In the pattern of mendelian inheritance, X- linked dominant inheritance means that all who inherit the X chromosome will present with the condition. In the case of maternal x-linked inheritance sons and daughters have an equal chance of inheriting the condition as there is a 50% chance of inheriting the defective X chromosome from their mother as opposed to a non affected X chromosome from their father. However in paternal X-linked dominant inheritance, sons will be unaffected as they inherit a Y chromosome from their father while daughters are sure to inherit the defective X-chromosome.

47,XYY syndrome is characterized by an extra copy of the Y chromosome in each of a male’s cells. Although many males with this condition are taller than average, the chromosomal change sometimes causes no unusual physical features. Most males with 47,XYY syndrome have normal production of the male sex hormone testosterone and normal sexual development, and they are usually able to father children.47,XYY syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills. Affected boys can have delayed development of motor skills or hypotonia.Other signs and symptoms of this condition include hand tremors or other involuntary movements (motor tics), seizures, and asthma. Males with 47,XYY syndrome have an increased risk of behavioural, social, and emotional difficulties compared with their unaffected peers. These problems include attention-deficit/hyperactivity disorder (ADHD); depression; anxiety; and autism spectrum disorder.Physical features related to 47,XYY syndrome can include increased belly fat, macrocephaly, macrodontia, flat feet (pes planus), fifth fingers that curve inward (clinodactyly), widely spaced eyes (ocular hypertelorism), and scoliosis. These characteristics vary widely among affected boys and men.

Treacher Collins syndrome is a congenital disorder transmitted in an autosomal dominant fashion. The basic abnormality is the TCOF 1 gene mutation, which leads to a multitude of clinical features, of which the most striking is the symmetrical craniofacial deformities. Important clinical features include conductive deafness, coloboma of lower eyelids, cleft palate, antimongoloid (short and downward slanting) palpebral fissures, deformed or absent ears, sleep apnoea, and airway issues. Hirsutism is not a recognized feature of this syndrome.

Somatropin is a recombinant human growth hormone used to treat growth failure conditions associated with natural growth hormone deficiencies. According to the UK National Institute for Health and Care Excellence, these conditions include, Prader-Willi syndrome, Noonan syndrome, short stature Homeobox (SHOX) gene deficiency, Turner’s syndrome, chronic renal insufficiency and children who are small for gestational age. Treatment of short stature in achondroplasia has not seen any significant changes with somatotropin.

Neurofibromatosis Type 1 (also known as von Recklinghausen disease) does not present with cataracts.The eye findings in NF1 are Lisch’s nodules, which are pigmentary lesions seen on the iris and constitute one of the major diagnostic features in this condition. Note:Interestingly, in another syndrome closely related to it, Neurofibromatosis type 2 (NF-2), cataracts can occur. Early detection in family members may be made by finding lens opacities (both congenital polar cataracts and posterior lenticular opacities). Other options:- Incontinentia pigmenti is an X-linked dominant disorder with pigmentary skin changes, mental retardation and eye involvement in 40% of cases. – Myotonic dystrophy is a triplet-repeat disorder with neurological symptoms and cataracts. – Lowe syndrome (oculo-cerebro-renal syndrome) is an X-linked recessive condition. Males with this X-linked recessive condition have cataracts, hypotonia, mental retardation, generalised aminoaciduria and renal tubular acidosis with hypophosphatemia. – Wilson disease is an inborn error of copper metabolism. The clinical features include hepatic involvement, progressive neurological features, eye involvement, including Kayser-Fleischer rings and cataracts.

Hereditary diseases follow specific inheritance patterns according to the type of gene involved. Mutations in the genes which are on the sex chromosome, chromosome X are called X-linked diseases. X-linked recessive conditions result when both the copies of X-chromosomes are defective, which happens in the case of females since males have only one X- chromosome. Thus, these conditions are more common among males, and examples include colour blindness, haemophilia, Lesch-Nyhan syndrome, and hunter’s syndrome, etc. Hunter’s syndrome is a type of lysosomal storage disease which is also known as mucopolysaccharidosis II. Vitamin D resistant rickets is a X-linked dominant disease. Neurofibromatosis type 1 and 2 are autosomal dominant conditions while Wilson’s disease is autosomal recessive.

