Due to various causes, including DDT poisoning, the population size of whooping cranes was reduced to about two dozen during the 1940s. The population size has since rebounded, and there are now hundreds of whooping cranes. Which would you not expect to see in the current populations of whooping cranes? A. very low levels of genetic variation
B. very few heterozygous individuals;
C. considerable genetic polymorphism.
D. all of the above would be seen in whooping cranes.

Single or simple nucleotide polymorphisms (from English Single Nucleotide Polymorphisms, SNPs), (pronounced as snips). They are the simplest and most common form of genetic polymorphism since they consist of changing a single nucleotide in the context of a genetic sequence. They are considered to determine most of the genetic variability among individuals, causing many of the phenotypic (observable) differences in them.

A SNP is a variation in the DNA sequence that affects a single nitrogenous base of a genome sequence: adenine (A), thymine (T), cytosine (C) or guanine (G). Some authors consider that changes of a few nucleotides, as well as small insertions and deletions (indels) can be considered as SNPs, if one of these variations occurs in at least 1% of the population; If 1% is not reached, it is not considered SNP and is a point mutation.

SNPs are distributed heterogeneously throughout the genome and are found in both the coding (exon) and non-coding regions (introns and promoter region) of the genes as well as in the areas of the genome where no known genes settle (sometimes called "junk DNA").