Infants and Children in Context. Tara L. Kuther

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 1 in every 250 births (Parazzini et al., 2016; Umstad et al., 2019). The causes of MZ twinning are not well understood. Rates of MZ twins are not related to maternal age or the number of births, but in vitro fertilization appears to increase the risk of MZ twins (Knopman et al., 2014; Umstad et al., 2019).

      Applying Developmental Science

      Prenatal Sex Selection

      Parents have long shown a preference for giving birth to a girl or boy, depending on circumstances such as cultural or religious traditions, the availability of males or females to perform certain kinds of work important to the family or society, or the sex of the couple’s other children. Until recently, the sex of an unborn child was a matter of hope, prayer, and folk rituals. It is only in the past generation that science has made it possible for parents to reliably choose the sex of their unborn child. The introduction of sex selection has been a boon to couples carrying a genetically transmitted disease (i.e., a disease carried on the sex chromosomes), enabling them to have a healthy baby of the sex unaffected by the disease they carried.

      Sperm cells can be sorted by whether they carry the X or Y chromosome. Through in vitro fertilization, a zygote with the desired sex is created.

      Brain light/Alamy Stock Photo

      Sex selection is generally conducted using two methods: preimplantation genetic diagnosis (PGD) or preconception sperm sorting (Bhatia, 2018). PGD creates zygotes within the laboratory by removing eggs from the woman and fertilizing them with sperm. This is known as in vitro (literally, “in glass”) fertilization because fertilization takes place in a test tube, outside of the woman’s body. After 3 days, a cell from the organism is used to examine the chromosomes and determine its sex. The desired male or female embryos are then implanted into the woman’s uterus. PGD is generally conducted only when the risk of family genetic disorders is high and is about 99% effective.

      Preconception sperm sorting entails spinning sperm in a centrifuge to separate those that carry an X or a Y chromosome. Because X sperm are denser than Y sperm, they are easily separated. Sperm with the desired chromosomes are then used to fertilize the ovum. Sperm sorting has been available and commonly used since the 1970s. The success rate is about 75% (Bhatia, 2018).

      The availability of sex selection procedures enables parents to choose the sex of their child because of personal desires, such as to create family balance or to conform to cultural valuing of one sex over the other, rather than simply to avoid transmitting genetic disorders (Robertson & Hickman, 2013). Critics argue that sex selection can lead down a “slippery slope” of genetic engineering and selecting for other characteristics, such as appearance, intelligence, and more (Dondorp et al., 2013). 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 Lives in Context: Cultural Context feature in Chapter 12 for more information; Bhatia, 2010, 2018).

      Most Canadian, U.K., and European countries restrict the use of PGD and prohibit it for nonmedical reasons (Bayefsky, 2016). The United States does not have a formal policy regarding sex selection (Deeney, 2013). Sex selection remains hotly debated in medical journals, by hospital and university ethics boards, and by the public.

      What Do You Think?

      1 Should parents be able to choose the sex of their baby? 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? ●

      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 but 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.

      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 2.3. 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 2.3) 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.

      Description

      Figure 2.3 Dominant–Recessive Inheritance

      Table 2.1

      Source: McKusick (1998) and McKusick-Nathans Institute of Genetic Medicine (2019).

      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. The alleles for blood types A and B do not 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