populations. unlike natural selection, genetic drift occurs when alleles decrease randomly over time and not as a result of selection bias. over a long history, this can cause significant shifts in the underlying genetic distribution of a population. we can model genetic drift with the wright-fisher model. in a population of n with 2n genes, there are two alleles with frequencies p and q. if the previous generation had an allele with frequency p, then the probability that the next generation has k of that allele is: ( 2 n k ) p k q 2 n − k p^q^ over time, one allele will be fixed when the frequency of that allele reaches 1 and the frequency of the other allele reaches 0. the probability that any allele is fixed is proportional to the frequency of that allele. for two alleles with frequencies p and q, the probability that p will be fixed is p. the expected number of generations for an allele with frequency p to be fixed is: t ¯ fixed = − 4 n e ( 1 − p ) ln ⁡ ( 1 − p ) p _=(1-p)\ln(1-p)} where ne is the effective population size. single-nucleotide polymorphism single-nucleotide polymorphism or snps are variations of a single nucleotide. snps can occur in coding or non-coding regions of genes and on average occur once every 300 nucleotides. snps in coding regions can cause synonymous, missense, and nonsense mutations. snps have shown to be correlated with drug responses and risk of diseases such as sickle-cell anemia, alzheimer's disease, cystic fibrosis, and more. dna fingerprinting main article: dna fingerprinting dna profiling, whereby a dna fingerprint is constructed by extracting a dna sample from body tissue or fluid. then, it is segmented using restriction enzymes and each segment marked with probes then exposed on x-ray film. the segments form patterns of black bars;the dna fingerprint. dna fingerprints are used in conjunction with other methods in order to individuals information in federal programs such as codis (combined dna index system for missing persons) in order to help identify individuals mitochondrial dna mitochondrial dna, which is only passed from mother to child. the first human population studies based on mitochondrial dna were performed by restriction enzyme analyses (rflps) and revealed differences between the four ethnic groups (caucasian, amerindian, african, and asian). differences in mtdna patterns have also been shown in communities with a different geographic origin within the same ethnic group alloenzymic variation alloenzymic variation, a source of variation that identifies protein variants of the same gene due to amino acid substitutions in proteins. after grinding tissue to release the cytoplasm, wicks are used to absorb the resulting extract and placed in a slit cut into a starch gel. a low current is run across the gel resulting in a positive and negative ends. proteins are then separated by charge and size, with the smaller and more highly charged molecules moving more quickly across the gel. this techniques does underestimate true genetic variability as there may be an amino acid substitution but if the amino acid is not charged differently than the original no difference in migration will appear it is estimated that approximately 1/3 of the true genetic variation is not expressed by this technique. structural variation structural variation, which can include insertions, deletions, duplications, and mutations in dna. within the human population, about 13% of the human genome is defined as structurally variant. phenotypic variation phenotypic variation, which accounts for both genetic and epigenetic factors that affect what characteristics are shown. for applications such as organ donations and matching, phenotypic variation of blood type, tissue type, and organ size are considered. civic measurement of human variation may also be initiated by governmental parties. a government may conduct a census, the systematic recording of an entire population of a region. the data may be used for calculating metrics of demography such as sex, gender, age, education, employment, etc.; this information is utilized for civic, political, economic, industrial, and environmental assessment and planning. commercial commercial motivation for understanding variation in human populations arises from the competitive advantage of tailoring products and services for a specific target market. a business may undertake some form of market research in order to collect data on customer preference and behavior and implement changes which align with the results. social significance and valuation both individuals and entire societies and cultures place values on different aspects of human variability; however, values can change as societies and cultures change. not all people agree on the values or relative rankings, and neither do all societies and cultures. nonetheless, nearly all human differences have a social value dimension. examples of variations which may be given different values in different societies include skin color and/or body structure. race and sex have a strong value difference, while handedness has a much weaker value difference. the values given to different traits among human variability are often influenced by what phenotypes are more prevalent locally. local valuation may affect social standing, reproductive opportunities, or even survival. differences may vary or be distributed in various ways. some, like height for a given sex, vary in close to a "normal" or gaussian distribution. other characteristics (e.g., skin color) vary continuously in a population, but the continuum may be socially divided into a small number of distinct categories. then, there are some characteristics that vary bimodally (for example, handedness), with fewer people in intermediate categories. classification and evaluation of traits when an inherited difference of body structure or function is severe enough to cause a significant hindrance in certain perceived abilities, it is termed a genetic disease, but even this categorization has fuzzy edges. there are many instances in which the degree of negative value of a human difference depends completely on the social or physical environment. for example, in a society with a large proportion of deaf people (as martha's vineyard in the 19th century), it was possible to deny that deafness is a disability. another example of social renegotiation of the value assigned to a difference is reflected in the controversy over management of ambiguous genitalia, especially whether abnormal genital structure has enough negative consequences to warrant surgical correction. furthermore, many genetic traits may be