Primogeniture

Primogeniture is the right, by law or custom, of the firstborn to inherit the entire estate, to the exclusion of younger siblings (compare to ultimogeniture). Historically, the term implied male primogeniture, to the exclusion of females. According to the Norman tradition, the first-born son inherited the entirety of a parent's wealth, estate, title or office and then would be responsible for any further passing of the inheritance to his siblings. In the absence of children, inheritance passed to the collateral relatives, in order of seniority of the males of collateral lines. The eligible descendants of deceased elder siblings take precedence over living younger siblings, such that inheritance is settled in the manner of a depth-first search.

The principle has applied in history to inheritance of real property (land) and inherited titles and offices, most notably monarchies.

Variations on primogeniture modify the right of the first-born son to the entirety of a family's inheritance (see appanage) or, in the West since World War II with the wider promotion of feminism, eliminate the preference for male over female siblings. Six monarchies in Europe have eliminated male preference: Sweden, Norway, Netherlands, Belgium, Denmark and Luxembourg.

Definitions

European monarchies by succession.

  equal primogeniture

  male primogeniture, to be changed to equal primogeniture

  male primogeniture

  agnatic primogeniture

  elective/appointed

The type of marriage prevalent in each culture plays a crucial role in the adoption of differing primogenitures. In Christian Europe, the church had a monopoly on the power to sanction marriage. It discouraged polygamy and divorce. Consequently, in Europe, it was extremely difficult to ensure succession solely by direct male line or even by direct offspring. In Islamic, and Oriental cultures, religion either sanctioned polygyny or use of consorts, or had no authority over marriage; monarchs could consequently ensure sufficient numbers of male offspring to confirm the succession. In such cultures, female heads of state were rare or non-existent. In Japan, the system still mandates that the heir to the throne must be a male, there is only one male grandchild of the current emperor.

Absolute cognatic primogeniture

Absolute, equal or lineal primogeniture, known in French as aînesse intégrale (integral primogeniture), is inheritance by the oldest surviving child without regard to gender. It is also known as (full) cognatic primogeniture today. This form of primogeniture was not practiced by any modern monarchy before 1980.

However, according to Poumarede (1972) the Basques of the Kingdom of Navarre transmitted title and property to the first-born, whatever the gender. This inheritance practice was followed by the high nobility and free families alike in the early and high middle ages. The Navarrese monarchy itself had been inherited by dynasties from outside of Navarre, which followed different succession laws (usually male preference primogeniture, until Navarre was absorbed into the French crown when it followed Salic law). Aînesse intégrale practices weakened among the high nobility of Navarre once Navarre became more exposed to and pressured by outside influences, and largely disappeared from use by the high nobility once the Kingdom of Navarre was merged with the French crown. Eventually only the Basque gentry and free families of the Basque Country and other regions continued to follow this practice until as late as the 19th century.

Sweden revised its constitution to adopt royal succession by absolute primogeniture in 1980, displacing King Carl XVI Gustaf's infant son, Carl Philip, in favor of his elder daughter, Victoria, in the process. Several other monarchies have since followed suit: Netherlands in 1983, Norway in 1990, Belgium in 1991, Denmark (see below) in 2009 and Luxembourg in 2011.

Recently other countries have changed or have considered changing to absolute primogeniture:

