Genetic diversity is a level of biodiversity that refers to the total number of genetic characteristics in the genetic makeup of a species. Biodiversity is the variation of Life forms within a given Ecosystem, Biome or for the entire Earth.
The academic field of population genetics includes several hypotheses regarding genetic diversity. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic The neutral theory of evolution proposes that diversity is the result of the accumulation of neutral substitutions. The neutral theory of molecular evolution is an influential theory that was introduced with provocative effect by Motoo Kimura in the late 1960s and early 1970s Diversifying selection is the hypothesis that two subpopulations of a species live in different environments that select for different alleles at a particular locus. Disruptive selection, also called diversifying selection, is a descriptive term used to describe changes in Population genetics that simultaneously favor individuals An allele (ˈæliːl (UK /əˈliːl/ (US (from the Greek αλληλος allelos, meaning each other) is one member of a pair or series of different forms This may occur, for instance, if a species has a large range relative to the mobility of individuals within it. Frequency-dependent selection is the hypothesis that as alleles become more common, they become less fit. Frequency dependent selection is the term given to an evolutionary process where the fitness of a Phenotype is dependent on its frequency relative to other phenotypes This is often invoked in host-pathogen interactions, where a high frequency of a defensive allele among the host means that it is more likely that a pathogen will spread if it is able to overcome that allele.
There are many different ways to measure genetic diversity. The modern causes for the loss of animal genetic diversity have also been studied and identified.  A September 14, 2007 study conducted by the National Science Foundation found that genetic diversity and biodiversity are dependent upon each other -- that diversity within a species is necessary to maintain diversity among species, and vice versa. Events 81 - Domitian becomes Emperor of the Roman Empire upon the death of his brother Titus. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The National Science Foundation (NSF is a United States Government agency that supports fundamental Research and Education in all the non-medical Biodiversity is the variation of Life forms within a given Ecosystem, Biome or for the entire Earth. According to the lead researcher in the study, Dr. Richard Lankauof, "If any one type is removed from the system, the cycle can break down, and the community becomes dominated by a single species. "
Genetic diversity plays a huge role in the survival and adaptability of a species. When a species’ environment changes, slight gene variations are necessary for it to adapt and survive. A species that has a large degree of genetic diversity among its individuals will have more variations from which to choose the most fitting allele. Species that have very little genetic variation are at a great risk. With very little gene variation within the species, healthy reproduction becomes increasingly difficult, and offspring often deal with similar problems to those of inbreeding. Inbreeding is breeding between close Relatives whether plant or animal 
When humans initially started farming, they used selective breeding to pass on desirable traits of the crops while omitting the undesirable ones. This article focuses on selective breeding in domesticated animals Selective breeding leads to monocultures: entire farms of nearly genetically identical plants. Monoculture is the agricultural practice of producing or growing one single crop over a wide area Little to no genetic diversity makes crops extremely susceptible to widespread disease. Bacteria morph and change constantly. When a disease causing bacteria changes to attack a specific genetic variation, it can easily wipe out vast quantities of the species. If the genetic variation that the bacterium is best at attacking happens to be that which humans have selectively bred to use for harvest, the entire crop will be wiped out. 
A very similar occurrence is the cause of the infamous Potato Famine in Ireland. Since new potato plants do not come as a result of reproduction but rather from pieces of the parent plant, no genetic diversity is developed, and the entire crop is essentially a clone of one potato, it is especially susceptible to an epidemic. In the 1840s, much of Ireland’s population depended on potatoes for food. They planted namely the “lumper” variety of potato, which was susceptible to a rot-causing mold called Phytophthora infestans.  This mold destroyed the vast majority of the potato crop, and left thousands of people to starve to death.
The natural world has several ways of preserving or increasing genetic diversity. Among oceanic plankton, viruses aid in the genetic shifting process. Plankton consist of any drifting Organisms ( Animals Plants Archaea, or Bacteria) that inhabit the Pelagic zone of Ocean viruses, which infect the plankton, carry genes of other organisms in addition their own. When a virus containing the genes of one cell infects another, the genetic makeup of the latter changes. This constant shift of genetic make-up helps to maintain a healthy population of plankton despite complex and unpredictable environmental changes. 
Cheetahs are a threatened species. The cheetah ( Acinonyx jubatus) is an atypical member of the cat family ( Felidae) that is unique in its speed while lacking climbing abilities Extremely low genetic diversity and resulting poor sperm quality has made breeding and survivorship difficult for Cheetahs – only about 5% of cheetahs make it to adulthood.  About 10,000 years ago, all but the jubatus species of cheetahs died out. The species encountered a population bottleneck and close family relatives were forced to mate with each other, resulting in inbreeding . A population bottleneck (or genetic bottleneck) is an Evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented  However, it has been recently discovered that female cheetahs can mate with more than one male per litter of cubs. They undergo induced ovulation, which means that a new egg is produced every time a female mates. By mating with multiple males, the mother increases the genetic diversity within a single litter of cubs. 
Genetic Diversity of a population can be assessed by some simple measures.
Gene Diversity is the proportion of polymorphic loci across the genome.
Heterozygosity is the mean number of individuals with polymorphic loci. Zygosity refers to the genetic condition of a Zygote. In genetics zygosity describes the similarity or dissimilarity of DNA between Homologous
Alleles per locus is also used to demonstrate variability.