Under a triploidy (to triploid , "triple") is in the genetics understood a feature in which an animal or a cell three ( lat. Tri = three) complete haploid sets of chromosomes has (3n) . Triploidy is known in the animal kingdom and in plants. In humans, triploidy leads to severe disability and, with a few exceptions, premature death.
Triploidy in humans
|Classification according to ICD-10|
|Q92.7||Triploidy and polyploidy|
|ICD-10 online (WHO version 2019)|
Due to a genome mutation, a person with triploidy has 69 genome carriers in his body cells instead of the usual 46 chromosomes due to the tripling of chromosomes and is in most cases not viable in the long term due to the resulting physical-organic peculiarities.
Embryos with triploidy usually die during pregnancy . The probability of a live birth is very low at around 1 in 50,000, and most children who are born alive die relatively soon after their birth . Rarely do they survive several months, and very rarely do they reach adulthood . Depending on the proportion of normally equipped cells, the prognosis for children with mosaic triploidy is often more favorable, as is the case for children with digynia (see below).
Forms of triploidy in humans
With triploidy, there are three complete sets of chromosomes in the nucleus of a cell . There are three types of triploidy:
Diandria ( type I / hyperandric triploidy )
- In type I triploidy, known as diandry , two of the three complete sets of chromosomes come from the paternal side. The reason for this can be the connection between two haploid sperm cells and one egg cell or fertilization of the egg cell with a diploid sperm cell . Most fetuses are rejected as spontaneous miscarriages in the very early stages of pregnancy .
Digynia ( type II / hypergynic triploidy )
- In type II triploidy, known as digynia , two of the three complete sets of chromosomes come from the maternal side. The reason for this can be a failure to eject the second pole body . In some cases, digynia can be traced back to intracytoplasmic sperm injection (ICSI). Affected children can sometimes survive after birth for up to several months.
- In mosaic triploidy, not all body cells have the triploid chromosome set of 69, but there is also a cell line with the usual chromosome set of 46. The presence of several karyotypes within an organism is called a mosaic in genetics . Depending on the proportion of disome cells, the symptoms of triploidy can be milder and life expectancy can be positively influenced. The karyotype in a mosaic triploid is e.g. B. 69, XXX / 46XX or 69, XXY / 46XY.
Frequency of occurrence in humans
Triploidy usually occurs sporadically (sporadically, randomly) and without familial accumulation. The peculiarity was first described from a scientific point of view in 1960. Since then, over 100 cases with complete triploidy (types I and II) and about 20 cases with mosaic triploidy have been documented. Slightly more male than female fetuses are affected.
Triploidy (mostly digynia / type II) can be found in around 1 in 50,000 live born babies .
However, the general frequency of occurrence is estimated to be 1 to 3: 100. These figures also include those fetuses that were rejected as early miscarriages by the pregnant woman's body in the very early stages of pregnancy . About three out of 25 spontaneous miscarriages are due to triploidy in the unborn child. In most cases, early rejected fetuses show type I triploidy as a result of double fertilization by dispermia (two sperm have fertilized one egg), whereas type II (digynia) can be detected more often in miscarriages at a comparatively late point in time .
Triploidy can be caused by the following features:
- During the fertilization of an egg cell it can happen in rare cases that a so-called double fertilization occurs, that is, that two sperm cells combine with the same egg cell.
- At the fertilization one can egg or sperm cell with diploid be involved chromosomes ( meiosis -II Disorder).
- A polar body may have been included in the fertilized egg cell .
- An unusual zygotic or postzygotic cell division of the fertilized egg cell may have taken place.
Features and diagnosis
During pregnancy , the following features can indicate triploidy in the unborn child, although not all symptoms occur in all children or are present to the same extent:
- Overweight, enlarged, hydatiform degenerated placenta ( placenta ) / partially moles (with triploidy of paternal origin) / with triploidy of maternal origin, the placenta is mostly developed as usual.
- abnormally low amount of amniotic fluid ( oligohydramnios or anhydramnios )
- increased level of alpha-1-fetoprotein (AFP) and / or low level of PAPP-A in maternal blood
- mostly asymmetrical growth retardation
- a comparatively small head ( microcephaly ) or a comparatively large head ( macrocephaly ), brachycephaly
- extensive lack of convolutions between the two large hemispheres of the brain ( agyria , bar agenesis )
- dilated lateral cerebral ventricles
- Expansion of the renal pelvis system (pyelectasia)
- Heart defects (cardiovascular changes, often atrioventricular septal defects)
- White spots in the heart (golf ball phenomenon)
- single umbilical artery ( singular umbilical artery )
- echogenic intestinal loops
- comparatively short tubular bones on the thigh (femur)
- Arnold Chiari malformation
- Spina bifida aperta
- Cleft palate / cleft lip and palate
- Holoprosencephaly (developmental disorder of the forebrain and face due to a mistake in separating the brain into two hemispheres )
- Ring and middle fingers have grown together (= syndactyly 3/4) / peridactyly
- Omphalocele (umbilical cord rupture: the umbilical cord on the baby is inflated like a bag and abdominal organs protrude through the navel / in approx. 25% of cases, with typically only the intestine in the hernial sac.)
