The central dogma of molecular biology states that DNA is transcribed into RNA, which is then translated into protein.

The human genome can code for 64 different codons, but only produces 20 different amino acids. This results in some amino acids having multiple codes in order to use the remaining 44 available codons.

This ability of a single amino acid to have more than one possible codon is called "degeneracy." Occasionally when DNA is mutated, it will results in a codon that encodes the same amino acid. For example, a mutation from CUU in the mRNA sequence to CUG will still code for leucine.

DNA nucleotides are organized to form codes. When DNA is transcribed into RNA, these codes are read by the ribosomes to create proteins. Each gene refers to a sequence of DNA that codes for a specific protein. Mutation to a specific gene will affect that protein coded for by that sequence.

A locus is the location of a gene on a chromosome, and an allele is an alternative form of a given gene. A genotype is the description of the alleles for a specific set of genes.

Following the P generation is the 1st offspring generation, called F1. When the F1 generation is crossed, the result is the 2nd offspring generation, called F2.

G1 and G2 actually refer to periods of growth during the cell cycle, and are not related to genetics and inheritance.

A gene codes for a certain protein product, which is often associated with a certain trait. Each gene is found at a specific location, or locus, on a chromosome.

Alleles refer to different forms of DNA that can appear at the same locus. In other words, an allele is an alternative form of a given gene. Different alleles often result in different phenotypes, such as changes in color or size.

Eukaryotes possess coding (exon) and non-coding (intron) seqeunces that allow for incredibly long DNA sequences to correspond with relatively short peptides. Only the exons are translated after the mRNA sequence, which is transcribed fully from the DNA, undergoes mRNA splicing. Furthermore, while a codon (three nucleoties) does correspond with one amino acid, it does not account for the dramatic difference stated in the question. Also, the cell does not translate an entire unmodified mRNA sequence and cleave afterwards as that would be incredibly wasteful and potentially harmful to the cell. 

The reading frame of a mRNA is established by AUG (start codon) which codes for methionine. 

A phenotype refers to the observable traits within an individual such as eye color, adult height, weight, and skin color. The genetic contribution to a phenotype is referred to as genotype. Some phonotypical traits are determined by the genotype, while others are shaped by environmental factors.