Guidepost 1: How does conception normally occur?

• Fertilization, the union of an ovum and a sperm, results in the formation of a one-celled zygote, which then duplicates itself by cell division.

Guidepost 2: What causes multiple births?

• Multiple births can occur either by the fertilization of two ova (or one ovum that has split) or by the splitting of one fertilized ovum. Larger multiple births result from either one of these processes or a combination of the two.
• Dizygotic (fraternal) twins have different genetic makeups and may be of different sexes; monozygotic (identical) twins have the same genetic makeup. Because of differences in prenatal and postnatal experience, "identical" twins may differ in temperament and other respects.

Guidepost 3: How does heredity operate in determining sex and transmitting normal and abnormal traits?

• The basic functional units of heredity are the genes, which are made of deoxyribonucleic acid (DNA). DNA carries the biochemical instructions, or genetic code, that governs bodily functions and determines inherited characteristics. Each gene seems to be located by function in a definite position on a particular chromosome. The complete sequence of genes in the human body is the human genome.
• At conception, each normal human being receives 23 chromosomes from the mother and 23 from the father. These form 23 pairs of chromosomes---22 pairs of autosomes and 1 pair of sex chromosomes. A child who receives an X chromosome from each parent will be a female. If the child receives a Y chromosome from the father, a male will be conceived.
• The simplest patterns of genetic transmission are dominant and recessive inheritance. Sometimes codominance occurs. When a pair of alleles are the same, a person is homoygous for the trait; when they are different, the person is heterozygous.
• Most normal human characteristics are the result of polygenic or multifactorial transmission. Except for monozygotic twins, each child inherits a unique genotype. Dominant inheritance and multifactorial transmission explain why a person's phenotype does not always express the underlying genotype.
• Birth defects and diseases may result from simple dominant, recessive, or sex-linked inheritance, from mutations, or from genome imprinting. Chromosomal abnormalities also can cause birth defects.
• Through genetic counseling, prospective parents can receive information about the mathematical odds of bearing children with certain defects.
• Genetic testing involves risks as well as benefits.

Guidepost 4: How do scientists study the relative influences of heredity and environment, and how do heredity and environment work together?

• Research in behavioral genetics is based on the assumption that the relative influences of heredity and environment can be measured statistically. If heredity is an important influence on a trait, genetically closer persons will be more similar in that trait. Family studies, adoption studies, and studies of twins enable researchers to measure the heritability of specific traits.
• The concepts of reaction range, canalization, genotype-environment interaction, genotype-environment correlation (or covariance), and niche-picking describe ways in which heredity and environment work together.
• Siblings tend to be more different than alike in intelligence and personality. According to behavior genetics research, heredity accounts for most of the similarity, and nonshared environmental effects account for most of the difference. Critics claim that this research, for methodological reasons, minimizes the role of parenting and the complexity of developmental systems.

Guidepost 5: What roles do heredity and environment play in physical health, intelligence, and personality?

• Obesity, longevity, intelligence, and temperament are influenced by both heredity and environment.
• Schizophrenia and autism are psychopathological disorders influenced by both heredity and environment.

Guidepost 6: What are the three stages of prenatal development, and what happens during each stage?

• Prenatal development occurs in three stages of gestation: the germinal, embryonic, and fetal stages.
• Growth and development both before and after birth follow the cephalocaudal principle (head to tail) and the proximodistal principle (center outward).
• Severely defective embryos usually are spontaneously aborted during the first trimester of pregnancy.

Guidepost 7: What can fetuses do?

• As fetuses grow, they move less, but more vigorously. Swallowing amniotic fluid, which contains substances from the mother's body, stimulates taste and smell. Fetuses seem able to hear, exercise sensory discrimination, learn, and remember.

Guidepost 8: What environmental influences can affect prenatal development?

• The developing organism can be greatly affected by its prenatal environment. The likelihood of a birth defect may depend on the timing and intensity of an environmental event and its interaction with genetic factors.
• Important environmental influences involving the mother include nutrition, physical activity, smoking, intake of alcohol or other drugs, transmission of maternal illnesses or infections, maternal age, and external environmental hazards, such as chemicals and radiation. External influences also may affect the father's sperm.

Guidepost 9: What techniques can assess a fetus's health and well-being, and what is the importance of prenatal care?

• Ultrasound, amniocentesis, chorionic villus sampling, embryoscopy, preimplantation genetic diagnosis, umbilical cord sampling, and maternal blood tests can be used to determine whether an unborn baby is developing normally.
• Early, high-quality prenatal care is essential for healthy development. It can lead to detection of defects and disorders and, especially if begun early and targeted to the needs of at-risk women, may help reduce maternal and infant death, low birthweight, and other birth complications.