mask the allele for developing sickle-shaped blood cells. About 5% of African American newborns (and relatively few Caucasians or Asian Americans) carry the recessive sickle cell trait (Ojodu, Hulihan, Pope, & Grant, 2014). Sickle cell alleles cause red blood cells to become crescent, or sickle, shaped. Cells that are sickle shaped cannot distribute oxygen effectively throughout the circulatory system (Ware, de Montalembert, Tshilolo, & Abboud, 2017). The average life expectancy for individuals with sickle cell anemia is 55 years in North America (Pecker & Little, 2018). Alleles for normal blood cells do not mask all of the characteristics of recessive sickle cell alleles, illustrating incomplete dominance. Sickle cell carriers do not develop full-blown sickle cell anemia (Chakravorty & Williams, 2015). Carriers of the trait for sickle cell anemia may function normally but may show some symptoms such as reduced oxygen distribution throughout the body and exhaustion after exercise. Only individuals who are homozygous for the recessive sickle cell trait develop sickle cell anemia.
Recessive sickle cell alleles cause red blood cells to become crescent shaped and unable to distribute oxygen effectively throughout the circulatory system. Alleles for normal blood cells do not mask all of the characteristics of recessive sickle cell alleles, illustrating incomplete dominance.
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Polygenic Inheritance
Whereas dominant–recessive and codominant–recessive patterns account for some genotypes, most traits are a function of the interaction of many genes, known as polygenic inheritance. Hereditary influences act in complex ways, and researchers cannot trace most characteristics to only one or two genes. Instead, polygenic traits are the result of interactions among many genes. Examples of polygenic traits include height, intelligence, personality, and susceptibility to certain forms of cancer (Bouchard, 2014; Kremen, Panizzon, & Cannon, 2016; Penke & Jokela, 2016). As the number of genes that contribute to a trait increases, so does the range of possible traits. Genetic propensities interact with environmental influences to produce a wide range of individual differences in human traits.
Genomic Imprinting
The principles of dominant–recessive and incomplete dominance inheritance can account for over 1,000 human traits (Amberger & Hamosh, 2017; McKusick, 2007). However, a few traits are determined by a process known as genomic imprinting. Genomic imprinting refers to the instance in which the expression of a gene is determined by whether it is inherited from the mother or the father (Kelly & Spencer, 2017; National Library of Medicine, 2019). For example, consider two conditions that illustrate genomic imprinting: Prader-Willi syndrome and Angelman syndrome. Both syndromes are caused by an abnormality in the 15th chromosome (Kalsner & Chamberlain, 2015). As shown in Figure 2.4, if the abnormality occurs on chromosome 15 acquired by the father, the individual—whether a daughter or son—will develop Prader-Willi syndrome, a set of specific physical and behavioral characteristics including obesity, insatiable hunger, short stature, motor slowness, and mild to moderate intellectual impairment (Butler, Manzardo, Heinemann, Loker, & Loker, 2016). If the abnormal chromosome 15 arises from the mother, the individual—again, whether a daughter or a son—will develop Angelman syndrome, characterized by hyperactivity, thin body frame, seizures, disturbances in gait, and severe learning disabilities, including severe problems with speech (Buiting, Williams, & Horsthemke, 2016). Prader-Willi and Angelman syndromes are rare, occurring on average in 1 in 12,000 to 20,000 persons (Kalsner & Chamberlain, 2015; Spruyt, Braam, & Curfs, 2018). Patterns of genetic inheritance can be complex, yet they follow predictable principles. For a summary of patterns of genetic inheritance, refer to Table 2.2.
Figure 2.4 Genomic Imprinting
Source: C. Cristofre Martin (1998).
Table 2.2
Thinking in Context 2.1
1 Consider the evolutionary developmental perspective discussed in Chapter 1. From an evolutionary developmental perspective, why are some characteristics dominant and others recessive? Is it adaptive for some traits to dominate over others? Why or why not?
2 Consider your own physical characteristics, such as hair and eye color. Are they indicative of recessive traits or dominant ones? Which of your traits are likely polygenic?
3 From an evolutionary developmental perspective, why do twins occur? Do you think twinning serves an adaptive purpose? Explain.
Chromosomal and Genetic Problems
Many disorders are passed through genetic inheritance or are the result of chromosomal abnormalities. Hereditary and chromosomal abnormalities can often be diagnosed prenatally. Others are evident at birth or can be detected soon after an infant begins to develop. Some are discovered only over a period of many years.
Genetic Disorders
Disorders and abnormalities that are inherited through the parents’ genes are passed through the inheritance processes that we have discussed. These include well-known conditions such as sickle cell anemia, as well as others that are rare. Some are highly visible and others go unnoticed during an individual’s life.
Dominant–Recessive Disorders
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