Inbreeding is a genetic term that refers to
reproduction as a result of the
mating of two animals which are
genetically related to each other. If the
relationship is a close one or it is practiced repeatedly,
inbreeding 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 called
inbreeding depression. Deleterious
alleles causing inbreeding depression can
subsequently be removed through
culling,
which is also known as genetic purging.
Livestock breeders often practice
inbreeding to "fix" desirable characteristics within a population.
However, they must then
cull unfit
offspring, especially when trying to establish the new and
desirable trait in their stock.
In
plant breeding, inbred lines are
used as stocks for the creation of
hybrid lines to make use of the
heterosis effect. Inbreeding in
plants also occurs naturally in the form of
self-pollination.
Results
Inbreeding may result in a far higher phenotypic expression of
deleterious
recessive genes
within a population than would normally be expected. As a result,
first-generation inbred individuals are more likely to show
physical and health defects, including:
Natural selection works to remove
individuals who acquire the above types of traits from the gene
pool. Therefore, many more individuals in the first generation of
inbreeding will never live to reproduce. Over time, with isolation
such as a
population
bottleneck caused by purposeful (assortative) breeding or
natural environmental
stress, the
deleterious inherited traits are culled.
The
cheetah once was reduced by disease,
habitat restriction, overhunting of prey, competition from other
predators (primarily
lions, competition from
human land use, etc.) to a very small number of individuals. All
cheetahs now come from this very small gene pool. Should a
virus appear that none of the cheetahs have resistance
to,
extinction is always a possibility.
Currently, the threatening virus is
feline infectious
peritonitis, which has a
disease
rate in domestic cats from 1%-5%; in the cheetah population it
is ranging between 50% to 60%. The cheetah is also known, in spite
of its small gene pool, for few genetic illnesses.
Island species are often very inbred, as their isolation from the
larger group on a mainland allows for natural selection to work
upon their population. This type of isolation may result in the
formation of race or even speciation, as the inbreeding first
removes many deleterious genes, and allows expression of genes that
allow a population to adapt to an ecosystem. As the adaptation
becomes more pronounced the new species or race radiates from its
entrance into the new space, or dies out if it cannot adapt and,
most importantly, reproduce.
The reduced genetic diversity that results from inbreeding may mean
a
species may not be able to adapt to
changes in environmental conditions. Each individual will have
similar immune systems, as immune systems are genetically based.
Where a species becomes
endangered, the population may fall below
a minimum whereby the forced interbreeding between the remaining
animals will result in
extinction.
In the
South American sea lion, there was concern that recent population
crashes would reduce genetic diversity. Historical analysis
indicated that a population expansion from just two matrilineal
lines were responsible for most individuals within the population.
Even so, the diversity within the lines allowed for great variation
in the gene pool that may inoculate the South American sea lion
from extinction.
Natural breedings include inbreeding by necessity, and most animals
only migrate when necessary. In many cases, the closest living mate
is a mother, sister, grandmother, father, grandfather... In all
cases the environment presents stresses to select or remove those
individuals who cannot survive because of illness from the
population.
In lions,
prides are often
followed by related males in bachelor groups. When the dominant
male is killed or driven off by one of these bachelors, a father
may be replaced with his son. There is no mechanism for preventing
inbreeding or to ensure outcrossing. In the prides, most lionesses
are related to one another. If there is more than one dominant
male, the group of
alpha males are
usually related. Two lines then are being "line bred". Also, in
some populations such as the Crater lions, it is known that a
population bottleneck has occurred. Researchers found far greater
genetic
heterozygosity than expected.
In fact, predators are known for low genetic variance, along with
most of the top portion of the tropic levels of an
ecosystem. Additionally, the alpha males of two
neighboring prides can potentially be from the same litter; one
brother may come to acquire leadership over another's pride, and
subsequently mate with his 'nieces' or cousins. However, killing
another male's cubs, upon the takeover, allows for the new selected
gene complement of the incoming alpha male to prevail over the
previous male. There are genetic
assays being
scheduled for lions to determine their genetic diversity. The
preliminary studies show results inconsistent with the outcrossing
paradigm based on individual environments of the studied
groups.
There was an assumption that wild populations do not inbreed; this
is not what is observed in some cases in the wild. However, in
species such as
horses, animals in
wild or
feral conditions
often drive off the young of both genders, thought to be a
mechanism by which the species instinctively avoids some of the
genetic consequences of inbreeding.
