If one human disease is caused by a change in one codon in a gene from GAA to GUA, then this disease is the result of a mutation. Genes have mutated in such a way that only one codon in a gene is changed, so as to give rise to a certain disease in a human being. Crossing-over, meiosis error, and polypoloidy cannot produce such mistakes.
The answer is: a mutation. The mutation is the alteration of a nucleotide sequence in DNA. There are different types of mutations. If one or more nucleotides are substituted by one or more nucleotides, it is an example of substitution. In this example, GAA is changed to GUA as the result of a substitution of the second nucleotide A by U.Hope I helped! :) Cheers!