• In July 2006, the Nepalese government proposed adopting equal primogeniture, but the monarchy was abolished 28 May 2008.
• In Thailand, the last two constitutions allow the succession of a female monarch. However after the birth of Crown Prince Maha Vajiralongkorn's son, Dipangkorn Rasmijoti to his third wife in 2005, the permission of a female monarch is unlikely.
• In Japan, there have been debates over whether to adopt absolute primogeniture, as Princess Aiko is the only child of Crown Prince Naruhito. However, the birth of Prince Hisahito, a son of Prince Akishino (younger brother of Crown Prince Naruhito, and next in line to the Chrysanthemum Throne after Naruhito) has temporarily side-lined the debate.
• In the United Kingdom, the Succession to the Crown Bill of 2004 proposed changing the line of succession to the British throne to absolute primogeniture. The same proposal is part of the Equality Bill announced in 2008, though shortly after the announcement, the Attorney General announced that there were "no immediate plans to legislate" on that part of the bill; one of the reasons concerning this was the difficulty in making it approved in the 15 other Commonwealth realms. With the agreement of the other Commonwealth realms, the UK government announced legislation to change to gender equal primogeniture in 2011.
• In 2006 and 2009, the Danish parliament voted almost unanimously, only with two abstentions, in favour of a new royal succession law that would allow a first-born child to ascend the throne. This was approved by a referendum held on 7 June 2009.
• In 2006, King Juan Carlos I of Spain issued a decree reforming the succession to noble titles from male preference primogeniture to absolute cognatic primogeniture, where the first born inherits title regardless of gender.

The order of succession for all noble dignitaries is determined in accordance with the title of concession and, if there is none, with that traditionally applied in these cases. When the order of succession to the title is not specified in the nobility title creation charter, the following rules apply:

• Absolute preference is given to the direct descending line over the collateral and ascending line, and, within the same line, the closest degree takes precedence over the more remote and, within the same degree, the elder over the younger, combined with the principles of first-born and representation.
• Men and women have an equal right of succession in Grandee of Spain and nobility titles, and no person may be given preference in the normal order of succession for reasons of gender.

A variation of this system allows women to succeed, as long as their offspring does not, in order to keep the throne in the family. Such were the cases of the few Empresses of Japan. In Norway, a nuance on this system excludes every member of the family who isn't a direct descendant of the last reigning monarch.

The motivation behind this system is recent and based on some views of equality between men and women. An ancient and alternative way in which women managed to rise to power, especially without displacing the direct male line descendants of the first monarchs, is the historical Consortium or Coregency between husband and wife or other relatives, the most notable of these being the cases of Hatshepsut and Thutmose III, as well as the Ptolemaic Dynasty's Kings and Queens.

Agnatic primogeniture

"Agnatic primogeniture" or "patrilineal primogeniture" is inheritance according to seniority of birth among the sons of a monarch or head of family, with sons and their male issue inheriting before brothers and their issue. Thus, inheritance could be traced to the male line only, to the complete exclusion of females. This is commonly referred to as the Salic law (see Terra salica).

By the beginning of the 19th century, only the royal houses of Bourbon and Savoy among Europe's historic national dynasties continued to exclude women from succession. Later, the new monarchies or dynasties of France (under the Bonapartes), Belgium, Denmark (from 1853), Sweden (from 1810), and the Balkan realms of Albania, Bulgaria, Montenegro, Romania, and Serbia introduced Salic law. During this era, Spain fought civil wars which pitted the Salic and female-line heirs of their dynasties against one another for possession of the crown.

Most British and French titles (particularly newer creations) of nobility descend to the senior male by primogeniture, to the exclusion of females, and agnatic cadets may bear courtesy titles.

A variation on Salic primogeniture allows the sons of women to inherit, but not women themselves, an example being succession to the throne of Spain from 1947–1978. This is the law in Liechtenstein and was in the former Archduchy of Austria. Another variation is the so-called Semi-Salic Law, which allows women to succeed only at the extinction of all the male descendants. Such were the cases of Bourbon Spain until 1833 and the dominions of Austria-Hungary, as well as the former Kingdom of Württemberg. This was the law of Luxembourg until equal primogeniture was introduced on 20 June 2011.

Agnatic-cognatic primogeniture

One's agnate may be male or female provided that the kinship is calculated patrilineally; i.e., only through males back to a common ancestor.

"Agnatic-cognatic primogeniture" allows female agnates (or their descendants) to inherit once there are no surviving male agnates. The term semi-Salic succession is used in the same meaning. Usually, women do not succeed by application of the same kind of primogeniture as was in effect among males in the family. Rather, the female who is nearest in kinship to the last male monarch of the family inherits, even if another female agnate of the dynasty is senior by primogeniture. Among sisters (and the lines of descendants issuing from them), the elder are preferred to the younger. In reckoning consanguinity or proximity of blood the law defines who among female relatives is "nearest" to the last male. Definitions varied among monarchies where semi-Salic succession was prevalent.

