Why did geneticists believe, even before direct experimental evidence was obtained, that the genetic code would turn out to be composed of triplet sequences and be non overlapping

Geneticists believe, even before direct experimental evidence was obtained, that the genetic code would turn out to be composed of triplet sequences and be non overlapping because a triplet code gives 64 possible combinations but we only have 20 essential amino acids in our human proteins so there's room for some redundancy. That way some point mutations will still be able to correctly code for the desired protein.

Explanation:

The Genetic Code would turn out to be composed of triplet sequences means that the -

Genetic Code is stored on one of the two strands of a DNA molecules as a linear, non-overlapping sequence of the nitrogenous bases Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). These are the of "letters of the alphabet,"  that are used to write the "code words."

The genetic code consists of a sequence of three letter "words," (sometimes called 'triplets', sometimes called 'codons'), written one after another along the length of the DNA strand.

Each code word is a unique combination of three letters (like the ones shown below in the attachment) that will eventually be interpreted as a single amino acid in a polypeptide chain. There are 64 code words possible from an 'alphabet' of four letters.

One of these code words, the 'start signal' begins all the sequences that code for amino acid chains. Three of these code words act as 'stop signals' that indicate that the message is over. All the other sequences code for specific amino acids.

However, some amino acids are only coded for by a single 'word,' while some others are coded for by up to four 'words.' The genetic code is redundant.

Genetic codes do not overlap: The genetic code is composed of nucleotide triplets. In other words, three nucleotides in mRNA (a codon) specify one amino acid in a protein. The code is non-overlapping. This means that successive triplets are read in order.

sorry, but you do not have posted the diagram.