Final answer:
Differences in alleles, which are alternative forms of a gene, are the major cause of variation in almost all genomes. Most genetic variation is the result of differences in individual DNA base pairs within alleles. These variations can be in the form of single nucleotide polymorphisms (SNPs) or structural variations.
Step-by-step explanation:
The major cause of variation in almost all genomes is differences in alleles, which are alternative forms of a gene. Most human genetic variation is the result of differences in individual DNA base pairs within alleles. These variations can be in the form of single nucleotide polymorphisms (SNPs) or structural variations, such as copy number variation or translocations. These genetic differences contribute to the diversity observed among individuals.