Diprosopus (Greek , "two-faced", from , ', "two" and , ' [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek , "skull", the other parts Latin), is an extremely rare congenital disorder whereby parts (accessories) or all of the face are duplicated on the head.

Sirenomelia, alternatively known as Mermaid syndrome, is a rare congenital deformity in which the legs are fused together, giving them the appearance of a mermaid's tail as the nickname suggests.

This condition is found in approximately one out of every 100,000 live births (about as rare as conjoined twins) and is usually fatal within a day or two of birth because of complications associated with abnormal kidney and urinary bladder development and function. More than half the cases of sirenomelia result in stillbirth and this condition is 100 times more likely to occur in identical twins than in single births or fraternal twins. It results from a failure of normal vascular supply from the lower aorta in utero. Maternal diabetes has been associated with caudal regression syndrome and sirenomelia, although a few sources question this association.

VACTERL-H is an expanded form of the VACTERL association that concludes that this diagnosis is a less severe form of sirenomelia. The disorder was formerly thought to be an extreme case of caudal regression syndrome; however, it was reclassified to be considered a separate condition.

Only a few individuals who did not have fatal kidney and bladder complications are known to have survived beyond birth with this condition.

The condition has an incidence of 1 in every 500 live births. Postaxial hand polydactyly is a common isolated disorder in African black children, and autosomal dominant transmission is suspected. Postaxial polydactyly is more frequent in native Africans living in the Eastern and Central than the Caucasians and Mongoloids and is more frequent in male children. In contrast, postaxial polydactyly seen in white children is usually syndromic and associated with an autosomal recessive transmission. One study by Finley et al. combined data from Jefferson County, Alabama, United States and Uppsala County, Sweden. This study showed incidence of all types of polydactyly to be 2.3 per 1000 in Caucasian males, 0.6 per 1000 in Caucasian females, 13.5 per 1000 in African males, and 11.1 per 1000 in African females.

Ectrodactyly can be caused by various changes to 7q. When 7q is altered by a deletion or a translocation ectrodactyly can sometimes be associated with hearing loss. Ectrodactyly, or Split hand/split foot malformation (SHFM) type 1 is the only form of split hand/ malformation associated with sensorineural hearing loss.

A large number of human gene defects can cause ectrodactyly. The most common mode of inheritance is autosomal dominant with reduced penetrance, while autosomal recessive and X-linked forms occur more rarely. Ectrodactyly can also be caused by a duplication on 10q24. Detailed studies of a number of mouse models for ectrodactyly have also revealed that a failure to maintain median apical ectodermal ridge (AER) signalling can be the main pathogenic mechanism in triggering this abnormality.

A number of factors make the identification of the genetic defects underlying human ectrodactyly a complicated process: the limited number of families linked to each split hand/foot malformation (SHFM) locus, the large number of morphogens involved in limb development, the complex interactions between these morphogens, the involvement of modifier genes, and the presumed involvement of multiple gene or long-range regulatory elements in some cases of ectrodactyly. In the clinical setting these genetic characteristics can become problematic and making predictions of carrier status and severity of the disease impossible to predict.

In 2011, a novel mutation in DLX5 was found to be involved in SHFM.

Ectrodactyly is frequently seen with other congenital anomalies. Syndromes in which ectrodactyly is associated with other abnormalities can occur when two or more genes are affected by a chromosomal rearrangement. Disorders associated with ectrodactyly include Ectrodactyly-Ectodermal Dysplasia-Clefting (EEC) syndrome, which is closely correlated to the ADULT syndrome and Limb-mammary (LMS) syndrome, Ectrodactyly-Cleft Palate (ECP) syndrome, Ectrodactyly-Ectodermal Dysplasia-Macular Dystrophy syndrome, Ectrodactyly-Fibular Aplasia/Hypoplasia (EFA) syndrome, and Ectrodactyly-Polydactyly. More than 50 syndromes and associations involving ectrodactyly are distinguished in the London Dysmorphology Database.

