Pedigree collapse
Encyclopedia
In genealogy
, pedigree collapse describes how reproduction between two individuals who knowingly or unknowingly share an ancestor causes the number of distinct ancestors in the family tree of their offspring
to be smaller than it could otherwise be. Robert C. Gunderson
coined the term which is also known by the German term Ahnenschwund.
, formed by the person, the parents (2), the grandparents (4), great-grandparents (8), and so on. However, the number of individuals in such a tree grows exponentially
and will eventually become impossibly high. For example, a single individual alive today would, over 30 generations going back to the High Middle Ages
, have
or roughly a billion ancestors, more than the total world population
at the time.
This apparent paradox is explained by shared ancestors. Instead of consisting of all unique individuals, a single individual may occupy multiple places in the tree. This typically happens when the parents of an ancestor are cousins (sometimes unbeknownst to themselves). For example, the offspring of two first cousins has at most only six great-grandparents instead of the normal eight. This reduction in the number of ancestors is pedigree collapse. It collapses the ancestor tree into a directed acyclic graph
.
In some cultures, cousins were encouraged or required to marry to keep kin bonds, wealth and property within a family (endogamy
). Among royalty
, the frequent requirement to only marry other royals resulted in a reduced gene pool
in which most individuals were the result of extensive pedigree collapse. Alfonso XII of Spain
, for example, had only four great-grandparents instead of the usual eight. Furthermore, two of these great-grandparents, Charles IV of Spain
and Maria Luisa of Parma
, were parents of another twice great-grandmother, Maria Isabella of Spain
. More generally, in many cultures intermarriage may frequently occur within a small village, limiting the available gene pool.
The House of Habsburg is an example of genetically-induced disease as the direct result of pedigree collapse. The last Habsburg King of Spain, Charles II
, makes an instructive case. In Charles' case, there were three uncle-niece marriages among the seven unions of his immediate ancestry (i.e. parents, grandparents and great-grandparents). His father and two of his great-grandfathers married their nieces. His paternal grandparents were first cousins once removed, but they comprised two of the seven marriages because they were also parents to his maternal grandmother. His maternal grandparents' marriage and the final marriage of great-grandparents was between first cousins.
The maximum pedigree collapse of 50% within a single generation is caused by procreation between full siblings; such children have only two different grandparents instead of the usual four. If two half-siblings procreate, their children have three grandparents instead of four (25%).
If a child and parent were to procreate, their offspring would have four grandparents, although one of these would also be a parent and therefore introduce no additional genes – thus procreation between parents and children would result in less pedigree collapse than procreation between full siblings.
If a person procreates with a full sibling of one of their parents, the offspring have four different persons as grandparents, and eight great-grandparents, but again some of these contribute no additional genes. (See inbreeding
.)
Small, isolated populations such as those of remote islands represent extreme examples of pedigree collapse, but the common historical tendency to marry those within walking distance, due to the relative immobility of the population before modern transport, meant that most marriage partners were at least distantly related. Even in America around the 19th century, the tendency of immigrants to marry among their ethnic, language or cultural group produced many cousin marriages.
If one considers as a function of time t the number of a given individual's ancestors who were alive at time t, it is likely that for most individuals this function has a maximum at around 1200 AD. Some geneticist
s believe that everybody on Earth is at least 50th cousin
to everybody else.
Genealogy
Genealogy is the study of families and the tracing of their lineages and history. Genealogists use oral traditions, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kinship and pedigrees of its members...
, pedigree collapse describes how reproduction between two individuals who knowingly or unknowingly share an ancestor causes the number of distinct ancestors in the family tree of their offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
to be smaller than it could otherwise be. Robert C. Gunderson
Robert C. Gunderson
Robert C. Gunderson was the first supervisor of the Genealogical Society of Utah's Royalty Identification Unit. He invented the term "pedigree collapse".-Career:...
coined the term which is also known by the German term Ahnenschwund.
How it works
Without pedigree collapse, a person's ancestor tree is a binary treeBinary tree
In computer science, a binary tree is a tree data structure in which each node has at most two child nodes, usually distinguished as "left" and "right". Nodes with children are parent nodes, and child nodes may contain references to their parents. Outside the tree, there is often a reference to...
, formed by the person, the parents (2), the grandparents (4), great-grandparents (8), and so on. However, the number of individuals in such a tree grows exponentially
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
and will eventually become impossibly high. For example, a single individual alive today would, over 30 generations going back to the High Middle Ages
High Middle Ages
The High Middle Ages was the period of European history around the 11th, 12th, and 13th centuries . The High Middle Ages were preceded by the Early Middle Ages and followed by the Late Middle Ages, which by convention end around 1500....
, have
or roughly a billion ancestors, more than the total world populationWorld population
The world population is the total number of living humans on the planet Earth. As of today, it is estimated to be billion by the United States Census Bureau...
at the time.
This apparent paradox is explained by shared ancestors. Instead of consisting of all unique individuals, a single individual may occupy multiple places in the tree. This typically happens when the parents of an ancestor are cousins (sometimes unbeknownst to themselves). For example, the offspring of two first cousins has at most only six great-grandparents instead of the normal eight. This reduction in the number of ancestors is pedigree collapse. It collapses the ancestor tree into a directed acyclic graph
Directed acyclic graph
In mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...
.
