Which of the following kinds of mutations is MOST likely to be a null loss-of-function?

Assume that a base-pair substitution mutation converts a DNA triplet (AAT)to another DNA triplet (AAA).A second mutation now changes the AAA triplet to the GAA triplet.(UUA and CUU code for leucine, and UUU codes for phenylalanine.)This second mutation is an example of a(n):

Some mutations cause new alleles that are helpful in a population, whereas some mutations can be deleterious, such as those that cause: