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Chromosomal abnormalities are the result of errors during cell reproduction, meiosis or mitosis, or damage caused afterward. Occurring in about 1 of every 1,500 births, the most widely known chromosome disorder is trisomy 21, more commonly called Down syndrome (de Graaf, Buckley, Dever, & Skotko, 2017; Morrison & McMahon, 2018). Down syndrome occurs when a third chromosome appears alongside the 21st pair of chromosomes. Down syndrome is associated with marked physical, health, and cognitive attributes, including a short, stocky build, and striking facial features mark the disorder, such as a round face, almond-shaped eyes, and a flattened nose, as shown in Figure 2.5 (Davis & Escobar, 2013; Kruszka et al., 2017). Children with Down syndrome tend to show delays in physical and motor development relative to other children and health problems, such as congenital heart defects, vision impairments, poor hearing, and immune system deficiencies (Ram & Chinen, 2011; Zampieri et al., 2014). Down syndrome is the most common genetic cause of intellectual developmental disability (Vissers, Gilissen, & Veltman, 2016), but children’s abilities vary. Generally, children with Down syndrome show greater strengths in nonverbal learning and memory relative to their verbal skills (Grieco, Pulsifer, Seligsohn, Skotko, & Schwartz, 2015). Expressive language is delayed relative to comprehension. Infants and children who participate in early intervention and receive sensitive caregiving and encouragement to explore their environment show positive outcomes, especially in the motor, social, and emotion areas of functioning (Næss, Nygaard, Ostad, Dolva, & Lyster, 2017; Wentz, 2017).

Description

Figure 2.5 Down Syndrome

Advances in medicine have addressed many of the physical health problems associated with Down syndrome so that today, the average life expectancy is 60 years of age, compared with about 25 in the 1980s (Glasson, Dye, & Bittles, 2014; National Association for Down Syndrome, 2017). However, Down syndrome is associated with premature aging and an accelerated decline of cognitive functioning (Covelli, Raggi, Meucci, Paganelli, & Leonardi, 2016; Ghezzo et al., 2014). Individuals with Down syndrome are at risk to show signs of Alzheimer’s disease very early relative to other adults (Hithersay, Hamburg, Knight, & Strydom, 2017; Wiseman et al., 2015). This is an example of how disorders and illnesses can be influenced by multiple genes and complex contextual interactions; in this case, Down syndrome and Alzheimer’s disease share genetic markers (Lee, Chien, & Hwu, 2017).

Some chromosomal abnormalities concern the 23rd pair of chromosomes: the sex chromosomes. These abnormalities result from either an additional or missing sex chromosome. Given their different genetic makeup, sex chromosome abnormalities yield different effects in males and females. They are summarized in Table 2.5.

Table 2.5

Source: Ammerman et al. (2015); McKusick-Nathans Institute of Genetic Medicine (2019); Pappas and Migeon (2017); Wigby et al. (2016); and Wistuba, Brand, Zitzmann, and Damm (2017).