Male-preference cognatic primogeniture

Male preference cognatic primogeniture (also known as mixed-female succession) allows a female to succeed if she has no living brothers and no deceased brothers who left surviving legitimate descendants. This was the most common primogeniture practiced in Western European feudalism, such as the Castilian Siete Partidas. Male-preferred primogeniture is currently practised in Monaco, Spain, Thailand and the sixteen Commonwealth Realms. It also used to be practised in Portugal. It is usually the rule for Scotland and baronies by writ in the United Kingdom; although baronies by writ go into abeyance if there are several surviving sisters or their legitimate descendants.

The Spanish Constitution of 1978 codifies an example of male preference primogeniture as it has been traditionally practiced in Spain.

The Crown of Spain shall be inherited by the successors of H.M. Juan Carlos I de Borbón, the legitimate heir of the historic dynasty. Succession to the throne shall follow the regular order of primogeniture and representation, the first line having preference over subsequent lines; and within the same line the closer grade over the more remote; and within the same grade the male over female, and in the same sex, the elder over the younger.

This system has religious Jewish-Christian roots. It appears at the end of the Book of Numbers 36:1-12, when five daughters claim the inheritance of their father at the lack of male sons and despite having other male relatives. Then they are allowed to inherit, as long as they marry a member of their tribe, in order to prevent the transfer of their share of the tribal lands to the hands of elements of other tribes and so they do, by marrying their uncles' sons.

Matrilineal primogeniture

"Matrilineal primogeniture" is a form of succession where the eldest female child inherits the throne to the total exclusion of males. The order of succession to the position of the Rain Queen is an example in an African culture of matrilineal primogeniture: not only is dynastic descent reckoned through the female line, but only females are eligible to inherit.

Uterine (or ovarian) primogeniture

A right of succession may also be inherited by a male through a female ancestor or spouse, to the exclusion of any female heir who might be older or of nearer proximity of blood; Spain's mid-twentieth century dynastic succession law has been mentioned. In such cases, inheritance was based on uterine kinship, so a king would typically be succeeded by his sister's son. This particular system of inheritance applied to the thrones of the Picts of Northern Britain and the Etruscans of Italy. Some Kingdoms and Tribes in Africa practice the same use. One of the reasons related to this usage might be related with the fact of the certainty of the relationship to the previous King and Kings, since the son, and daughter, of a sister are, even if they don't have the same father, his relation (Mater semper certa est).

Debate

Primogeniture prevents the subdivision of estates and diminishes internal pressures to sell property (for example, if two children inherit a house and neither can afford to buy out the other's share). In Western Europe, most younger sons of the nobility had no prospect of inheriting property, and were obliged to seek careers in the Church, in military service, or in government. Wills often included bequests to a monastic order who would take the disinherited son. Many of the Spanish Conquistadors were younger sons who had to make their fortune in war. In the late 17th and early 18th centuries, many younger sons of English aristocrats specifically chose to leave England for Virginia in the Colonies. Many of the early Virginians who were plantation owners were such younger sons who had left England fortuneless due to primogeniture laws.

In Japan, the Imperial chronologies include eight reigning empresses from ancient times up through the Edo period; however, their successors were most often selected from amongst the males of the paternal Imperial bloodline, which is why some conservative scholars argue that the women's reigns were temporary and that male-only succession tradition must be maintained. Japanese empresses such as Empress Genshō (680-748), who succeeded her mother the Empress Gemmei (661-721) on the throne (but only because she was a Princess of the Imperial family, daughter of Prince Kusakabe), remain the sole exceptions to this conventional argument.

The fact that the eldest son "scooped the pool" often led to ill-feeling amongst younger sons and daughters. Through marriage, estates inherited by primogeniture were combined and some nobles achieved wealth and power sufficient to pose a threat even to the crown itself. Finally, nobles tended to complain about and resist rules of primogeniture (though this opposition might indicate primogeniture among nobles was good for the king).