Calculation
The inbreeding is computed as a percentage of chances for two
alleles to be identical by descent. This percentage is called
"inbreeding coefficient". There are several methods to compute this
percentage, the two main ways are the path method
[8796] and the tabular method
[8797].
Typical inbreeding percentages are as follows:
- Father/daughter – mother/son – brother/sister → 25%
- Half-brother/half-sister → 12.5%
- Uncle/niece – aunt/nephew → 12.5%
- Cousin → 6.25%
An inbreeding calculation may be used to determine the general
genetic distance among
relatives by multiplying by 2, because any progeny would have a
1 in 2 risk of actually inheriting the identical alleles from both
parents. For instance, the parent/child or sibling/sibling have 50%
identical genetics.NOTE: For siblings, the degree of genetic
relationship is not an automatic 50% (as it is with parents and
their children), but a range from 100% at one extreme – as in the
case of identical twins [who obviously could not mate as they are
the same sex] – to an exceedingly unlikely 0%. Siblings share an
average of 50% of their genes, but unlike the 50% ratio between
parents and children, the actual ratio between siblings in any
given case can vary widely.
Domestic animals
Breeding in domestic animals is assortative breeding primarily (see
selective breeding). Without the
sorting of individuals by trait, a breed could not be established,
nor could poor genetic material be removed.
Inbreeding is used by breeders of domestic animals to fix desirable
genetic traits within a population or to attempt to remove
deleterious traits by allowing them to manifest phenotypically from
the genotypes. Inbreeding is defined as the use of close relations
for breeding such as mother to son, father to daughter, brother to
sister. Breeders must cull unfit breeding suppressed individuals
and/or individuals who demonstrate either homozygosity or
heterozygosity for genetic based diseases. The issue of casual
breeders who inbreed irresponsibly is discussed in the following
quotation on cattle:Meanwhile, milk production per cow per
lactation increased from 17,444 lbs to 25,013 lbs from 1978 to 1998
for the Holstein breed. Mean breeding values for milk of Holstein
cows increased by 4,829 lbs during this period. High producing cows
are increasingly difficult to breed and are subject to higher
health costs than cows of lower genetic merit for production
(Cassell, 2001).
Intensive selection for higher yield has increased relationships
among animals within breed and increased the rate of casual
inbreeding.
Many of the traits that affect profitability in crosses of modern
dairy breeds have not been studied in designed experiments. Indeed,
all crossbreeding research involving North American breeds and
strains is very dated (McAllister, 2001) if it exists at all.
Linebreeding, which is a milder form of inbreeding is
accomplished through breeding of cousins, aunt to nephew, half
brother to half sister. This was used to isolate breeds within the
companion and livestock industry. For instance an animal with a
desirable colour is bred back within the lines with identified
selection traits whether it be milk production or adherence to
breed standard of appearance or behavior. Breeders must then cull
unfit individuals, and in some cases the breeders will then
outbreed to increase the level of genetic
diversity. Again casual breeding is problematic as it is without
the requisite culling of individuals who are either maladaptive,
not to breed standard or carriers of poor genetic material that
must be removed from a healthy breeding program.
Outcrossing is where two unrelated
individuals have been crossed to produce progeny. In outcrossing,
unless there is verifiable genetic information, one may find that
all individuals are distantly related to an ancient progenitor. If
the trait carries throughout a population, all individuals can have
this trait. This is called the founder's effect. In the well
established breeds, that are commonly bred,a large gene pool is
present. For example, in 2004, over 18,000 Persian cats were
registered. A possibility exists for a complete outcross, if no
barriers exist between the individuals to breed. However it is not
always the case, and a form of distant linebreeding occurs. Again
it is up to the assortative breeder to know what sort of traits
both positive and negative exist within the diversity of one
breeding. This diversity of genetic expression, within even close
relatives, increases the variability and diversity of viable
stock.
The two dog sites above also point out that in the registered dog
population, the onset of large numbers of casual breeders has
corresponded with an increase in the number of genetic illnesses of
dogs by not understanding how, why and which traits are inherited.
The dog sites indicate that the largest percentage of dog breeders
in the US are
casual breeders.
