Hi all
The idea of one gene for one trait, predates any understanding of genetics at the microscopic level. People recognized that some things were inherited, like eye color, and spun theories from there. 

W. Johannsen wrote in Amer. Naturalist 45 132  (1911) 
>  I have proposed the terms ‘gene’ and ‘genotype’..to be used in the science of genetics. The ‘gene’ is nothing but a very applicable little word, easily combined with others, and hence it may be useful as an expression for the ‘unit-factors’, ‘elements’ or ‘allelomorphs’ in the gametes, demonstrated by modern Mendelian researches.

We now know that the genetic code is a complicated hodge podge of instructions, some critical for survival and some apparently serving no function. In this way it resembles legacy computer programs where there are parts that nobody knows why they are there, but they *might* be critical. 

At one time there was the concept that a gene equals a protein. This led to the discovery of so called genes that can be inserted into organisms to produce proteins like the Bt toxin, or fluorescence. Most traits are multi-factorial however. Again, they are somewhat like computer programs with various modules, subroutines, and escape sequences. 

All the same, it is true that the substitution of one base pair (single nucleotide polymorphism or SNP for short) can lead to devastating effects. When I worked in the genomics lab we had a mouse line that carried such a SNP. If either the male or the female had it, the offspring appeared normal. If both parents had it, the resultant homozygous animal would be born with a wide range of birth defects, including sterility.

PLB

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