Neurofibromatosis type 2, also known as schwannomin, is an autosomal dominant genetic disease caused by mutation of the merlin gene located on chromosome 22. Merlin is a tumour suppressor protein, which is responsible for controlling cell shape, growth and adhesion, and is predominantly found in nervous tissue. Its mutation increases the risk of tumour development especially bilateral vestibular schwannomas, the hallmark of neurofibromatosis 2.

Most cases of Prader-Willi syndrome are not inherited, particularly those caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy. These genetic changes occur as random events during the formation of reproductive cells (eggs and sperm) or in early embryonic development. Affected people typically have no history of the disorder in their family.Rarely, a genetic change responsible for Prader-Willi syndrome can be inherited. For example, it is possible for a genetic change that abnormally inactivates genes on the paternal chromosome 15 to be passed from one generation to the next.Prader-Willi syndrome is a complex genetic condition that affects many parts of the body. In infancy, this condition is characterized by weak muscle tone (hypotonia), feeding difficulties, poor growth, and delayed development. Beginning in childhood, affected individuals develop an insatiable appetite, which leads to chronic overeating (hyperphagia) and obesity. Some people with Prader-Willi syndrome, particularly those with obesity, also develop type 2 diabetes (the most common form of diabetes).

Lesch-Nyhan syndrome is a condition that occurs almost exclusively in males. This condition is inherited in an X-linked recessive pattern It is characterized by neurological and behavioural abnormalities and the overproduction of uric acid. Uric acid is a waste product of normal chemical processes and is found in blood and urine. Excess uric acid can be released from the blood and build up under the skin and cause gouty arthritis (arthritis caused by an accumulation of uric acid in the joints). Uric acid accumulation can also cause kidney and bladder stones.The nervous system and behavioural disturbances experienced by people with Lesch-Nyhan syndrome include abnormal involuntary muscle movements, such as tensing of various muscles (dystonia), jerking movements (chorea), and flailing of the limbs (ballismus). People with Lesch-Nyhan syndrome usually cannot walk, require assistance sitting, and generally use a wheelchair. Self-injury (including biting and head banging) is the most common and distinctive behavioural problem in individuals with Lesch-Nyhan syndrome.

In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome.

Most cases of Trisomy 18, also called Edwards syndrome, are not inherited but occur as random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 18. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body’s cells.Mosaic trisomy 18 is also not inherited. It occurs as a random event during cell division early in embryonic development. As a result, some of the body’s cells have the usual two copies of chromosome 18, and other cells have three copies of this chromosome.Partial trisomy 18 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 18. Although they do not have signs of trisomy 18, people who carry this type of balanced translocation are at an increased risk of having children with the condition.Canavan disease is a type of leukodystrophy and a degenerative disorder that causes progressive damage to nerve cells in the brain. It is inherited in an autosomal recessive manner.Cystic Fibrosis is the most common lethal recessive disease of white people, with a carrier frequency of 1:25 and is inherited in an autosomal recessive manner.Galactosaemic is autosomal recessive and due to mutations in the GALT gene.Phenylketonuria is the most common inborn error of metabolism in the UK with an incidence of 1:10,000 and a carrier rate of 1:50.

Marfan syndrome is a disorder that affects the connective tissue in many parts of the body. Connective tissue provides strength and flexibility to structures such as bones, ligaments, muscles, blood vessels, and heart valves. The signs and symptoms of Marfan syndrome vary widely in severity, timing of onset, and rate of progression.This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.At least 25 percent of Marfan syndrome cases result from a new mutation in the FBN1 gene. These cases occur in people with no history of the disorder in their family.The two primary features of Marfan syndrome are vision problems caused by a dislocated lens (ectopia lentis) in one or both eyes and defects in the large blood vessel that distributes blood from the heart to the rest of the body (the aorta).Associations include:Tall statureLearning disability (50%)Lens subluxation (usually upwards)Femoral herniaHemivertebraeCardiac problems (Aortic/mitral regurgitation, aneurysms)Joint hypermobilityArachnodactyly

Duchenne muscular dystrophy has an X-linked recessive pattern of inheritance. Since the first son is affected by the disease, it means that the mother is a carrier. The male children will inherit the Y chromosome from their father and the X chromosome from their mother, having 50% chances of inheriting the X chromosome with the affected gene.

