Indirect evidence for a mutation by RFLP analysis

Indirectly, a mutation can be demonstrated if
there is an individual difference (polymorphism)
in the base sequences of the mutant and
the normal gene segments (restriction fragment
length polymorphism, RFLP, see p. 64). For
instance, if two of the same DNA segments
differ in a polymorphism for the recognition
sequence of a restriction enzyme, then DNA
fragments of different sizes (here, either 7 kb
and 6 kb, or 13 kb) result after cleavage with the
enzyme (1). If the mutation has occurredwithin
the 13 kb fragment, then this fragment indicates
presence of the mutation. In the given
DNA segment, there are three possibilities
(genotypes): two fragments of 7 kb without
mutation; one fragment of 7 kb (normal) and
one fragment of 13 kb (which carries the mutation);
and two mutation-carrying fragments of
13 kb (3). The Southern blot (4) shows whether
the person being examined is homozygous normal
(has no 13 kb fragment), is heterozygous (a
7 kb and a 13 kb fragment), or is homozygous
for the mutation (two 13 kb fragments).
The prerequisite for this indirect analysis is previous
knowledge of which of the DNA fragments
contains the mutation. The observed
difference is not the result of the mutation, as in
A. If the Southern blot pattern of affected and
unaffected individuals does not differ, then this
method will not be informative for the disorder.