Two languages are considered to be genetically related if one is descended from the other or if both are descended from a common ancestor. For example, the Romance languages are all descended from the spoken Latin of ancient Rome,[note 1] and so languages like Spanish, Italian, Portuguese, Romanian, and French are said to be genetically related to one another as well as to Latin. Similarly, Danish, Swedish and Norwegian are genetically related as members of the North Germanic language family because of their shared descent from Ancient Norse.
The Romance languages and the North Germanic languages are both subfamilies of the Indo-European language family since both Latin and Old Norse are believed to be descended from an even more ancient language, Proto-Indo-European, and are therefore genetically related to each other as well.
One controversial theory concerning the genetic relationships among languages is monogenesis, the idea that all known languages, with the exceptions of creoles, pidgins and sign languages, are descendant from a single ancestral language. If that is true, it would mean all languages (other than pidgins, creoles, and sign languages) are genetically related, but in many cases, the relationships may be too remote to be detectable.
The traditional term genetic relationship is increasingly replaced by genealogical relationship in recent literature to avoid confusion with the unrelated use of the term in biological genetics.
In some cases, such as the Romance and North Germanic examples described above, the shared derivation of a group of related languages from a common ancestor is attested in the historical record. In other cases, genetic relationships between languages are established through use of the comparative method of linguistic analysis.
In order to test the hypothesis that two languages are related, the comparative method begins with the collection of pairs of words that are hypothesized to be cognates: i.e., words in related languages that are derived from the same word in the shared ancestral language. Pairs of words that have similar pronunciations and meanings in the two languages are often good candidates for hypothetical cognates. The researcher must rule out the possibility that the two words are similar merely due to chance, or due to one having borrowed the words from the other (or from a language related to the other). Chance resemblance is ruled out by the existence of large collections of pairs of words between the two languages showing similar patterns of phonetic similarity. Once coincidental similarity and borrowing have been eliminated as possible explanations for similarities in sound and meaning of words, the last explanation is common origin: it is inferred that the similarities occurred due to descent from a common ancestor, and the words are actually cognates, implying the languages must be related.
When languages are in contact with one another, either of them may influence the other through linguistic interference such as borrowing. For example, French has influenced English, Arabic has influenced Persian, Sanskrit has influenced Tamil and Chinese has influenced Japanese in this way. However, such influence does not constitute (and is not a measure of) a genetic relationship between the languages concerned. Linguistic interference can occur between languages that are genetically closely related, between languages that are distantly related (like English and French, which are distantly related Indo-European languages) and between languages that have no genetic relationship.
A common visual representation of a language family is given by a genetic language tree. The tree model is sometimes termed a dendrogram or phylogeny. The family tree shows the relationship of the languages within in a family, much as a family tree of an individual shows their relationship with their relatives. There are criticisms to the family tree model. Critics focus mainly on the claim that the internal structure of the trees is subject to variation based on the criteria of classification. Even among those who support the family tree model, there are debates over which languages should be included in a language family. For example, within the dubious Altaic language family, there are debates over whether the Japonic and Koreanic languages should be included or not.
The wave model has been proposed as an alternative to the tree model. The wave model uses isoglosses to group language varieties; unlike in the tree model, these groups can overlap. While the tree model implies a lack of contact between languages after derivation from an ancestral form, the wave model emphasizes the relationship between languages that remain in contact, which is more realistic. Historical glottometry is an application of the wave model, meant to identify and evaluate genetic relations in linguistic linkages.
Some problems encountered by the genetic relationship group of languages include language isolates and mixed, pidgin and creole languages. Mixed languages, pidgins and creole languages constitute special genetic types of languages. They do not descend linearly or directly from a single language and have no single ancestor. Language isolates are languages that are unrelated to other languages. Each language isolate is considered to be a single language family with one language according to the Ethnologue. Including language isolates when counting language families considerably increases the number of language families.