of the genetic predisposition and exposure to phenylalanine from the environment (Blau, 2016). Children with PKU can process only very small amounts of phenylalanine. If the disease is discovered, the infant is placed on a diet low in phenylalanine. Yet it is very difficult to remove nearly all phenylalanine from the diet. Individuals who maintain a strict diet usually attain average levels of intelligence, although tend to score lower than those without PKU (Jahja et al., 2017). Some cognitive and psychological problems may appear in childhood and persist into adulthood, particularly difficulty in attention and planning skills, emotional regulation, depression, and anxiety (Hawks, Strube, Johnson, Grange, & White, 2018; Jahja et al., 2017). The emotional and social challenges associated with PKU, such as the pressure of a strict diet and surveillance from parents, may worsen these symptoms, and dietary compliance tends to decline in adolescence (Medford, Hare, & Wittkowski, 2017).
Table 2.3
Sources: Kahn et al. (2016); McKusick-Nathans Institute of Genetic Medicine (2017).
X-Linked Disorders
A special instance of the dominant–recessive pattern occurs with genes that are located on the X chromosome (Shah, DeRemigis, Hageman, Sriram, & Waggoner, 2017). Recall that males (XY) have both an X and a Y chromosome. Some recessive genetic disorders, like the gene for red-green colorblindness, are carried on the X chromosome. Males are more likely to be affected by X-linked genetic disorders because they have only one X chromosome and therefore any genetic marks on their X chromosome are displayed. Females (XX) have two X chromosomes; a recessive gene located on one X chromosome will be masked by a dominant gene on the other X chromosome. Females are thereby less likely to display X-linked genetic disorders because both of their X chromosomes must carry the recessive genetic disorder for it to be displayed.
Fragile X syndrome is an example of a dominant–recessive disorder carried on the X chromosome (Hagerman et al., 2017). Because the gene is dominant, it need appear on only one X chromosome to be displayed. That means that fragile X syndrome occurs in both males and females. Males with fragile X syndrome typically have a long, narrow face; large ears; and large testes. Fragile X syndrome is the most common known inherited form of intellectual disability (Doherty & Scerif, 2017), and children with fragile X syndrome tend to show moderate to severe intellectual disability (Raspa, Wheeler, & Riley, 2017). Cardiac defects are common as well as several behavioral mannerisms, including poor eye contact and repetitive behaviors such as hand flapping, hand biting, and mimicking others, behaviors common in individuals with autistic spectrum disorders (Hagerman et al., 2017). Fragile X syndrome is often codiagnosed with autism, with estimates of 30% to 54% of boys and 16% to 20% of girls with fragile X syndrome meeting the diagnostic criteria for autism (Kaufmann et al., 2017). As carriers, females may show some characteristics of the disorder but tend to display levels of intelligence within the normal or near-normal range.
Hemophilia, a condition in which the blood does not clot normally, is another example of a recessive disease inherited through genes on the X chromosome (Shah et al., 2017). Daughters who inherit the gene for hemophilia typically do not show the disorder because the gene on their second X chromosome promotes normal blood clotting and is a dominant gene. Females, therefore, can carry the gene for hemophilia without exhibiting the disorder. A female carrier has a 50/50 chance of transmitting the gene to each child. Sons who inherit the gene will display the disorder because the Y chromosome does not have the corresponding genetic information to counter the gene. Daughters who inherit the gene, again, will be carriers (unless their second X chromosome also carries the gene).
Table 2.4 illustrates diseases acquired through X-linked inheritance.
Down syndrome is the most common cause of intellectual disability. Interventions that encourage children to interact with their physical and social environment can promote motor, social, and emotional development.
Agencja Fotograficzna Caro / Alamy Stock Photo
Table 2.4
Source: McKusick-Nathans Institute of Genetic Medicine (2017).
Chromosomal Abnormalities
Chromosomal abnormalities are the result of errors during cell reproduction, meiosis or mitosis, or damage caused afterward. Occurring in about 1 of about 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 2.6 (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 including 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 emotional areas of functioning (Næss, Nygaard, Ostad, Dolva, & Lyster, 2017; Wentz, 2017).
As recently as the early 1980s, individuals with Down syndrome lived to an average age of only 25. 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 and many live into their 70s and beyond (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). As more adults age with Down syndrome, we have discovered a link between Down syndrome and Alzheimer’s disease, a brain degenerative disease that typically strikes in older adulthood (Hithersay, Hamburg, Knight, & Strydom, 2017; Wiseman et al., 2015). Individuals with Down syndrome are at risk to show signs of Alzheimer’s disease very early relative to other adults. 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).
Figure 2.6 Down Syndrome
Some chromosomal abnormalities concern the 23rd pair of chromosomes: the sex chromosomes. Given their different genetic makeup, sex chromosome abnormalities yield different effects in males and females. They are summarized in Table 2.5.
One of the most common sex chromosome abnormalities, with prevalence estimates between 1 in 500 and 1 in 1,000 males, is Klinefelter syndrome, in which
