Trisomy 21 is not inherited Instead it is due to a nondis junction that is a

Trisomy 21 is not inherited instead it is due to a

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Trisomy 21 is not inherited. Instead, it is due to a nondis- junction —that is, a failure of the chromosome to separate during meiosis. In most cases, the mother contributes two chromosomes instead of one, but cases of paternal nondisjunction have also been reported. Down syndrome can also occur when the child inherits one chromosome 21 from each parent and an abnormally fused chromosome (usually consisting of chro- mosomes 21 and 15) from one of the parents. This abnor- mality, called a translocation , results in additional genetic material passed on to the child. It occurs in approximately 3% of youths with Down syndrome. Down syndrome caused by translocation is inherited. Usually, the parents are unaffected carriers of abnormally fused chromosomes, and they unknowingly pass them on to their children. Finally, Down syndrome can occur when some cells fail to separate during mitosis. This causes the child to have some normal cells and some cells with an abnormal amount of genetic information. The mix of normal and abnormal genetic information is called chromosomal mosa- icism . Just as a mosaic is made up of different colored tiles, people with chromosomal mosaicism have cells of differ- ent genetic makeups. Chromosomal mosaicism accounts for approximately 2% of cases of Down syndrome. Children with Down syndrome have characteristic facial features including flattened face, slanting eyes, wide nasal bridge, and low-set ears (see Image 5.4). Other phys- ical features include short stature and poor muscle tone. Children with Down syndrome show small overall brain size and fewer folds and convolutions than in the brains of typically developing children. Fewer folds suggest reduced surface area of the cortex and may be partially responsible for low intelligence (Key & Thornton-Wells, 2012). Children with Down syndrome are almost always diagnosed with ID; few of these children earn IQ scores greater than 60. Cognitive development progresses in a typical fashion for the first few months of life. After the child’s first birthday, however, intellectual development slows and falls further behind typically developing peers. Figure 5.2 The Causes of Intellectual Disability. Genetic and metabolic disorders explain 50% of cases. Genetic Disorders 40% Metabolic Disorders 10% Teratogens 10% Perinatal Problems or Injury 10% Unknown Causes 30% “ORGANIC” “CULTURAL-FAMILIAL” Source: Based on Toth, deLacy, and King (2015). Copyright ©2018 by SAGE Publications, Inc. This work may not be reproduced or distributed in any form or by any means without express written permission of the publisher. Do not copy, post, or distribute
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126 PART II DEVELOPMENTAL DISORDERS AND DISABILITIES Young children with Down syndrome are usually described as happy, social, and friendly. It is extremely rewarding to volunteer as a tutor for a child with Down syndrome because these children are often socially out- going and affectionate. Children with Down syndrome are less likely to develop psychiatric disorders than other children with ID (Dykens et al., 2006). However, in ado-
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  • T. Keneni
  • Intelligence quotient, SAGE Publications, Developmental disability, Mental retardation

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