The sex chromosome abnormality known as Turner syndrome occurs when a female is born with only one X chromosome (National Library of Medicine, 2017). Girls with Turner syndrome show abnormal growth patterns. As adults, they are short in stature and often have small jaws with extra folds of skin around their necks (webbing) and lack prominent female secondary sex characteristics such as breasts. Their ovaries do not develop normally, and they do not ovulate (Culen, Ertl, Schubert, Bartha-Doering, & Haeusler, 2017). Girls with Turner syndrome are at risk for precocious puberty (in middle to late childhood), often with spontaneous onset instead of the gradual changes that typically accompany puberty (Improda et al., 2012), as well as thyroid disease, vision and hearing problems, heart defects, diabetes, and autoimmune disorders. Current estimates of its frequency range from 1 in 2,500 worldwide (National Library of Medicine, 2017). If Turner syndrome is diagnosed early, regular injections of human growth hormones can increase stature, and hormones administered at puberty can result in some breast development and menstruation (Christopoulos, Deligeoroglou, Laggari, Christogiorgos, & Creatsas, 2008; Culen et al., 2017).
Mutation
Not all inborn characteristics are inherited. Some result from mutations, sudden changes and abnormalities in the structure of genes that occur spontaneously or may be induced by exposure to environmental toxins such as radiation and agricultural chemicals in food (Lewis, 2017). A mutation may involve only one gene or many. It is estimated that as many as one half of all conceptions include mutated chromosomes (Plomin et al., 2013). Most mutations are fatal—the developing organism often dies very soon after conception, often before the woman knows she is pregnant (Sadler, 2015).
Lives in Context
Genes as Protective Factors in Development
Not all children exposed to adversity experience negative outcomes. Genes, such as MAOA, influence children’s sensitivity to maltreatment.
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Children who are maltreated or abused by their parents are at risk for developing many problems, including aggression and violent tendencies. Yet not all children who are maltreated become violent adolescents and adults. Why? A classic study examined this question.
Caspi and colleagues (2002) followed a sample of males from birth until adulthood and observed that not all maltreated boys developed problems with violence. Only boys who carried a certain type of gene were at risk for becoming violent after experiencing maltreatment. Specifically, there are two versions of a gene that controls monoamine oxidase A (MAOA), an enzyme that regulates specific chemicals in the brain; one produces high levels of the enzyme, and the other produces low levels. Boys who experienced abuse and other traumatic experiences were about twice as likely to develop problems with aggression, violence, and to even be convicted of a violent crim—but only if they carried the low-MAOA gene. Maltreated boys who carried the high-MAOA gene were no more likely to become violent than nonmaltreated boys. In addition, the presence of the low-MAOA gene itself was not associated with violence. The low-MAOA gene predicted violence only for boys who experience abuse early in life. These findings have been replicated in another 30-year longitudinal study of boys (Fergusson, Boden, Horwood, Miller, & Kennedy, 2011) as well as a meta-analysis of 27 studies (Byrd & Manuck, 2014).
Similar findings of a MAOA gene × environment interaction in which low MAOA, but not high MAOA, predicts negative outcomes in response to childhood adversity has been extended to include other mental health outcomes such as antisocial personality disorder and depression (Beach et al., 2010; Cicchetti, Rogosch, & Sturge-Apple, 2007; Manuck & McCaffery, 2014; Nikulina, Widom, & Brzustowicz, 2012). Many of these studies have examined only males. Females show a more mixed pattern, with some studies showing that girls display the MAOA gene × environment interaction but to a much lesser extent than boys, whereas other studies suggest no relationship (Byrd & Manuck, 2014).
In addition, some genes might increase our sensitivity to, and the effectiveness of, environmental interventions (Bakermans-Kranenburg & van IJzendoorn, 2015; Chhangur et al., 2017). Just as we may adjust contextual factors to contribute to successful developmental outcomes and resilience, in the future we might learn how to “turn on” protective genes and “turn off” those that contribute to risk.
What Do You Think?
1 In your view, how important are genetic contributors to development?
2 If some genes may be protective in particular contexts, should scientists learn how to turn them on? Why or why not? What about genes that may be harmful in particular contexts?
Sometimes mutations are beneficial. This is especially true if the mutation is induced by stressors in the environment and provides an adaptive advantage to the individual. For example, the sickle cell gene is a mutation that originated in areas where malaria is widespread, such as Africa (Ware, de Montalembert, Tshilolo, & Abboud, 2017).
Children who inherited a single sickle cell allele were more resistant to malarial infection and more likely to survive and pass it along to their offspring (Croke et al., 2017; Gong, Parikh, Rosenthal, & Greenhouse, 2013). The sickle cell gene is not helpful in places of the world where malaria is not a risk. The frequency of the gene is decreasing in areas of the world where malaria is uncommon. For example, only 8% of African Americans are carriers, compared with as much as 30% of Black Africans in some African countries (Maakaron & Taher, 2012). Therefore, the developmental implications of genotypes—and mutations—are context specific, posing benefits in some contexts and risks in others, as illustrated in the Lives in Context feature.
Thinking in Context 2.2
1 Identify risk factors for genetic and chromosomal disorders. What can prospective parents do to minimize the risks? What specific advice do you give?
2 Discuss how PKU illustrates the following two themes in human development: (1) the role of nature and nurture in development and (2) interactions among domains of development.
Reproductive Choices
The likelihood of genetic disorders often can be predicted before conception. Moreover, advances in technology permit abnormalities to be detected earlier than ever before. In this section, we start with a discussion of genetic counseling. We then consider why couples turn to reproductive technology methods such as artificial insemination, in vitro fertilization, and surrogacy. We also look at the option of adoption that prospective parents consider. Finally, we discuss the methods of prenatal diagnosis and the prenatal treatment of genetic disorders.
Genetic Counseling
The growing understanding of genetic inheritance has led many couples to wonder about their own genetic inheritance and what genes they will pass on to their children. Many prospective parents seek genetic counseling to determine the risk that their children will inherit genetic defects and chromosomal abnormalities (Ioannides, 2017; Uhlmann, Schuette, & Yashar, 2009). Candidates for genetic counseling include those whose relatives have a genetic condition, couples who have had difficulties bearing children, women over the age of 35, and couples from the same ethnic group. Genetic tests can also determine whether a couple’s difficulty conceiving or recurrent miscarriages are influenced by sperm chromosomal abnormalities in the male (Kohn, Kohn, Darilek, Ramasamy, & Lipshultz, 2016).
Parents meet with a genetic counselor to