Diphallia, penile duplication (PD), diphallic terata, or diphallasparatus, is a rare developmental abnormality in which a male is born with two penises. The first reported case was by Johannes Jacob Wecker in 1609. Its occurrence is 1 in 5.5 million boys in the United States.

When diphallia is present, it is usually accompanied by renal, vertebral, hindgut, anorectal or other congenital anomalies. There is also a higher risk of spina bifida. Infants born with PD and its related conditions have a higher death rate from various infections associated with their more complex renal or colorectal systems.

It is thought diphallia occurs in the fetus between the 23rd and 25th days of gestation when an injury, chemical stress, or malfunctioning homeobox genes hamper proper function of the caudal cell mass of the fetal mesoderm as the urogenital sinus separates from the genital tubercle and rectum to form the penis.

Although classically considered conjoined twinning (which it resembles), this anomaly is not normally due to the fusion or incomplete separation of two embryos. It is the result of abnormal activity by the protein SHH (sonic hedgehog).

SHH and its corresponding gene have been found to play an important role in signaling craniofacial patterning during embryonic development. Among other things, SHH governs the width of facial features. In excess it leads to widening of facial features and to duplication of facial structures. The greater the widening, the more structures are duplicated, often in a mirror image form. This has been demonstrated in the laboratory by introducing pellets of the SHH protein into chicken embryos, resulting in chickens with duplicate beaks. Inadequate amounts of that protein lead to opposite conditions such as cyclopia where facial features are insufficiently developed.

Healthy brain development is also dependent on the signaling function of SHH. During embryonic development, SHH directs embryonic cells to organize in specific areas that later become specialized neural tissues, thus controlling the size and shape of brain structures.

These lesions usually present in neonates, although they may not come to clinical attention until adulthood (for cosmetic reasons). There is no gender predilection. They are present in approximately 3-6 per 1000 live births.

Only ten cases of craniopagus parasiticus have been reported in the medical research literature. Of those cases, only three have survived birth. The first case on record is that of Everard Home's Two-Headed Boy of Bengal, whose skull is preserved at the Hunterian Museum at the Royal Society of Surgeons.

Type VII of radial polydactyly is associated with several syndromes:

Holt–Oram syndrome, Fanconi anemia (aplastic anemia by the age of 6), Townes–Brocks syndrome, and Greig cephalopolysyndactyly (also known to occur with ulnar polydactyly).

With so few individuals actually surviving until birth, the only treatment option is surgery to try to remove the parasitic twin. Surgery, however, is very dangerous and has been successful only once. The problem with surgical intervention is that the arterial supplies of the head are so intertwined that it is very hard to control the bleeding, and it has been suggested that cutting off the parasitic twin's arterial supply might improve the odds of the developed twin's survival.

It has been said by many dog owners that limber tail had been caused shortly (24 hours) after swimming in water that is too cold or on rare occasions too warm and indeed this has certainly produced this very condition. The actual cause is unknown but it may be caused by the narrowing of the space through which the spinal cord passes, typically due to degenerative change to the intervertebral disk spaces. These underlying changes may not lead to visible change until the problem is suddenly exacerbated, such as during physical activity, after trauma, etc. Occasionally other changes are seen prior to or in conjunction with limber tail disease, such as urinary or fecal incontinence, postural abnormalities in the pelvic limb, or pain in response to touching the lower back.

Caudal regression syndrome or sacral agenesis (or hypoplasia of the sacrum) is a congenital disorder in which there is abnormal fetal development of the lower spine—the caudal partition of the spine.

It occurs at a rate of approximately one per 25,000 live births.

The condition arises from some factor or set of factors present during approximately the 3rd week to 7th week of fetal development. Formation of the sacrum/lower back and corresponding nervous system is usually nearing completion by the 4th week of development. Due to abnormal gastrulation, the mesoderm migration is disturbed. This disturbance results in symptoms varying from minor lesions of the lower vertebrae to more severe symptoms such as complete fusion of the lower limbs. While the exact cause is unknown, it has been speculated that the condition may be associated with certain dietary deficiencies including a lack or insufficient amounts of folic acid.