Cystic fibrosis has an autosomal recessive pattern of inheritance, meaning that a person might be a carrier of the disease without developing it. If the unaffected partner is a carrier, then there is a 50% chance of inheritance and another 50% chance of having a child who is a carrier. However, if the partner is not a carrier, the offspring will not develop the disease but the possibility of being a carrier raises up to 100%.

In the pattern of mendelian inheritance, X- linked dominant inheritance means that all who inherit the X chromosome will present with the condition. In the case of maternal x-linked inheritance, sons and daughters have an equal chance of inheriting the condition as there is a 50% chance of inheriting the defective X chromosome from their mother as opposed to a non affected X chromosome from their father. However, in paternal X-linked dominant inheritance, sons will be unaffected as they inherit a Y chromosome from their father while daughters are sure to inherit the defective X-chromosome.

Cystic fibrosis is a progressive, genetic disease. A defective gene causes a thick, sticky build-up of mucus in the lungs, pancreas, and other organs. In the lungs, the mucus clogs the airways and traps bacteria leading to infections, extensive lung damage, and eventually, respiratory failure. In the pancreas, the mucus prevents the release of digestive enzymes that allow the body to break down food and absorb vital nutrients. Since the digestive enzymes are not being made, the food is not completely digested making them bulky, smelly, and hard to flush away. Finger clubbing is a prominent feature of cystic fibrosis on most patients.

Pierre Robin sequence is a condition present at birth, in which the infant has micrognathia, a tongue that is placed further back than normal (glossoptosis), and cleft palate. This combination of features can lead to difficulty breathing and problems with eating early in life. Pierre Robin sequence may occur isolated or be associated with a variety of other signs and symptoms (described as syndromic). The exact causes of Pierre Robin syndrome are unknown. The most common otic anomaly is otitis media, occurring 80% of the time, followed by auricular anomalies in 75% of cases. Hearing loss, mostly conductive, occurs in 60% of patients, while external auditory canal atresia occurs in only 5% of patients.

CF is inherited in an autosomal recessive manner.This means that to have CF, a person must have a mutation in both copies of the CFTR gene in each cell. People with CF inherit one mutated copy of the gene from each parent, who is referred to as a carrier.CF carriers can pass their copy of the CFTR gene mutation to their children. Each time two CF carriers have a child together, the chances are: 25 percent (1 in 4) the child will have CF50 percent (1 in 2) the child will be a carrier but will not have CF25 percent (1 in 4) the child will not be a carrier of the gene and will not have CFThis male’s parents are obligate carrier because he has a sibling with CFSo the chances he is a carrier too are 2 in 3The risk of having an affected child is the chance of both himself and his partner being carriers: (2/3 x 1/25 = 2/75) x 1/4 = 1/150.