advantageous in certain circumstances and disadvantageous in others. being a heterozygote or carrier of the sickle-cell disease gene confers some protection against malaria, apparently enough to maintain the gene in populations of malarial areas. in a homozygous dose it is a significant disability. each trait has its own advantages and disadvantages, but sometimes a trait that is found desirable may not be favorable in terms of certain biological factors such as reproductive fitness, and traits that are not highly valued by the majority of people may be favorable in terms of biological factors. for example, women tend to have fewer pregnancies on average than before and therefore net worldwide fertility rates are dropping. moreover, this leads to the fact that multiple births tend to be favorable in terms of number of children and therefore offspring count; when the average number of pregnancies and the average number of children was higher, multiple births made only a slight relative difference in number of children. however, with fewer pregnancies, multiple births can make the difference in number of children relatively large. a hypothetical scenario would be that couple 1 has ten children and couple 2 has eight children, but in both couples, the woman undergoes eight pregnancies. this is not a large difference in ratio of fertility. however, another hypothetical scenario can be that couple 1 has three children and couple 2 has one child but in both couples the woman undergoes one pregnancy (in this case couple 2 has triplets). when the proportion of offspring count in the latter hypothetical scenario is compared, the difference in proportion of offspring count becomes higher. a trait in women known to greatly increase the chance of multiple births is being a tall woman (presumably the chance is further increased when the woman is very tall among both women and men). yet very tall women are not viewed as a desirable phenotype by the majority of people, and the phenotype of very tall women has not been highly favored in the past. nevertheless, values placed on traits can change over time. such an example is homosexuality. in ancient greece, what in present terms would be called homosexuality, primarily between a man and a young boy, was not uncommon and was not outlawed. however, homosexuality became more condemned. attitudes towards homosexuality alleviated in modern times. acknowledgement and study of human differences does have a wide range of uses, such as tailoring the size and shape of manufactured items. see ergonomics. controversies of sociocultural and personal implications possession of above average amounts of some abilities is valued by most societies. some of the traits that societies try to measure by perception are intellectual aptitude in the form of ability to learn, artistic prowess, strength, endurance, agility, and resilience. each individual's distinctive differences, even the negatively valued or stigmatized ones, are usually considered an essential part of self-identity. membership or status in a social group may depend on having specific values for certain attributes. it is not unusual for people to deliberately try to amplify or exaggerate differences, or to conceal or minimize them, for a variety of reasons. examples of practices designed to minimize differences include tanning, hair straightening, skin bleaching, plastic surgery, orthodontia, and growth hormone treatment for extreme shortness. conversely, male-female differences are enhanced and exaggerated in most societies. in some societies, such as the united states, circumcision is practiced on a majority of males, as well as sex reassignment on intersex infants, with substantial emphasis on cultural and religious norms. circumcision is highly controversial because although it offers health benefits, such as less chance of urinary tract infections, stds, and penile cancer, it is considered a drastic procedure that is not medically mandatory and argued as a decision that should be taken when the child is old enough to decide for himself. similarly, sex reassignment surgery offers psychiatric health benefits to transgender people but is seen as unethical by some christians, especially when performed on children. much controversy surrounds the assigning or distinguishing of some variations, especially since differences between groups in a society or between societies is often debated as part of either a person's "essential" nature or a socially constructed attribution. for example, there has long been a debate among sex researchers on whether sexual orientation is due to evolution and biology (the "essentialist" position), or a result of mutually reinforcing social perceptions and behavioral choices (the "constructivist" perspective). the essentialist position emphasizes inclusive fitness as the reason homosexuality has not been eradicated by natural selection. gay or lesbian individuals have not been greatly affected by evolutionary selection because they may help the fitness of their siblings and siblings' children, thus increasing their own fitness through inclusive fitness and maintaining evolution of homosexuality. biological theories for same gender sexual orientation include genetic influences, neuroanatomical factors, and hormone differences but research so far has not provided any conclusive results. in contrast, the social constructivist position argues that sexuality is a result of culture and has originated from language or dialogue about sex. mating choices are the product of cultural values, such as youth and attractiveness, and homosexuality varies greatly between cultures and societies. in this view, complexities, such as sexual orientation changing during the course of one's lifespan, are accounted for. controversy also surrounds the boundaries of "wellness", "wholeness," or "normality." in some cultures, differences in physical appearance, mental ability, and even sex can exclude one from traditions, ceremonies, or other important events, such as religious service. for example, in india, menstruation is not only a taboo subject but also traditionally considered shameful. depending on beliefs, a woman who is menstruating is not allowed to cook or enter spiritual areas because she is "impure" and "cursed". there has been large-scale renegotiation of the social significance of variations which reduce the ability of a person to do one or more functions in western culture. laws have been passed to alleviate the reduction of social opportunity available to those with disabilities. the concept of "differently abled" has been pushed by those persuading society to see