be impaired. This would lead to beta-thalassemia, characterized by a reduced synthesis rate of the β-chain of globin and severe anemia due to the low levels of hemoglobin and the reduced lifespan of red blood cells.