Individuals with Down syndrome are at increased risk for a variety of eye and vision disorders. Fortunately, many of these eye problems can be treated, especially if discovered at an early age. The quality of life can be further enhanced by the proper assessment and correction of eye problems. The most common eye findings include:- Refractive errors – Children with Down syndrome are more likely to need glasses than are other children. This may be due to myopia (near-sightedness), hyperopia (far-sightedness), and/or astigmatism. Refractive error may develop early in life or later on.- Strabismus – Between 20% and 60% of individuals with Down syndrome have eyes that are misaligned (strabismus). Esotropia (eyes that drift in) is most common while exotropia(eyes that drift out) occurs less frequently. Strabismus may be treated with glasses, patching and/or eye muscle surgery.- Keratoconus – A cone shaped distortion of the cornea (front layer of the eye), occurs in up to 30% of those with Down syndrome. Keratoconus is usually diagnosed around puberty and should be monitored regularly. Blurred vision, corneal thinning, or corneal haze may result from keratoconus. Keratoconus is worsened by eye rubbing; therefore, eye rubbing should be discouraged.- Cataracts – There is an increased incidence of congenital cataracts (present at birth) as well as acquired cataracts (develop later). Cataracts may progress slowly and should be monitored regularly, with surgical treatment performed when appropriate.- Glaucoma- There is an increased risk of infantile glaucoma (elevated pressure within the eye).- Blepharitis – Inflammation of the eyelids with redness at the edge of the lids and crusting around the lashes may occur and cause a feeling of dryness or burning. Treatment is with eyelid hygiene and topical antibiotics.- Tearing – Excessive tears or watering of the eyes may occur because the drainage channels are blocked or narrow (nasolacrimal duct obstruction). This may require surgical intervention.- Nystagmus – This is an involuntary back-and-forthť movement or shaking of the eyes. It can affect vision to a mild or severe degree.

Ataxic gait is not a direct association of Down’s syndrome. All other given responses are associated with Down’s syndrome.

Advanced paternal age is associated with poor quality sperm leading to a reduction in overall fertility along with an increase in the occurrence of various medical disorders in the fetus. Advanced paternal age has been implicated as a risk factor for diseases like retinoblastoma, Marfan’s syndrome, neurofibromatosis, achondroplasia, autism spectrum disorders, schizophrenia, and acute lymphoblastic leukaemia. Down’s syndrome is mostly considered a consequence of advanced maternal age, but advanced paternal age has also been implicated as a risk factor. Turner syndrome is not found to be caused by advanced paternal age.

These cardiovascular findings are suggestive of coarctation of the aorta, which is common among patients with Turner’s syndrome. Physical features of Turner’s syndrome include a short neck with a webbed appearance, a low hairline at the back of the head, low-set ears, and narrow fingernails and toenails that are turned upward.Atrioventricular septal defect is the most common cardiac anomaly in Down’s syndrome. Structural heart defects at birth (i.e., ventricular septal defect, atrial septal defect, patent ductus arteriosus) are common in Edward’s syndrome. Pulmonary stenosis with or without dysplastic pulmonary valve and hypertrophic cardiomyopathy are common in Noonan’s syndrome. Supravalvar aortic stenosis and peripheral pulmonary stenosis are found in William’s syndrome.

Trisomy 21 (Down’s syndrome)is a genetic disorder characterized by the presence of an extra copy of chromosome 21 in all the body cells, mostly due to non-disjunction during gametogenesis. Robertsonian translocation occurs only in about 2-4% of the cases. A variety of clinical features are found in trisomy 21 due to multi-system involvement but the hallmark is hypotonia, which is present in almost all the cases of trisomy 21. Different types of leukemias are also found in association with down’s syndrome, but they are not the hallmark. Atlantoaxial subluxation is secondary to hypotonia, which induces joint and ligament laxity. Other clinical findings are hypothyroidism, Alzheimer’s disease, Hirschsprung’s disease, and pulmonary hypertension.

Tuberous sclerosis is an autosomal dominant neurocutaneous condition associated with increased psychiatric co-morbidity. It results from the mutation of TSC1 or TSC2 tumour suppressor genes. Multiple benign tumours of the brain and other vital organs are characteristic of this disease. Important findings include subependymal nodules, bone cysts, cardiac rhabdomyoma, and learning difficulties. Lisch nodules of the iris occur in neurofibromatosis.

Haemophilia is a X-linked recessive disorder. In X-linked recessive inheritance only males are affected. An exception to this seen in examinations are patients with Turner’s syndrome, who are affected due to only having one X chromosome. X-linked recessive disorders are transmitted by heterozygote females (carriers) and male-to-male transmission is not seen. Affected males can only have unaffected sons and carrier daughters.

