SUMMARY

The Beginning of Life

  1. Genes are the foundation for all development. Human conception occurs when two gametes (an ovum and a sperm, each with 23 chromosomes) combine to form a zygote, 46 chromosomes in a single cell.

  2. Every cell of every human being has a unique genetic code made up of about 20,000 to 23,000 genes, some in variations called alleles. The environment interacts with the genetic instructions for every trait.

  3. The sex of an embryo depends on the sperm: Y sperm create XY (male) embryos; X sperm creates XX (female) embryos. Twins occur if a zygote splits in two (monozygotic, or identical, twins) or if two ova are fertilized by two sperm (dizygotic, or fraternal, twins).

  4. Genes interact in various ways: sometimes additively, with multiple genes contributing to a trait, and sometimes in a dominant–recessive pattern. The genotype of each person is set by the genotypes of the parents, but the phenotype (apparent characteristics) may be quite different from the genotype.

From Zygote to Newborn

  1. The first two weeks of prenatal growth are the germinal period. The cells differentiate, and the developing organism implants itself in the lining of the uterus.

  2. The period from the third through the eighth week after conception is called the embryonic period. The heart begins to beat, and the eyes, ears, nose, and mouth form. By the eighth week, the embryo has the basic organs and features of a human, with the exception of the sex organs.

  3. The fetal period extends from the ninth week until birth. By the 12th week, all the organs and body structures have formed. The fetus attains viability at 22 weeks, when the brain is sufficiently mature to regulate basic body functions.

  4. The average fetus gains approximately 4½ pounds (2,000 grams) during the last three months of pregnancy. Maturation of the brain, lungs, and heart ensures survival of virtually all full-term babies.

  5. Medical intervention can speed contractions, dull pain, measure infant health via the Apgar scale, and save lives.

  6. Some women feel unhappy, incompetent, or unwell after giving birth. Postpartum depression gradually disappears with appropriate help. Fathers are affected by pregnancy and birth as well, and are vulnerable to depression after birth.

Problems and Solutions

  1. Often a zygote has more or fewer than 46 chromosomes. Such zygotes usually do not develop; the main exceptions are those with three chromosomes at the 21st location (Down syndrome, or trisomy-21) or an odd number of sex chromosomes.

  2. Thousands of teratogens, especially drugs and alcohol, have the potential to harm the embryo or fetus. Actual harm occurs because of a cascade: Genes, critical periods, dose, and frequency all have an impact.

  3. Birth complications, such as an unusually long and stressful labor that includes anoxia (a lack of oxygen to the fetus), have many causes. Low birthweight (less than 5½ pounds, or 2,500 grams) may result from multiple fetuses, placental problems, maternal illness, genes, malnutrition, smoking, drinking, and drug use.

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Nature and Nurture

  1. Alcohol use disorder is partly genetic, but not completely. It is particularly crucial that children who are genetically vulnerable avoid early exposure to alcohol.

  2. Nearsightedness is also partly genetic, but understanding heritability (the impact of genes within a population, not necessarily within an individual) helps show that the relatively recent increase in nearsightedness is affected by outdoor light.

  3. Nature and nurture interact to cause virtually all human problems; understanding genes, prenatal development, birth, and childhood increase the odds that a newborn will have a long and healthy life.