Sacral agenesis syndrome (agenesis of the lumbar spine, sacrum, and coccyx, and hypoplasia of the lower extremities) is a well-established congenital anomaly associated with maternal diabetes mellitus (not gestational diabetes). However, other causes are presumably involved, as demonstrated by the rare incidence of caudal regression syndrome (1:60,000) compared to diabetes.

The dominant inherited sacral agenesis (also referred to as Currarino syndrome) is very often correlated with a mutation in the Hb9 (also called HlxB9) gene (shown by Sally Ann Lynch, 1995, Nature Genetics).

It may be the cause of sirenomelia ("Mermaid syndrome").

An accessory auricle is considered a developmental anomaly resulting from the persistence of a structure which variably recapitulates the normal external ear.

Opitz G/BBB Syndrome is a rare genetic condition caused by one of two major types of mutations: MID1 mutation on the short (p) arm of the X chromosome or a mutation of the 22q11.2 gene on the 22nd chromosome. Since it is a genetic disease, it is an inherited condition. However, there is an extremely wide variability in how the disease presents itself.

In terms of prevention, several researchers strongly suggest prenatal testing for at-risk pregnancies if a MID1 mutation has been identified in a family member. Doctors can perform a fetal sex test through chromosome analysis and then screen the DNA for any mutations causing the disease. Knowing that a child may be born with Opitz G/BBB syndrome could help physicians prepare for the child’s needs and the family prepare emotionally. Furthermore, genetic counseling for young adults that are affected, are carriers or are at risk of carrying is strongly suggested, as well (Meroni, Opitz G/BBB syndrome, 2012). Current research suggests that the cause is genetic and no known environmental risk factors have been documented. The only education for prevention suggested is genetic testing for at-risk young adults when a mutation is found or suspected in a family member.

With rest, the tail returns to normal within a few days. Pain relief, such as a nonsteroidal anti-inflammatory drug may be administered. The symptoms may reoccur.

It is likely that this syndrome is inherited in an autosomal dominant fashion, however there may be a recessive form with hypotonia and developmental delay.

The original report was of a family in Cardiff, United Kingdom. There are subsequent reports of patients from the USA, France, Australia, UAE, India and from Cuba.

Occurring at a rate between 1 in 10,000 to 1 in 50,000 with a male-to-female ratio of 2.3-6:1, bladder exstrophy is relatively rare. For those individuals with bladder exstrophy who maintain their ability to reproduce, the risk of bladder exstrophy in their children is approximately 500-fold greater than the general population.

The Wassel classification is used to categorise radial polydactyly, based upon the most proximal level of skeletal duplication.

Duane-radial ray syndrome is caused by mutations in the "SALL4" gene which is a part of a group of genes called the SALL family. This gene plays an important role in embryonic development by providing instructions to make proteins that are involved in the formation of tissues and organs. SALL proteins act as transcription factors in that they attach themselves to certain regions in DNA in order to help control certain gene activities. Due to the mutations in the "SALL4" gene, proteins can not be made because one copy of the gene in each cell is stopped from performing its duty. These mutations are heterozygous and can be nonsense, short duplications, or deletions. At this time, there is no clear reason as to why a reduced amount of the SALL4 protein causes the symptoms of Duane-radial ray syndrome and similar conditions.

Duane-radial ray syndrome is inherited through autosomal dominance meaning that a mutation in one copy of the SALL 4 gene is all it takes to cause this syndrome. Those with this condition can have affected parents, but it can also manifest for the first time with no family history which is called de novo. Since Duane-radial ray syndrome is an autosomal dominant disorder, there is a 50% chance of passing the mutation on to offspring.

Tsukuhara syndrome is an infrequently occurring skeletal dysplasia characterised by a caudal synostosis of the vertebra at birth.

Sugarman syndrome is the common name of autosomal recessive oral-facial-digital syndrome type III, one of ten distinct genetic disorders that involve developmental defects to the mouth.

Alternative names for this condition include: Brachydactyly of the hands and feet with duplication of the first toes, Sugarman brachydactyly and Brachydactyly with major proximal phalangeal shortening.