Types of DNA mutations and their impactpoint mutation:Substitution: One base is incorrectly added during replication and replaces the pair in the corresponding position on the complementary strand as in Sickle cell anaemiaInsertion: One or more extra nucleotides are inserted into replicating DNA, often resulting in a frameshift as in one form of beta-thalassemiaDeletion: One or more nucleotides is skipped during replication or otherwise excised, often resulting in a frameshift as in Cystic fibrosis.Chromosomal mutation:Inversion: One region of a chromosome is flipped and reinserted as in Opitz-Kaveggia syndromeDeletion: A region of a chromosome is lost, resulting in the absence of all the genes in that area as in Cri du chat syndromeDuplication: A region of a chromosome is repeated, resulting in an increase in dosage from the genes in that region as in some cancers and Charcot-Marie toothTranslocation: A region from one chromosome is aberrantly attached to another chromosome as in One form of leukaemiaCopy number variation:Gene amplification: The number of tandem copies of a locus is increased as in Some breast cancersExpanding trinucleotide repeat: The normal number of repeated trinucleotide sequences is expanded as in Fragile X syndrome, Huntington’s disease

Rett syndrome is a disorder that affects the brain and occurs most commonly in girls. It presents with a period of normal development followed by severe problems with language and communication, learning, coordination, and other brain functions.

Marfan syndrome is a disorder that affects the connective tissue found throughout the body, Marfan syndrome can affect many systems, often causing abnormalities in the heart, blood vessels, eyes, bones, and joints. The two primary features of Marfan syndrome are vision problems caused by a dislocated lens (ectopia lentis) in one or both eyes and aortic root disease, leading to aneurysmal dilatation, aortic regurgitation and dissection is the main cause of morbidity and mortality in Marfan syndrome.

The physical features and developmental delay are the key aspects in the given scenario. The child in question shows features of gross motor and social developmental delay and has physical features indicative of Prader-Willi syndrome (hypopigmentation, esotropia, disproportionately small hands and feet, loss of central muscle tone and undescended testes). Children with Prader-Willi syndrome can present with failure to thrive until ,12-18 months, at which point, hyperphagia and obesity become more prominent. Other options:- Although Klinefelter syndrome can present with delayed development, undescended/small testes and reduced muscle power, the presence of small hands/feet, hypopigmentation and failure to thrive are not characteristic features. – Marfan syndrome presents with different physical features (arachnodactyly, cardio-respiratory complications and skin changes, amongst others) than those associated with Prader-Willi syndrome. – DiGeorge can manifest with developmental delay, hypotonia and feeding difficulties. However, this clinical scenario does not report any of the typical facial features, hearing abnormalities or cardiac abnormalities that are typically caused by DiGeorge syndrome. – Russell-Silver syndrome can cause developmental abnormalities, poor muscle tone and power (poor head control and muscle function), feeding difficulties and poor growth during the post-natal period and infancy. However, characteristic facial (small, triangular face, blue sclerae) and skeletal abnormalities (limb asymmetry, finger abnormalities) are not present. Therefore, Prader-Willi syndrome is the most appropriate diagnosis for this patient.Note:Prader-Willi syndrome is an example of genetic imprinting where the phenotype depends on whether the deletion occurs on a gene inherited from the mother or father: In Prader-Willi syndrome, it is the paternal gene that is deleted from the long arm of chromosome 15, while in Angelman syndrome the maternal gene is deleted. Prader-Willi syndrome can occur due to the microdeletion of paternal 15q11-13 (70% of cases) maternal uniparental disomy of chromosome 15.

The autosomal dominant pattern of inheritance involves the presence of at least one of the defected copies of the gene in an individual to develop a specific diseased condition. Examples of this mode of inheritance include myotonic dystrophy, neurofibromatosis, achondroplasia, etc. Still’s disease, also called juvenile idiopathic arthritis, is not a heritable disease. Tay-Sachs disease is an autosomal recessive disease., while spinocerebellar ataxia and Friedreich’s ataxia are caused by trinucleotide repeat expansion.