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Body does not produce

enough insulin, which causes

abnormal metabolism of


Early onset can be fatal unless

treated with insulin. 1 in 2,500 births


Delayed blood clotting causes

internal and external bleeding.

Blood transfusions/injections can

reduce or prevent damage due

to internal bleeding. 1 in 10,000 males



Central nervous system

deteriorates, producing

problems in muscle

coordination and mental


Does not usually appear until age

35 or older; death likely 10 to 20

years after symptoms appear. 1 in 20,000 births



Metabolic disorder that, left

untreated, causes intellectual


Special diet can result in average

intelligence and normal life span.

1 in 10,000 to 1 in

20,000 births



Blood disorder that limits the

body’s oxygen supply; it can

cause joint swelling, as well as

heart and kidney failure.

Penicillin, medication for pain,

antibiotics, and blood


1 in 400 African

American children

(lower among other


Spina bifida

Neural tube disorder that

causes brain and spine


Corrective surgery at birth,

orthopedic devices, and

physical/medical therapy. 2 in 1,000 births



Deceleration of mental and

physical development caused

by an accumulation of lipids in

the nervous system.

Medication and special diet are

used, but death is likely by 5

years of age.

1 in 30 American Jews

is a carrier.

Figure 5 Some Gene-Linked Abnormalities

Phenylketonuria (PKU) is a genetic disorder in which the individual cannot properly metabolize phenylalanine, an amino acid that naturally occurs in many food sources. It results from a recessive gene and occurs about once in every 10,000 to 20,000 live births. Today, phenylketonuria is easily detected in infancy, and it is treated by a diet that prevents an excess accumulation of phenylalanine (Rohde & others, 2014). If phenylketonuria is left untreated, however, excess phenylalanine Page 45 builds up in

the child, producing intellectual disability and hyperactivity. Phenylketonuria accounts for approximately 1 percent of individuals who are institutionalized for intellectual disabilities, and it occurs primarily in Whites.

How Would You…?

As a health-care professional, how would you explain the heredity-environment interaction to

new parents who are upset when they discover that their child has a treatable genetic defect? Sickle-cell anemia, which occurs most often in African Americans, is a genetic disorder that impairs functioning of the body’s red blood cells. Red blood cells, which carry oxygen to the body’s other cells, are usually shaped like a disk. In sickle-cell anemia, a recessive gene causes the red blood cell to become a hook-shaped “sickle” that cannot carry oxygen properly and dies quickly. As a result, the body’s cells do not receive adequate oxygen, causing anemia and early death (Derebail & others, 2014). About 1 in 400 African American babies is affected by sickle-cell anemia. One in 10 African Americans is a carrier, as is 1 in 20 Latin Americans. Recent research strongly supports the use of hydroxyurea therapy for infants with sickle-cell anemia beginning at 9 months of age (Yawn & John-Sowah, 2015). Other diseases that result from genetic abnormalities include cystic fibrosis, some forms of diabetes, hemophilia, Huntington disease, Alzheimer disease, spina bifida, and Tay- Sachs disease. Someday, scientists may be able to determine why these and other genetic abnormalities occur and discover how to cure them (Capurro & others, 2015; Tai & others, 2015; Wang & others, 2016; Williams & others, 2016).

Genetic counselors, usually physicians or biologists who are well-versed in the field of medical genetics, may specialize in providing information to individuals who are at risk of giving birth to children with the kinds of genetic abnormalities just described (Stilwell, 2016). They can evaluate the degree of risk involved and offer helpful strategies for offsetting some of the effects of these diseases (Paneque, Sequeiros, & Skirton, 2015; Redlinger-Grosse & others, 2016). To read about the career and work of a genetic counselor, see Careers in Life-Span Development.

Careers in life-span development

Holly Ishmael, Genetic Counselor Holly Ishmael is a genetic counselor at Children’s Mercy Hospital in Kansas City. She obtained an undergraduate degree in psychology and then a master’s degree in genetic counseling from Sarah Lawrence College.

Genetic counselors work as members of a health-care team, providing information and support to families with birth defects or genetic disorders. They identify families at risk by analyzing inheritance patterns and explore options with the family. Some genetic counselors, like Holly, become specialists in prenatal and pediatric genetics; others might specialize in cancer genetics or psychiatric genetic disorders.

Holly says, “Genetic counseling is a perfect combination for people who want to do something science-oriented, but need human contact and don’t want to spend all of their time in a lab or have their nose in a book” (Rizzo, 1999, p. 3).

Genetic counselors hold specialized graduate degrees in the areas of medical genetics and counseling. They enter graduate school with undergraduate backgrounds from a variety of disciplines, including biology, genetics, psychology, public health, and social work. There are approximately 30 graduate genetic counseling programs in the United States. If you are interested in this profession, you can obtain further information from the National Society of Genetic Counselors at www.nsgc.org.

Holly Ishmael (left) in a genetic counseling session.© Holly Ishmael Welsh

Prenatal Development We turn now to a description of how the process of development unfolds from its earliest moment—the moment of conception—when two parental cells, with their unique genetic contributions, merge to create a new individual.

Conception occurs when a single sperm cell from a male unites with an ovum (egg) in a female’s fallopian tube in a process called fertilization. Over the next few months the genetic code discussed earlier directs a series of changes in the fertilized egg, but many events and hazards will influence how that egg develops and becomes a person.

The Course of Prenatal Development Prenatal development lasts approximately 266 days, beginning with fertilization and ending with birth. Pregnancy can be divided into three periods: germinal, embryonic, and fetal.

The Germinal Period The germinal period is the period of prenatal development that takes place in the first two weeks after conception. It includes the creation of the fertilized egg (the zygote), cell division, and the attachment of the multicellular organism to the uterine wall. Rapid cell division by the zygote begins the germinal period. (Recall from earlier in the chapter that this cell division occurs through a process called mitosis.) Within one week after conception, the differentiation of Page 50these cells—their specialization for different tasks—has already begun. At this stage the organism, now called the blastocyst, consists of a hollow ball of cells that will eventually develop into the embryo, and the trophoblast, an outer layer of cells that later provides nutrition and support for the embryo. Implantation, the embedding of the blastocyst in the uterine wall, takes place during the second week after conception. Figure 7 summarizes these significant developments in the germinal period.

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