Human Development: A Life-Span View - Biological Foundations

This chapter aims to describe human chromosomes, genetic disorders, and the interactions between heredity and environment. It covers the periods of the zygote, embryo, and fetus, the impact of general risk factors and teratogens on prenatal development, and the three stages of childbirth, complications, and infant mortality.

Mechanisms of Heredity: Human eggs and sperm each contain 23 chromosomes. Their union forms a zygote with 46 chromosomes (23 pairs: 22 autosomes and 1 pair of sex chromosomes—XY for males, XX for females).
Genotype and Phenotype: Genotype is the complete set of inherited genes; phenotype is the expression of these traits, influenced by both genotype and environment.
Alleles: Varied gene forms (alleles) can be homozygous (identical) or heterozygous (different). Dominant alleles express over recessive ones, which are only expressed if both are recessive. Incomplete dominance results in blended phenotypes.
Genetic Disorders: Stem from inherited recessive alleles (e.g., sickle-cell disease), dominant alleles (e.g., Huntington's disease), or abnormal chromosomes (e.g., Down syndrome).
Heredity, Environment, and Development: Behavioral genetics studies the inheritance of behavioral traits, often polygenetic (influenced by multiple genes). Twin studies (dizygotic vs. monozygotic) help measure hereditary influences. Concepts like heritability coefficients, niche-picking (seeking environments aligned with traits), and nonshared environmental influences highlight the dynamic interaction between genes and environment.

Pregnancy lasts an average of 38 weeks, divided into three stages:
- Period of the Zygote (Weeks 1-2): Zygote implants in the uterine wall; the germ disc forms the baby, and surrounding cells develop into the placenta.
- Period of the Embryo (Weeks 3-8): Major body structures form from three layers: ectoderm (skin, nervous system), mesoderm (muscles, bones, circulatory system), and endoderm (digestive system, lungs). The amniotic sac and umbilical cord develop. Growth follows cephalocaudal (head-to-toe) and proximodistal (center-outward) principles.
- Period of the Fetus (Weeks 9-38): Characterized by significant growth, maturation of bodily systems, and developmental milestones like sex differentiation (Week 9), circulatory system activation (Week 12), and the start of viability (Weeks 22-28).

Teratogens: Factors causing abnormal prenatal development. These include drugs (e.g., alcohol causing FASD, nicotine), diseases (e.g., AIDS, rubella), and environmental hazards (e.g., pollution, lead exposure).
Impact Factors: Severity depends on the fetus's genotype, timing and dosage of exposure, and specific aspects affected. Effects may be delayed.
Prenatal Diagnosis and Treatment: Methods include genetic counseling, diagnostic techniques (ultrasound, amniocentesis), and fetal therapy (medication, surgeries).

Stages of Labor:
- Stage 1: Cervix dilates (12-24 hours).
- Stage 2: Baby moves through the birth canal and is delivered (less than an hour).
- Stage 3: Placenta is expelled.
Childbirth Approaches: Prepared childbirth classes teach relaxation and coping techniques. Home births are rare (approx. 1%).
Adjustment to Parenthood: New parents may experience irritation and resentment, with 10-15% facing postpartum depression. Breastfeeding can help stabilize mood.
Birth Complications: Include hypoxia (insufficient blood flow), C-section necessity, prematurity (before week 36), and low birth weight ( < 5.5 lbs).
Infant Mortality: The U.S. rate is $5/1,000$ births, higher than in many industrialized nations due to factors like inadequate prenatal care and maternity leave policies.

The chapter summarizes human chromosomes, genetic disorders, heredity-environment interaction, prenatal development stages (zygote, embryo