Therefore the investment in a papered animal,with an expected short
term profit, motivates some to ignore the practice of culling.
Casual breeders in companion animals often ignore breeding
restrictions within their contracts with source companion animal
breeders. The casual breeders breed the very culls that a genetics
based breeder has released as a pet. The casual breeder also was
cited in the quotes above on cattle raising.
Inbreeding is also deliberately induced in
laboratory mice in order to guarantee a
consistent and uniform
animal model for
experimental purposes.
Humans
Genetic disorders
The offspring of
consanguineous
relationship are at greater risk of certain genetic disorders.
These
autosomal recessive
disorders occur in individuals who are
homozygous for a particular recessive gene
mutation. This means that they carry two
copies (
alleles) of the same gene. Except in
certain rare circumstances (new mutations or
uniparental disomy) both parents of an
individual with such a disorder will be carriers of the gene. Such
carriers are not affected and will not display any signs that they
are carriers, and so may be unaware that they carry the mutated
gene. As relatives share a proportion of their genes, it is much
more likely that related parents will be carriers of an autosomal
recessive gene, and therefore their children are at a higher risk
of an autosomal recessive disorder. The extent to which the risk
increases depends on the degree of genetic relationship between the
parents; so the risk is greater in mating relationships where the
parents are close relatives, but for relationships between more
distant relatives, such as second cousins, the risk is lower
(although still greater than the general population).
Prohibitions to inbreeding
The taboo of
incest has been discussed by
many social scientists.
Anthropologists
attest that it exists in most cultures. As inbreeding within the
first generation often produces expression of recessive traits, the
prohibition has been discussed as a possible functional response to
the requirement of culling those born deformed, or with undesirable
traits. Some biologists like
Charles
Davenport advocated the traditional forms of assortative
breeding, i.e.
eugenics, to form better
"human stock".
Ancient Egypt
In
ancient Egypt, some
Pharaohs married their sisters; in such cases a
special combination between
endogamy and
polygamy is found. Normally the old ruler's
eldest son and daughter (who could be either siblings or
half-siblings) became the new rulers. All rulers of the
Ptolemaic dynasty from
Ptolemy II were married to their brothers and
sisters, so as to keep the Ptolemaic blood "pure" and to strengthen
the line of succession.
Cleopatra VII
and
Ptolemy XIII, who married and
became co-rulers of
ancient Egypt
following their father's death, are the most widely known
example.
Royalty and nobility
In discussing humans, the term
inbreeding is highly
offensive and judgmental. Roughly 20% of the world's marriages are
between
first cousins, and such marriages are not illegal
in most of the world. Although it is an undisputed fact that cousin
marriages increase the probability of genetic disease, the level of
statistical increase varies with the degree of relationship, and
the frequency of the marriages. The casual use of the term
inbred implies that some degree of degradation exists,
when in fact there may be no effect at all. The family
relationships of royalty are usually very well known leading
observers to view royalty as highly
inbred, but they are
often comparable to many ethnic groups where the relationships are
not publicized as well. The royal and noble families of
Europe have traditionally been prone to
royal intermarriage, as it protected
property, wealth, and position.
European monarchies
Jean V of
Armagnac formed an extremely rare brother-sister marriage. In
particular, the
Habsburg up until
the year 1700 had a great deal of intermarriage. The line died out
leading to the
War of the
Spanish Succession. Also other royal houses, such as the
Wittelsbachs had marriages among aunts,
uncles, nieces, and nephews.
The British
royal family
had several marriages as close as the first cousin, but none
closer.
The most famous example of a genetic disorder aggravated by royal
family intermarriage was the
House of
Habsburg, which inmarried particularly often. Famous in this
case is the
Habsburger (Unter) Lippe (Habsburg
jaw/Habsburg lip/"Austrian lip"), typical for many Habsburg
relatives over a period of six centuries. The condition progressed
through the generations to the point that the last of the Spanish
Habsburgs,
Charles II of Spain,
could not properly chew his food. (See mandibular
prognathism.)
Besides the jaw deformity, Charles II also had a huge number of
other genetic physical, intellectual, sexual, and emotional
problems. It is speculated that the simultaneous occurrence in
Charles II of two different genetic disorders: combined pituitary
hormone deficiency and distal renal tubular acidosis could explain
most of the complex clinical profile of this king, including his
impotence/infertility which in last instance led to the extinction
of the dynasty.
