Lifespan Development. Tara L. Kuther

to cultural valuing of one sex over the other, rather than to avoid transmitting genetic disorders (Robertson & Hickman, 2013). Critics argue that sex selection can lead down a “slippery slope” of selecting for other characteristics—hair color, eye color, intelligence, and more (Dondorp et al., 2013). Might children born from gender selection be expected to act in certain sex-typical ways, and if they do not, might that disappoint parents? Others express concerns about societal sex ratio imbalances if sex selection becomes widely practiced (Colls et al., 2009; Robertson & Hickman, 2013). Such sex ratio imbalances favoring males have occurred in India and China because of female infanticide, gender-driven abortion, and China’s one-child family policy (see the Cultural Influences on Development feature in Chapter 10 for more information; Bhatia, 2010; Ethics Committee of the American Society for Reproductive Medicine, 2001).

      Should selecting an embryo’s sex be a matter of parental choice? A review of 36 countries, including 25 in Europe, revealed that many had no policies regarding selection; those that did prohibited sex selection for nonmedical reasons (Darnovsky, 2009). The European Union bans socially, nontherapeutically motivated sex selection (Council of Europe, 1997). The United States does not have a formal policy regarding sex selection (Deeney, 2013). Sex selection remains hotly debated in medical journals, hospital and university ethics boards, and the public.

      What Do You Think?

      1 What do you think about parents choosing the sex of their children? In your view, under what conditions is sex selection acceptable?

      2 If you were able to selectively reproduce other characteristics, apart from sex, what might you choose? Why or why not?

      Monozygotic (MZ) twins, or identical twins, originate from the same zygote, sharing the same genotype with identical instructions for all physical and psychological characteristics. MZ twins occur when the zygote splits into two distinct separate but identical zygotes that develop into two infants. It is estimated that MZ twins occur in 4 of every 1,000 U.S. births (American College of Obstetricians and Gynecologists & Society for Maternal-Fetal Medicine, 2014). The causes of MZ twinning are not well understood. Temperature fluctuations are associated with MZ births in animals, but it is unknown whether similar effects occur in humans (Aston, Peterson, & Carrell, 2008). In vitro fertilization and advanced maternal age (35 and older) may increase the risk of MZ twins (Knopman et al., 2014).

      Monozygotic, or identical, twins share 100% of their DNA.

      Ray Evans / Alamy Stock Photo

      Patterns of Genetic Inheritance

      Although the differences among various members of a given family may appear haphazard, they are the result of a genetic blueprint unfolding. Researchers are just beginning to uncover the instructions contained in the human genome, but we have learned that traits and characteristics are inherited in predictable ways.

      Dominant–Recessive Inheritance

      Lynn has red hair while her brother, Jim, does not—and neither do their parents. How did Lynn end up with red hair? These outcomes can be explained by patterns of genetic inheritance, how the sets of genes from each parent interact. As we have discussed, each person has 23 pairs of chromosomes, one pair inherited from the mother and one from the father. The genes within each chromosome can be expressed in different forms, or alleles, that influence a variety of physical characteristics. When alleles of the pair of chromosomes are alike with regard to a specific characteristic, such as hair color, the person is said to be homozygous for the characteristic and will display the inherited trait. If they are different, the person is heterozygous, and the trait expressed will depend on the relations among the genes (Lewis, 2017). Some genes are passed through dominant–recessive inheritance in which some genes are dominant and are always expressed regardless of the gene they are paired with. Other genes are recessive and will be expressed only if paired with another recessive gene. Lynn and Jim’s parents are heterozygous for red hair; both have dark hair, but they each carry a recessive gene for red hair.

      Description

      Figure 2.4 Dominant-Recessive Inheritance

      When an individual is heterozygous for a particular trait, the dominant gene is expressed, and the person becomes a carrier of the recessive gene. For example, consider Figure 2.4. Both parents have nonred hair. People with nonred hair may have homozygous or heterozygous genes for hair color because the gene for nonred hair (symbolized by N in Figure 2.4) is dominant over the gene for red hair (r). In other words, both a child who inherits a homozygous pair of dominant genes (NN) and one who inherits a heterozygous pair consisting of both a dominant and recessive gene (Nr) will have nonred hair, even though the two genotypes are different. Both parents are heterozygous for red hair (Nr). They each carry the gene for red hair and can pass it on to their offspring. Red hair can result only from having two recessive genes (rr); both parents must carry the recessive gene for red hair. Therefore, a child with red hair can be born to parents who have nonred hair if they both carry heterozygous genes for hair color. As shown in Table 2.1, several characteristics are passed through dominant–recessive inheritance.

      Table 2.1

      Sources: McKusick (1998); McKusick-Nathans Institute of Genetic Medicine (2017).

      Incomplete Dominance

      In most cases, dominant–recessive inheritance is an oversimplified explanation for patterns of genetic inheritance. Incomplete dominance is a genetic inheritance pattern in which both genes influence the characteristic (Finegold, 2017). For example, consider blood type. Neither the alleles for blood type A and B dominate each other. A heterozygous person with the alleles for blood type A and B will express both A and B alleles and have blood type AB.

      A different type of inheritance pattern is seen when a person inherits heterozygous alleles in which one allele is stronger than the other yet does not completely dominate. In this situation, the stronger allele does not mask all of the effects of the weaker allele. Therefore, some, but not all, characteristics of the recessive allele appear. For example, the trait for developing normal blood cells does not completely 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