Provide the full forms of the following abbreviations:
$1.$ $\text{YAC}$
$2.$ $\text{SNPs}$
$3.$ $\text{VNTR}$
$4.$ $\text{PCR}$

(N/A) $1.$ $\text{YAC}$: $\text{Yeast Artificial Chromosome}$
$2.$ $\text{SNPs}$: $\text{Single Nucleotide Polymorphisms}$
$3.$ $\text{VNTR}$: $\text{Variable Number of Tandem Repeats}$
$4.$ $\text{PCR}$: $\text{Polymerase Chain Reaction}$