Sickle-cell anemia is a genetic disorder in which red blood cells take on an abnormal crescent shape.

People who do not have sickle-cell anemia possess the following nucleotide and amino acid sequences:

CTG ACT CCT GAG GAG AAG TCT
Leu Thr Pro Glu Glu Lys Ser


People who do have sickle-cell anemia possess the following nucleotide and amino acid sequences:

CTG ACT CCT GTG GAG AAG TCT
Leu Thr Pro Val Glu Lys Ser


Sickle-cell anemia is an example of
--
A.
crossing-over.
B.
a gene mutation.
C.
nondisjunction.
D.
a chromosomal mutation.