In some cultures, cousins were encouraged or required to marry to keep kin bonds, wealth and property within a family (endogamy
Endogamy
Endogamy is the practice of marrying within a specific ethnic group, class, or social group, rejecting others on such basis as being unsuitable for marriage or other close personal relationships. A Greek Orthodox Christian endogamist, for example, would require that a marriage be only with another...
). Among royalty
Royal family
A royal family is the extended family of a king or queen regnant. The term imperial family appropriately describes the extended family of an emperor or empress, while the terms "ducal family", "grand ducal family" or "princely family" are more appropriate to describe the relatives of a reigning...
, the frequent requirement to only marry other royals resulted in a reduced gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
in which most individuals were the result of extensive pedigree collapse. Alfonso XII of Spain
Alfonso XII of Spain
Alfonso XII was king of Spain, reigning from 1874 to 1885, after a coup d'état restored the monarchy and ended the ephemeral First Spanish Republic.-Early life and paternity:Alfonso was the son of Queen Isabella II of Spain, and...
, for example, had only four great-grandparents instead of the usual eight. Furthermore, two of these great-grandparents, Charles IV of Spain
Charles IV of Spain
Charles IV was King of Spain from 14 December 1788 until his abdication on 19 March 1808.-Early life:...
and Maria Luisa of Parma
Maria Luisa of Parma
Maria Luisa of Parma was Queen consort of Spain from 1788 to 1808 as the wife of King Charles IV of Spain. She was the youngest daughter of Duke Philip of Parma and his wife, Louise-Élisabeth of France, the eldest daughter of King Louis XV.She was christened Luisa Maria Teresa Ana, but was known...
, were parents of another twice great-grandmother, Maria Isabella of Spain
Maria Isabella of Spain
María Isabella of Spain was an Infanta of Spain and the Queen of the Two Sicilies. She was the second wife of Francis I and the youngest surviving daughter of the Spanish king Charles IV and his consort Maria Luisa of Parma...
. More generally, in many cultures intermarriage may frequently occur within a small village, limiting the available gene pool.
The House of Habsburg is an example of genetically-induced disease as the direct result of pedigree collapse. The last Habsburg King of Spain, Charles II
Charles II of Spain
Charles II was the last Habsburg King of Spain and the ruler of large parts of Italy, the Spanish territories in the Southern Low Countries, and Spain's overseas Empire, stretching from the Americas to the Spanish East Indies...
, makes an instructive case. In Charles' case, there were three uncle-niece marriages among the seven unions of his immediate ancestry (i.e. parents, grandparents and great-grandparents). His father and two of his great-grandfathers married their nieces. His paternal grandparents were first cousins once removed, but they comprised two of the seven marriages because they were also parents to his maternal grandmother. His maternal grandparents' marriage and the final marriage of great-grandparents was between first cousins.
The maximum pedigree collapse of 50% within a single generation is caused by procreation between full siblings; such children have only two different grandparents instead of the usual four. If two half-siblings procreate, their children have three grandparents instead of four (25%).
If a child and parent were to procreate, their offspring would have four grandparents, although one of these would also be a parent and therefore introduce no additional genes – thus procreation between parents and children would result in less pedigree collapse than procreation between full siblings.
If a person procreates with a full sibling of one of their parents, the offspring have four different persons as grandparents, and eight great-grandparents, but again some of these contribute no additional genes. (See inbreeding
Inbreeding
Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...
.)
Small, isolated populations such as those of remote islands represent extreme examples of pedigree collapse, but the common historical tendency to marry those within walking distance, due to the relative immobility of the population before modern transport, meant that most marriage partners were at least distantly related. Even in America around the 19th century, the tendency of immigrants to marry among their ethnic, language or cultural group produced many cousin marriages.
If one considers as a function of time t the number of a given individual's ancestors who were alive at time t, it is likely that for most individuals this function has a maximum at around 1200 AD. Some geneticist
Geneticist
A geneticist is a biologist who studies genetics, the science of genes, heredity, and variation of organisms. A geneticist can be employed as a researcher or lecturer. Some geneticists perform experiments and analyze data to interpret the inheritance of skills. A geneticist is also a Consultant or...
s believe that everybody on Earth is at least 50th cousin
Cousin chart
In kinship terminology, a cousin is a relative with whom one shares one or more common ancestors. The term is rarely used when referring to a relative in one's immediate family where there is a more specific term . The term "blood relative" can be used synonymously and establishes the existence of...
to everybody else.
See also
- Coefficient of relationshipCoefficient of relationshipIn population genetics, Sewall Wright's coefficient of relationship or coefficient of relatedness or relatedness or r is defined as 2 times the Coefficient of Inbreeding...
- ConsanguinityConsanguinityConsanguinity refers to the property of being from the same kinship as another person. In that respect, consanguinity is the quality of being descended from the same ancestor as another person...
- Cousin marriageCousin marriageCousin marriage is marriage between two cousins. In various jurisdictions and cultures, such marriages range from being considered ideal and actively encouraged, to being uncommon but still legal, to being seen as incest and legally prohibited....
- HaplotypeHaplotypeA haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...
- Identical ancestors pointIdentical ancestors pointIn genetic genealogy, the identical ancestors point is that point in a given population's past where each individual then alive turned out to be either the ancestor of every individual alive now, or to have no living descendants at all...
- InbreedingInbreedingInbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...
- Most recent common ancestorMost recent common ancestorIn genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
- The Seven Daughters of EveThe Seven Daughters of EveThe Seven Daughters of Eve is a book by Bryan Sykes that presents the theory of human mitochondrial genetics to a general audience...