In Democracy in America, Alexis de Tocqueville argues that the abolition of the laws of primogeniture and entail in the law of inheritance of private property (as opposed to inheritance of a monarchy) result in the more rapid division of land and thus force landed people to seek wealth outside the family estate in order to maintain their previous standard of living, accelerating the death of the landed aristocracy and also quickening the shift to democracy.

History

Biblical

Main article: Bechor

The earliest account of primogeniture to be widely known in modern times involved Isaac's son Jacob being born second and Isaac's son, Esau being born first and entitled to the "birthright" (bekhorah בְּכוֹרָה), but eventually selling it to Isaac's second son, Jacob, for a small amount of food. Although the veracity of this account has not been established through other sources, its widespread acceptance shows that primogeniture was sufficiently common in the Middle East for the account to seem plausible to the people living there prior to the Roman Empire.

Roman Law

During the Roman Empire, Roman law, which governed much of Europe, made no real distinction between the oldest or youngest, male or female when it came to the law of inheritance.

Reemergence in medieval and modern times

The law of Primogeniture in Europe has its origins in Medieval Europe; which due to the feudal system necessitated that the estates of land-owning feudal lords be kept as large and united as possible to maintain social stability as well as the wealth, power and social standing of their families.

Adam Smith, in his book An Inquiry into the Nature and Causes of the Wealth of Nations, explains the origin of primogeniture in Europe in the following way:

[W]hen land was considered as the means, not of subsistence merely, but of power and protection, it was thought better that it should descend undivided to one. In those disorderly times, every great landlord was a sort of petty prince. His tenants were his subjects. He was their judge, and in some respects their legislator in peace and their leader in war. He made war according to his own discretion, frequently against his neighbours, and sometimes against his sovereign. The security of a landed estate, therefore, the protection which its owner could afford to those who dwelt on it, depended upon its greatness. To divide it was to ruin it, and to expose every part of it to be oppressed and swallowed up by the incursions of its neighbours. The law of primogeniture, therefore, came to take place, not immediately indeed, but in process of time, in the succession of landed estates, for the same reason that it has generally taken place in that of monarchies, though not always at their first institution.

Historical examples

A case of agnatic primogeniture is exemplified in the French royal milieu, where the Salic law (attributed to the Salian Franks) forbade any inheritance of a crown through the female line. This rule was adopted to solve the dispute over the legitimate successor of Charles IV of France (Edward III of England or Philip VI of France, though the former would have a stronger claim should proximity of blood be considered). Conflict between the Salic law and the male-preferred system was also the genesis of Carlism in Spain.

The crowns of Hanover and Great Britain, which had been in personal union since 1714, were separated in 1837 upon the death of King William IV: his niece Victoria inherited the British crown under male-preference primogeniture but, because of semi-Salic law, was ineligible to that of Hanover, which passed to William's eldest surviving brother, Ernest I.

In 1890, the divergence of the thrones of Luxembourg and the Netherlands, both ruled by semi-Salic law, was caused by the fact that the Luxembourg line of succession went back more generations than the Dutch one. The Luxembourg succession was ruled by the provisions of the Nassau House Treaty of 1783. Where the succession is concerned, Luxembourg is the successor state to the Principality of (Orange-)Nassau-Dietz. The Dutch succession only went back to King William I (1815–1840). Therefore Luxembourg still had agnatic heirs from another branch of the House of Nassau left to succeed, while in the Netherlands the male line starting with William I was depleted.

Since the Middle Ages, the semi-Salic principle was prevalent for the inheritance of feudal land in the Holy Roman Empire: inheritance was allowed through females when the male line expired. Females themselves did not inherit, but their male issue could. For example, a grandfather without sons was succeeded by his grandson, the son of his daughter, although the daughter still lived. Likewise, an uncle without sons of his own was succeeded by his nephew, a son of his sister, even if the sister still lived.

