Genetic Abnormalities

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Turner Syndrome

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Turner syndrome or Ullrich-Turner syndrome encompasses several conditions, of which monosomy X is the most common. It is a chromosomal disorder affecting females in which all or part of one of the X chromosomes is absent. Occurring in 1 out of every 2500 girls, the syndrome manifests itself in a number of ways. There are characteristic physical abnormalities, such as short stature, lymphoedema, broad chest, low hairline, low-set ears, and webbed neck. Girls with TS typically experience gonadal dysfunction with subsequent amenorrhea and infertility. Concurrent health concerns are also frequently present, including congenital heart disease, hypothyroidism, ophthalmological problems, and otological concerns. Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematic, and memory areas. Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. This may also manifest itself as a difficulty with motor control or with mathematics. While it is non-correctable, in most cases it does not cause difficulty in daily living.

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Klinefelter Syndrome

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Klinefelter's syndrome, 47,XXY or XXY syndrome is a condition caused by a chromosome aneuploidy. Affected individuals have at least two X chromosomes and at least one Y chromosome.[1]
The principal effects are development of small testicles and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms. The second most common extra chromosome condition.

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Fragile-X Syndrome

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Fragile X syndrome, or Martin-Bell syndrome, is a genetic syndrome which results in a spectrum (from none to severe) of characteristic physical, intellectual, emotional and behavioural features. The syndrome is associated with the expansion (excessive repetition) of a single trinucleotide gene sequence (CGG) on the X chromosome, and results in a failure to express the FMR-1 protein which is required for normal neural development. There are four generally accepted forms of Fragile X syndrome which relate to the length of the repeated CGG sequence; Normal (29-31 CGG repeats), Premutation (55-200 CGG repeats), Full Mutation (more than 200 CGG repeats), and Intermediate or Gray Zone Alleles (40 - 60 repeats).

Aside from intellectual disability, prominent characteristics of the syndrome include an elongated face, large or protruding ears, flat feet, larger testicles in men (macroorchidism), and low muscle tone. Speech may include cluttered speech or nervous speech[7]. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) and atypical social development, particularly shyness and limited eye contact. Some individuals with the fragile X syndrome also meet the diagnostic criteria for autism. Most females experience symptoms to a lesser degree because of their second X-chromosome, however they can develop just as severe symptoms. While full mutation males tend to present with severe intellectual disability, the symptoms of full mutation females run the gamut of minimally affected to severe intellectual disability, which may explain why females are underdiagnosed relative to males.

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Down Syndrome

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Down syndrome, Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome.

Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate developmental disabilities. A small number have severe to profound mental disability. The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births.

Many of the common physical features of Down syndrome also appear in people with a standard set of chromosomes. They may include a single transverse palmar crease (a single instead of a double crease across one or both palms, also called the Simian crease), an almond shape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebral fissures (the separation between the upper and lower eyelids), shorter limbs, poor muscle tone, a larger than normal space between the big and second toes, and protruding tongue.

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Huntington's Disease

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Huntington's disease, also called Huntington's chorea, chorea major, or HD, is a genetic neurological disorder characterized after onset by uncoordinated, jerky body movements called chorea and a decline in some mental abilities, which can lead to affected aspects of behavior. As the disorder progresses, these symptoms can cause complications that significantly reduce life expectancy.

Huntington's disease is one of several trinucleotide repeat disorders, caused by the length of a repeated section of a gene exceeding the normal range. The huntingtin gene (HTT) normally provides the information to produce Huntingtin protein, but when affected, produces mutant Huntingtin (mHTT) instead. The presence of mHTT causes an increase in the rate of neuronal cell death in select areas of the brain, affecting neurological functions.

The Huntingtin gene (HTT) is located on the short arm of chromosome 4 (4p16.3). HTT contains a sequence of three DNA bases—cytosine-adenine-guanine (CAG)—repeated multiple times. The polyQ region contains fewer than 36 glutamines results in production of the cytoplasmic protein called huntingtin (HTT). In the general population this count rarely exceeds 27 glutamines. A sequence of 36 or more glutamines, however, results in the production of form of HTT which has different characteristics, this form is called mutant HTT or more commonly mHTT. The presence of mHTT increases the rate of neuronal decay in select types of neurons in various regions of the brain. Generally, the number of CAG repeats is related to how much this process is affected, and correlates with age at onset and the rate of progression of symptoms.

The areas affected are mainly in the striatum, but also the frontal and temporal cortices.[57] The striatum's subthalamic nuclei send control signals to the globus pallidus, which initiates and modulates motion. The weaker signals from subthalamic nuclei thus cause reduced initiation and modulation of movement, resulting in the characteristic movements of the disorder.[