Section II Beginnings Chapter 3 Biological Beginnings Summary

1. Images of Life-Span Development: The Jim and Jim Twins

The Minnesota Study of Twins Reared Apart investigates various aspects of the lives of identical twins and fraternal twins from all over the world. Although the study's conclusions must be evaluated in light of several methodological criticisms, the more important point is that this study illustrates a growing interest in the genetic basis of human development.

2. The Evolutionary Perspective

Natural Selection. Natural selection is an evolutionary process that favors individuals within a species best adapted to survive and reproduce. Darwin believed that evolutionary change occurs at a slow pace. Although no dramatic evolutionary changes have occurred since the human lineage appeared in the fossil record about 50,000 years ago, there have been sweeping cultural changes. Biological evolution shaped humans into a culture-making species for whom change can occur rapidly.

Sociobiology. Sociobiology is an evolutionary view that argues behavior is motivated by a desire to dominate the gene pool. Sociobiology is controversial. Critics say that Sociobiology ignores the environmental determinants of behavior, lacks predictive power, and manifests a bias against females.

3. Heredity

What Are Genes? Human development begins with the genetic code we receive from our parents. The nucleus of each human cell contain 46 chromosomes, composed of DNA, that provide the blueprint for the development of an individual. Genes are the building blocks of chromosomes. Gametes are human reproductive cells.

Reproduction. Genes are transmitted from parents to offspring by gametes, the ovum or egg from the female and sperm from the male. The process of meiosis produces the gametes. In human beings each gamete has 23 chromosomes. Reproduction occurs when a sperm fertilizes an ovum, producing a zygote. Male and female sperm can be separated by a centrifuge, making it possible to control the sex of the offspring. Approximately 10-15 percent of American couples are infertile. In vitro fertilization, conception outside the body, allows otherwise infertile couples to bear children; however, adoption provides an alternative choice.

Abnormalities in Genes and Chromosomes. A number of genetic problems can occur. PKU syndrome is a genetic problem resulting in the failure to produce an enzyme necessary for metabolism and can cause severe mental retardation if not treated by dietary restrictions. Down syndrome is a form of mental retardation caused by the presence of an extra choromosome, and is accompanied by physical abnormalities. Sickle-cell anemia is a disease of the red blood cells, existing primarily among blacks, and causes early death. Several disorders are associated with sex chromosome abnormalities. These include Klinefelter syndrome, males with an extra X chromosome; Turner syndrome, females missing an X chromosome; and XYY syndrome, males with an extra X chromosome. Genetic counseling provides information to couples about the risks of having a genetically defective child. Four procedures can be used to determine the presence of genetic defects: amniocentesis, ultrasound sonography, the chorionic villus test, and the maternal blood test.

4. Genetic Principles and Methods

Some Genetic Principles. Genetic transmission is a complex process because the underlying genetic structure, or genotype, must be inferred from the actual physical characteristics of a person, the phenotype. Nonetheless, some general principles have been identified. Some genes are dominant, and will always be expressed phenotypically when present, whereas other genes are recessive, and will be expressed phenotypically only when paired with another recessive gene on the other chromosome, the principle of dominant-recessive genes. Some characteristics are determined by the interaction of many different genes, the principle of polygenetic inheritance. The reaction range refers to the possible extent of phenotypes that could be expressed by a given genotype. The genetic codes set broad limits on the range of possible outcomes, which are then affected by the environment. Canalization refers to the extent to which certain genotypes might be immune to environmental influences.

Methods Used by Behavior Geneticists. The field of behavior genetics studies the nature and degree of the hereditary basis for behavior. Twin studies examine identical and fraternal twins for the degree of similarity. Adoption studies compare the adopted child with both the adoptive and biological parents.

Heredity's Influence on Development. The two most widely investigated aspects of heredity's influence on development concern intelligence and temperament. Jensen argues that intelligence is primarily inherited and that environment and culture play minimal roles. Jensen reviewed twin studies and claimed that the heritability of intelligence, as measured by IQ, is about 80 percent. Criticisms of Jensen's work include: (a) 10 tests measure only a narrow range of intelligence, and (b) investigations do not include environments that differ radically.

5. Heredity-Environment Interaction and Development

Behavior geneticists study the influences of both genes and environment on development, the heredity-environment interaction.

Passive Genotype-Environment, Evocative Genotype-Environment, and Active Genotype-Environment Interactions. Sandra Scarr believes that parental genotypes influence the environment they provide for their children. Three ways behavior geneticists believe that heredity and environment interact in this manner are passively, evocatively, and actively. Passive genotype-environment interactions occur when parents, who are genetically related to the child, provide a rearing environment for the child. Evocative genotype-environment interactions occur because a child's genotype elicits certain types of physical and social environments. Active (niche-picking) genotype-environment interactions ocur when children seek out environment they find compatible and stimulating. The relative importance of these three forms of interaction changes as children develop.

Shared and Nonshared Environmental Influences. Shared environmental experiences are children's common experiences, such as their parents' personalities and intellectual orientation, the family's social class, and the neighborhood in which they live. Nonshared environmental experiences refer to the child's own unique experiences both within a family and outside the family, that are not shared with another sibling. Robert Plomin argues nonshared environmental experiences produce different personalities for siblings.

The Contemporary Heredity/Environment Controversy. Sandra Scarr's view that genotypes drive experience have been criticized by Baumrind, Maccoby, and Jackson who stress the role of experience as the primary determinants of development.

Conclusions about Heredity-Environment Interaction. Both genes and experience are necessary for development. The two factors are said to interact because the environmental effects depend on individual's genetic endowment.

6.0 Contemporary Concerns

Sociocultural Worlds of Development 3.1: The Human Species is a Culture-Making Species. The biological evolution of humans is quite slow relative to cultural evolution. Cultural evolution makes it possible for humans to shape the world to fit human needs.

Perspectives on Parenting and Education 3.1: The Effects of Early Intervention on Intelligence. Early intervention programs try to provide favorable outcomes early in the life of children who are judged at risk for impaired intelligence. One study of the babies of mothers with IQs averaging 80 produced higher IQs at age three for children who attended an educational day-care center as compared to home-reared children. A second study demonstrated a successful intervention for Mexican American families who received parent education and educational day care when compared to control families.

Life-Span Practical Knowledge 3.1: Being Adopted. The authors review the influence of being adopted on people's lives and explore the relationship between being adopted and Erik Erikson's stages of psychosocial development.

Life-Span Practical Knowledge 3.2: Prenatal Tests. Robin Blatt's book is a comprehensive guide to the benefits and risks of various prenatal tests. It also addresses emotional and ethical issues facing couples who consider prenatal testing.

Life-Span Health and Well-Being: Genetic Counseling. Genetic counselors are typically physicians or biologists who are versed in medical genetics. These individuals are familiar with the potential genetic problems, their likelihood, and ways to minimize their effects. Final decisions always depend on a couple’s ethical and religious beliefs.