The most famous genetic disease that circulated among European
royalty was hemophilia. Because the progenitor, Queen Victoria, was
in a first cousin marriage, it is often mistakenly believed that
the cause was consanguinity. However, this disease is not
aggravated by cousin marriages.
Many more examples of royal couples are included in the article
list of coupled
cousins.
Intermarriage within European royal families has declined in
relation to the past. Inter-nobility
marriage was used as a method of forming
political alliances among elite
power-brokers. These ties were often sealed only upon the birth of
progeny within the
arranged
marriage. Thus marriage was seen as a union of lines of
nobility, not of a contract between individuals as it is seen
today.
Isolated groups
Among genetic populations that are isolated, opportunities for
exogamy are reduced, however may not intend
to inbreed. Isolation may be geographical, leading to inbreeding
among people in remote mountain valleys. Or isolation may be
social, induced by the lack of appropriate partners, such as
Protestant princesses for Protestant royal heirs, in which case
inbreeding is desired. Since the late
Middle
Ages, it is the urban
middle class
that has had the widest opportunity for
outbreeding and the least desire to inbreed.
- Some Peruvian Sapa Incas married their
sisters; in such cases a special combination between endogamy and polygamy is
found. Normally the son of the old ruler and the ruler's oldest
(half-)sister became the new ruler.
- The Inca had an unwritten rule that the new
ruler must be a son of the Inca and his wife and sister. He then
had to marry his sister (not half-sister), which ultimately led to
the catastrophic Huascars reign, culminating
in a civil war and then fall of the
empire.
Icelandic study
A recent study in Iceland by the deCODE genetics company, published
by the journal Science, found that third cousins had the highest
rate of genetic success and children, suggesting a minimal
relationship to each other is favorable in humans pairing off and
reproducing.
Allele exposure
Genes express themselves according to the way they pair with each
other as alleles. One case of this is known as homozygosity. It is
the case where similar or identical alleles combine to express a
trait that is not otherwise expressed (recessiveness). In other
words, inbreeding, through homozygosity, exposes recessive alleles.
Culling is a case of a use of this.
Evolution
Inbreeding has a variety of consequences. Allele exposure can cause
genes to be expressed that are not otherwise expressed. This fact,
combined with the fact that most mutations are recessive may
indicate that inbreeding drives evolution. Speciation, a key
process in evolution, depends on reproductive barriers, a necessary
feature of which is inbreeding.
See also
References
- M Menotti-Raymond and S J O'Brien. "Dating the
genetic bottleneck of the African cheetah." Proc Natl Acad Sci U S
A. 1993 April 15; 90(8): 3172–3176.
- CHARLES F. LECK. "ESTABLISHMENT OF NEW POPULATION CENTERS
WITH CHANGES IN MIGRATION PATTERNS." Journal of Field
Ornithology, Spring 1980 Vol. 51, No. 2
-
http://www.dur.ac.uk/anthropology.journal/vol13/iss1/posters/freilich.pdf
-
http://services.oxfordjournals.org/cgi/tslogin?url=http://www.oxfordjournals.org%2Fjnls%2Flist%2Fjhered%2Ffreepdf%2F82-378.pdf
-
http://www.iupac.org/publications/pac/1998/pdf/7011x2079.pdf
- "ADVS 3910 Wild Horses Behavior," web page accessed June 22,
2007 at
http://www.advs.usu.edu/academics/pdf/ADVS3910WildHorses.pdf
- How to compute and inbreeding coefficient (the path method)
http://www.braquedubourbonnais.info/en/inbreeding-calculation.htm
- G2036 Culling the Commercial Cow Herd: BIF Fact
Sheet, MU Extension
- Homepage: S1008
- Inbreeding and linebreeding Retrieved on
2009-8-10
-
http://web.archive.org/web/20070608052840/http://showcase.netins.net/web/royalair/libreeding.htm
- Top Cat Breeds for 2004
- Preserving Quality and Genetic Diversity in a Dog
Breed
- Kingston H M, "ABC of Clinical Genetics", Page 7, 3rd Edition
(2002), BMJ Books, London, 0-7279-1627-0
- "The Habsburg Lip", Topics in the History of Genetics
and Molecular Biology, Fall 2000
- Iceland's 'Kissing Cousins' Breed More
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