Common in feudal Europe outside of Germany was land inheritance based on a form of primogeniture: A lord was succeeded by his eldest son but, failing sons, either by daughters or sons of daughters. In most medieval Western European feudal fiefs, females (such as daughters and sisters) were allowed to succeed, brothers failing. But usually the husband of the heiress became the real lord, assuming his wife's title with the suffix jure uxoris.

In more complex medieval cases, the sometimes conflicting principles of proximity of blood and primogeniture competed, and outcomes were at times unpredictable. Proximity meant that an heir closer in degree of kinship to the lord in question was given precedence although that heir was not necessarily the heir by primogeniture.

• The Burgundian succession in 1361 was resolved in favor of John, son of a younger daughter, on basis of blood proximity, being a nearer cousin of the dead duke than Charles, grandson of the elder daughter. Proximity sometimes favored younger lines (directly contrary to the outcome from applying primogeniture), since it was more probable that from a younger line, a member of an earlier generation was still alive compared with the descendants of the elder line.

• In dispute over the Scottish succession, 1290–91, the Bruce family pleaded tanistry and proximity of blood, whereas Balliol argued his claim based on primogeniture. The arbiter, Edward I of England, decided in favor of primogeniture. But later, the Independence Wars reverted the situation in favor of the Bruce, due to political exigency.

• The Earldom of Gloucester (in the beginning of 14th century) went to full sisters of the dead earl, not to his half-sisters, though they were elder, having been born of the father's first marriage, while the earl himself was from second marriage. Full siblings were considered higher in proximity than half-siblings.

However, primogeniture increasingly won legal cases over proximity in later centuries.

Later, when lands were strictly divided among noble families and tended to remain fixed, agnatic primogeniture (practically the same as Salic Law) became usual: succession going to the eldest son of the monarch; if the monarch had no sons, the throne would pass to the nearest male relative in the male line.

Some countries however accepted female rulers early on, so that if the monarch had no sons, the throne would pass to the eldest daughter. For example, in 1632 Queen Christina of Sweden succeeded to the throne after the death of her father, King Gustav II Adolf.

In England, primogeniture was mandatory for inheritance of land. Until the Statute of Wills was passed in 1540, a will could control only the inheritance of personal property. Real estate (land) passed to the eldest male descendant by operation of law. The statute added a provision that a landowner could "devise" land by the use of a new device called a "testament". The rule of primogeniture in England was not changed until the Administration of Estates Act in 1925.

In law, the rule of inheritance whereby land descends to the oldest son. Under the feudal system of medieval Europe, primogeniture generally governed the inheritance of land held in military tenure (see knight). The effect of this rule was to keep the father’s land for the support of the son who rendered the required military service. When feudalism declined and the payment of a tax was substituted for military service, the need for primogeniture disappeared. In England, consequently, there was enacted the Statute of Wills (1540), which permitted the oldest son to be entirely cut off from inheriting, and in the 17th century military tenure was abolished; primogeniture is, nevertheless, still customary in England. In the United States primogeniture never became widely established.

Reasons for which male lines are more commonly used than female lines

The preference for males in most systems of primogeniture (and in other mechanisms of hereditary succession) comes mostly from the perceived nature of the tasks and role of the monarch: A monarch most usually was, first and foremost, a military protector. It is also related to perceived social and cultural roles defined for men in documented societies.

• It was very useful, or even requisite, that the monarch be a warrior and military commander. Also, war troops (consisting typically only of males) were perceived to approve only males as their commanders.
• Additionally, in such monarchies as France, the monarch held a certain mystical position, some best described as priestly position (high priest or demigod). That sort of position was, depending on the tradition in question, often denied to females. In the French monarchy, one of the official explanations for the Salic Law was that the monarch was obliged to use certain sacred instruments, which females are forbidden even to touch.

An alternative theory posits that the preference for males arose out of a desire to maximize reproductive success. It was thought that, because of a sexual double standard, in which males were able to produce illegitimate and legitimate children, a son would ultimately increase one's posterity more than a daughter. However, as history informs us, some women have also produced illegitimate children not necessarily of peerage lineage.

