Mutations and phenotype

The location of a mutation in the gene influences
the phenotype. Generally, mutations
in the 3! region aremore serious than mutations
in the 5! region (position effect). Mutations of
the pro!1(I) chain are more severe than those
in the pro!2(I) chain (chain effect). The substitution
of a larger amino acid for glycine, which
is indispensable for the formation of the triple
helix, leads to severe disorders (size effect).
Different types of mutations may occur, such as
deletions, mutations in the promoter or enhancer,
and splicing mutations. The codons
(AAG, AAA) for the amino acid lysine, which occurs
frequently in collagen, are readily transformed
into a stop codon by substitution of the
first adenine by a thymine (TAG or TAA), so that
a short, unstable procollagen is formed.