Biology 11th Edition by Cecie Starr ,Ralph Taggart

Exercise 13

Occasionally, a few of the families in a remote region of Kentucky produce blue offspring , a condition caused by an autosomal recessive disorder. Skin of affected individuals appears dark blue. Homozygous individuals do not have the enzyme that maintains hemoglobin in its normal molecular form. Without it, a blue form of hemoglobin accumulates in blood and shows through the skin.
Formulate a hypothesis to explain the recurrence of the blue offspring trait among a cluster of families.