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Recall that in dominant–recessive inheritance, dominant genes are always expressed regardless of the gene they are paired with and recessive genes are expressed only if paired with another recessive gene. Table 2.3 illustrates diseases that are inherited through dominant–recessive patterns. Few severe disorders are inherited through dominant inheritance because individuals who inherit the allele often do not survive long enough to reproduce and pass it to the next generation. One exception is Huntington disease, a fatal disease in which the central nervous system deteriorates (National Library of Medicine, 2019). Individuals with the Huntington allele develop normally in childhood, adolescence, and young adulthood. Symptoms of Huntington disease do not appear until age 35 or later. By then, many individuals have already had children, and one-half of them, on average, will inherit the dominant Huntington gene.

Table 2.3

Source: McKusick-Nathans Institute of Genetic Medicine (2019).

Phenylketonuria (PKU) is a common recessive disorder that prevents the body from producing an enzyme that breaks down phenylalanine, an amino acid, from proteins (Kahn et al., 2016; Romani et al., 2017). Without treatment, the phenylalanine builds up quickly to toxic levels that damage the central nervous system, contributing to intellectual developmental disability, once known as mental retardation, by 1 year of age. The United States and Canada require all newborns to be screened for PKU (Blau, Shen, & Carducci, 2014).

PKU illustrates how genes interact with the environment to produce developmental outcomes. Intellectual disability results from the interaction 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 they 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 as young people push boundaries and seek independence (Medford, Hare, & Wittkowski, 2017).

A newborn’s blood is tested for phenylketonuria (PKU), a genetic disorder in which the body lacks the enzyme that breaks down phenylalanine. Without treatment, the phenylketonuria builds up to toxic levels and can damage the central nervous system.

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