In modern times, Hakubun Shimomura argued that the Chrysanthemum Throne should be inherited only by those whose paternal ancestors had held it, to preserve a genetic bloodline.

Genetics and preference for males (or females)

Although gender preference originated for other reasons, the modern science of genetics has made it possible to recognize that a strict gender preference in succession rules leads to heirs who share considerably more DNA with the distant ancestors who precede them in the line than would otherwise be the case. This was not known until modern times and did not influence the practice in antiquity, but it has recently begun to do so. For example, Hakubun Shimomura gave "preserving that Y chromosome" as his reason for opposing a proposal to allow a future child of Princess Aiko to become Emperor one day, even if there were no grandsons of Emperor Akihito.

Because the human Y chromosome changes relatively slowly over time and is only passed along the direct male line, it may be used to trace paternal lineage. The human Y chromosome is unable to recombine with the X chromosome, except for small pieces of pseudoautosomal regions at the telomeres (which comprise about 5% of the chromosome's length).

Both sons and daughters inherit 22 chromosomes of autosomal DNA from each parent. Daughters also inherit one X chromosome from each parent, but sons inherit a Y chromosome from their fathers and an X chromosome from their mothers. Mitochondrial DNA is inherited almost exclusively through the female line; all children inherit it only from their mothers.

Because Y chromosomes are smaller than X chromosomes, sons inherit a slightly smaller proportion of the genome from their fathers—half of the autosomal DNA, plus a Y chromosome) than daughters (half of the autosomal DNA, plus one X chromosome—which is in turn slightly smaller than the proportion that children of either gender inherit from their mothers—half of the autosomal DNA, plus one X chromosome, plus all mitochondrial DNA). Sons inherit slightly more than half of their nuclear DNA (all DNA except mitochrondrial DNA) from their mothers and slightly less than half from their fathers; daughters inherit equal amounts from each parent. If mitochrondrial DNA is considered, children of both genders inherit slightly more total DNA from their mothers than from their fathers. However, mitochrondria contain so much less DNA than chromosomes that the daughters can be assumed to inherit approximately 50% of their total DNA from each parent, even though this is technically true only of autosomal DNA.

Because children inherit equal amounts of autosomal DNA from each parent, the amount inherited from any particular distant ancestor is extremely tiny. Although 1/2 is inherited from each parent, only 1/4 is inherited from each grandparent (unless the parents are siblings), 1/8 from each third generation ancestor (great-grandparent), and so forth. By the tenth generation, the genetic contribution of each ancestor is less than 0.1%. After another ten generations, it is less than one part in one million. In successive generations, the genetic resemblance of any particular ancestor becomes no greater than that between any two individuals of the same ethnicity. For example, the similarity in autosomal DNA between the Emperor Keitai of Japan, and the current Emperor, approximately 1,500 years later, is no more than would be expected between any two random persons of purely Japanese ancestry.

Y chromosomes

In a strictly agnatic line of succession, in which only a son, grandson, great-grandson, etc., or a brother, a half-brother who has the same father, a cousin who shares a paternal grandfather, etc., may inherit, every member of the line would theoretically have identical Y chromosomes, except for the pseudoautosomal region and any mutations that may have happened. An example is the Chrysanthemum Throne in Japan, where every male Emperor from the year 507 (Emperor Keitai) to the present (as of 2010) theoretically shares a nearly identical Y chromosome, which is one of the reasons for opposition in Japan to adopting a gender-neutral system.

However, if any succession occurs through a female line in any generation, then none of the Y chromosome DNA continues through the line. For example, although there would be the same Y chromosome in the princes expected (as of 2010) to be the next two Kings of the United Kingdom of Great Britain and Northern Ireland (HRH Prince Charles, the Prince of Wales, son of Her Majesty Queen Elizabeth, and HRH Prince William, son of HRH Princess of Wales Diana Spencer), that DNA would be inherited from the father of the current (as of 2010) Prince of Wales (HRH Prince Charles), and not from the Queen or the prior Kings. Similarly, although HSH Prince Albert II of Monaco and his predecessor Rainer III would have the same Y chromosomes, any similarity to that of his predecessor Louis II would be purely coincidence, because Rainer III inherited the throne through his mother, Princess Charlotte, and not through her husband Prince Pierre. Although a male-line relationship is suggested by the fact that Louis II and Rainer III shared the last name Grimaldi, this actually resulted from Prince Pierre taking his wife's maiden surname as his own surname, which is a form of "false paternal event" (a situation in which a male's surname differs from that of his genetic father).

