Guidepost 1 How does conception normally occur, and how have beliefs about conception changed?

• Early beliefs about conception reflected incorrect beliefs about nature and about male and female anatomy.
• 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 What causes infertility, and what are alternative ways of becoming parents?

• The most common cause of infertility in men is a low sperm count; the most common cause in women is blockage of the fallopian tubes. Infertile couples now have several options for assisted reproduction, but these techniques may involve difficult thorny ethical and practical issues.

Guidepost 4 What genetic mechanisms determine sex, physical appearance, and other characteristics?

• 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 form the mother and 23 from the father. These form the 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. When a pair of alleles are the same, a person is homozygous for the trait; when they are different, the person is heterozygous.
• Most normal human characteristics are the result of polygenic inheritance 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.

Guidepost 5 How are birth defects and disorders transmitted?

• 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 6 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 certain traits.
• Critics claim that traditional behavior genetics is too simplistic. They study complex developmental systems, reflecting a confluence of constitutional, economic, social and cultural influences.
• The concepts of reaction range, canalization, genotype-environment interaction, genotype-environment correlation (or covariance), and niche-picking describes 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, many experiences that strongly affect development are different for each sibling.

Guidepost 7 What roles do heredity and environment play in physical health, intelligence, and personality?

• Health, longevity, intelligence, and temperament are influenced by both heredity and environment. The relative influences of heredity and environment may vary across the life span.
• Schizophrenia and autism are psychopathological disorders influenced by both heredity and environment.