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What is Bardet–Biedl syndrome and explain in detail?
Bardet–Biedl syndrome (BBS) is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized by rod/cone dystrophy, polydactyly, central obesity, hypogonadism, and kidney dysfunction in some cases. Historically, slower mental processing has also been considered a principal symptom but is now not regarded as such.

Signs and symptoms
Bardet–Biedl syndrome is a pleiotropic disorder with variable expressivity and a wide range of clinical variability observed both within and between families.  The most common clinical features are rod–cone dystrophy, with childhood-onset night-blindness followed by increasing visual loss; postaxial polydactyly; truncal obesity that manifests during infancy and remains problematic throughout adulthood; varying degrees of learning disabilities; male hypogenitalism and complex female genitourinary malformations; and renal dysfunction, a major cause of morbidity and mortality.There is a wide range of secondary features that are sometimes associated with BBS: 147–148  including: 153–154 
Strabismus, cataracts, astigmatism, pigmentary retinopathy, poor visual acuity, low vision, and/or blindness caused by an impaired photoreceptor transport mechanism in the retina.
"Brachydactyly, syndactyly of both the hands and feet is common, as is partial syndactyl (most usually between the second and third toes)"
Polyuria/polydipsia (nephrogenic diabetes insipidus)
Ataxia/poor coordination/imbalance
Mild hypertonia (especially lower limbs)
Diabetes mellitus
Hepatic involvement
Anosmia
Auditory deficiencies
Hirschsprung disease and subsequent bowel obstruction has been described.
Hypertrophy of interventricular septum and left ventricle and dilated cardiomyopathy.
Hypogonadism, kidney failure, urogenital sinuses, ectopic urethra, uterus duplex, septate vagina, and hypoplasia of the uterus, ovaries, and fallopian tubes
Speech disorder/delay
Developmental delay, especially of fine and gross motor skills

Relation to other rare genetic disorders
Findings in genetic research published in 2006 have suggested that a large number of genetic disorders, both genetic syndromes and genetic diseases, that were not previously identified in the medical literature as related, may be, in fact, highly related in the genetypical root cause of the widely varying, phenotypically observed disorders.  BBS is one such syndrome that has now been identified to be caused by defects in the cellular ciliary structure.  Thus, BBS is a ciliopathy