Demonstration of a point mutation from an altered restriction site

A restriction site may be altered by a mutation.
For example, a sickle cell mutation in codon 6 of
the " gene of hemoglobin (see p. 340) (1) causes
loss of a restriction site for the enzyme MstII
(CCTNAGG instead of CCTNTGG) because the A
(adenine) has been replaced by a T (thymine)
(2). The normal allele ("A) in this area produces
a 1.15 kb fragment after MstII digestion,
whereas the mutation eliminates the restriction
site in the middle so that a 1.35 kb fragment
results. The 1.35 kb fragment in the Southern
blot indicates (3) the presence of the sickle cell
mutation ("S). Thus, homozygous normal individuals
(AA), heterozygotes (AS), and homozygotes
for the sickle cell mutation (SS) can
be clearly distinguished; each of the three genotypes
can be precisely diagnosed.