- Unusually pronounced accumulation of fluid in the neck area (high neck transparency )
In babies who survive pregnancy and childbirth or are stillborn in a stage that allows a physical examination of the child, the following particularities can be determined, although not all symptoms in all children or all symptoms in all children to the same extent are:
- eyes that are comparatively far apart ( hypertelorism )
- comparatively small eyes ( microphthalmia )
- Malformation of the iris and / or choroid and / or the optic nerve in the eye ( coloboma )
- specially shaped (dysplastic) auricles
- unusually large red blood cells in the blood of the child
A presumptive diagnosis can prenatally (= prenatally ) including through appropriate ultrasound findings ( sonographic soft markers , in particular peculiarities of the placenta , physical malformations , any existing syndactyly 3/4), as well as serological soft markers such due to unusual blood tests of pregnant women (. As increased α1- Fetoprotein).
A reliable diagnosis can be made through a chromosome analysis , for example as part of an amniocentesis . A positive result prompts most pregnant women or parents-to-be to have an abortion carried out for medical reasons, especially since most children would not be viable in the long term, even if they were born alive.
Most babies are not viable in the long term. They usually die intrauterine (= within the uterus ) and are born dead as a miscarriage or stillbirth , or they are born severely underdeveloped (immature) and with severe malformations and usually die shortly after delivery.
A life expectancy of several months is sometimes given in digynia (triploidy type II), that is, two of the three sets of chromosomes must come from the maternal side. In mosaic triploidy, too, i.e. if only some of the body cells have the triploid chromosome set of 69 and the other part has the usual chromosome set of 46, the symptoms can be somewhat milder, depending on the proportion of normally equipped cells, which affects the lifespan can positively influence. Cases have been documented of people with mosaic triploidy reaching adulthood. They mostly have an asymmetrical build.
No form of triploidy is causally curable. Babies who are born alive and who receive medical and social care often survive longer than babies who are denied adequate care.
Triploidy usually occurs sporadically (sporadically, accidentally); no noticeable familial clusters are known. The probability of a triploidy is only 2% higher than the average probability if the pregnant woman or one of her blood relatives was previously pregnant with a child with triploidy.
Triploidy in the animal kingdom
In most species it leads to sterility. It is common in some animal species. For example, the Batura toad (Bufo pseudoraddei baturae) is always triploid. Triploid pond frogs enable pond frog populations to survive independently . In fish, triploidy is common and not lethal . In some cases, animals (e.g. trout ) are also deliberately bred triploid, often to avoid negative effects on reproductive capacity. Triploid (and therefore not reproductive) grass carp are used, for example, to regulate plant growth in canals. The oyster farming partly used triploid oysters that grow faster and - unlike normal diploid oysters - all year around have the same quality.
Triploidy in plants
Triploidy is not uncommon in some plant groups; triploid plants can arise, for example, when pollen from a tetraploid plant reaches the flower of a diploid plant (or vice versa). The germ cells of the diploid plant (2n) are haploid (n), those of the tetraploid plant (4n) are diploid (2n). This creates a triploid (3n) descendant. Triploid sets of chromosomes often lead to irregularities during meiosis . Many triploid plants are therefore unable to produce seeds themselves. Examples are cultivated varieties of pineapple , dessert bananas , saffron and seedless grapes , but also potatoes and watermelon and apple ( Boskop ).
- indianpediatrics.net: Photo of a baby with triploidy ( Memento from September 29, 2007 in the Internet Archive )
- thefetus.net: 2D ultrasound images of a child with triploidy ( Memento from January 27, 2004 in the Internet Archive )
- centrus.com.br: 2D ultrasound image of a child with triploidy in the 13th week of pregnancy ( Memento from June 17, 2004 in the Internet Archive )
- science. orf.at : vertebrates with three sets of chromosomes ( Memento from May 26, 2015 in the Internet Archive )
- ivf1.com: Karyogram of a girl with triploidy ( Memento from December 6, 2019 in the Internet Archive )
- Genome mutation | Disomy | Monoploidy | Trisomy | Polyploidy | Euploidy | Aneuploidy |