Additionally, if the mother of any of the males in the line was actually impregnated by someone other than the previous member of the male line, then the Y chromosomes in her son and all subsequent members of the line would be that of the man who impregnated her, and not that of any of the previous members of the line. For example, if a man's wife (or, in lines that are not restricted to legitimate children, his mistress) was impregnated by a rapist or another lover, or was already pregnant when she married him, then a resulting son could be the next in the line, but have no genetic relationship to his predecessor.

Mitochondrial DNA

In strict ovarian primogeniture (which was the same as uterine primogeniture until the development of "in vitro fertilization" in the 1970s, and still is the same in nearly all cases) and in strict matrilineal primogeniture, all members of a line would have identical mitochondrial DNA, except for mutations. (Each child's mitochrondrial DNA is inherited from its mother, who inherited it from her mother, who inherited it from her mother, etc.) This applies both in all-female lines, such as the Rain Queen in Africa, and when a throne is held by males, but inherited by the son of the sister of a prior King (so that the maternal grandmother of the King in one generation is the mother of the King in the previous generation), not by the sons of Kings (except in cases of incest). In lines where a Kings's son-in-law may inherit a throne, that son-in-law would not share any DNA with his predecessors (except by coincidence), but his legitimate sons would have the same mitochondrial DNA as the previous King (the son of their maternal grandmother).

Unlike autosomal DNA and Y chromosome DNA, inheritance of mitochondrial DNA is not affected by adultery, since mitochondrial DNA of a naturally conceived child is inherited only from the mother. Prior to the development of in vitro fertilization in the 1970s, all babies inherited their mitochondrial DNA from the woman in whose uterus they had developed. However, when an egg is removed from one woman, fertilized in vitro, and then placed in another woman, the resulting child inherits its mitochrondrial DNA from the woman in whose ovary the egg originated, and not the woman in whose uterus the child developed. This occurs in cases of egg donation and in cases of gestational surrogacy. Therefore, even in a line following strict uterine succession, it is possible for different members of the line born after the development of in vitro fertilization to have different mitochrondrial DNA, if either (a) an heir was conceived through egg donation, or (b) strict application of a rule of "uterine" succession allowed a child to inherit from the surrogate who bore it, rather than from those who supplied the egg and sperm and raised the child as their own. It is unknown whether in vitro fertilization, surrogacy, and egg donation will jeopardize inheritance rights in the future. One solution is for the genetic parents to "adopt" the child to ensure that they are legally recognized as parents.

X chromosomes

No rule-based form of primogeniture preserves X chromosome DNA in a line over many generations. This is due to several reasons:

• the X chromosome that a child (of either gender) inherits from its mother may come from either of the mother's parents, but not both;
• the X chromosome that a daughter inherits from her father may come from either of the father's mother's parents, but not both; and
• sons do not inherit X chromosomes from their fathers at all, any great-grandchild of the founder of a line may lack X chromosome DNA from that founder.

However, it is now possible to determine which great-grandchildren have an ancestor's X chromosome DNA through genetic testing. In theory, a line could adopt a rule that only children shown by DNA testing to have inherited the DNA of the founder of the line, or some other ancestor, could inherit a throne. No line is believed to have adopted this practice. Even without DNA testing, it is possible to determine which ancestor's X chromosome was inherited by certain royalty because the gene causing hemophilia is found on the X chromosome (for example, hemophilic Prince Waldemar inherited an X chromosome of his maternal grandmother, Princess Alice, and not that of his maternal grandfather).