2.5 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 20 000–23 000 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: A Y sperm creates an XY (male) embryo; an X sperm creates an XX (female) embryo. 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. While the genotype of each person is always determined by the combined genotypes of the parents, the phenotype (apparent characteristics) may be quite different from the genotype.

From Zygote to Newborn

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

6. 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.

7. 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.

8. The average fetus gains approximately 2000 grams during the last three months of pregnancy. Maturation of brain, lungs, and heart ensures survival of virtually all full-term babies.

9. Medical intervention can speed contractions, dull pain, measure health via the Apgar scale, and save lives. However, some measures seem unnecessary. The goal is a balance, protecting the baby but also allowing parental involvement and control.

10. Many women feel unhappy, incompetent, or unwell after giving birth. Postpartum depression gradually disappears with appropriate help; fathers are particularly crucial to the well-being of mother and child, although they, too, are vulnerable to depression.

Problems and Solutions

11. 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.

12. 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.

13. Birth complications, such as unusually long and stressful labour that includes anoxia (a lack of oxygen to the fetus), have many causes. Low birth weight (under or 2500 grams) may arise from multiple births, placental problems, maternal illness, genes, malnutrition, smoking, drinking, and drug use.

Nature and Nurture Connected

14. Alcoholism is partly genetic, but not completely. It is particularly crucial that children who are genetically vulnerable avoid early exposure to alcohol.

15. Nearsightedness is also partly genetic, but understanding heritability (the impact of genes within a population, not necessarily within an individual) helps show that the increase in nearsightedness is affected by the childhood environment.

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