Certain variables strongly influence the type of deficiency alleles observed at a locus, e.g., the size of the relevant gene, its chromosomal location, and whether deficiency alleles are under the influence of positive selection. In the case of the β-globin locus, the gene is small, the locus is autosomal in location, and β-thalassemia alleles are under positive selection in regions endemic for malaria that are populated by 50% of mankind. Thus, not all deficiency alleles of β-globin may be seen because (1) individuals carry two copies of the gene and (2) selection may result in the oldest deficiency mutations attaining the highest gene frequency.
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Kazazian et al. (1986) studied this question.