Biological Foundations: Heredity, Prenatal Development, and Birth

Biological Foundations: Heredity, Prenatal Development, and Birth

Mechanisms of Heredity

This section explores how hereditary information is transmitted from one generation to the next, focusing on chromosomes, genes, and their organization.

• Chromosomes: Structures within cells that contain genetic material. Humans have 46 chromosomes (23 pairs), with half contributed by each parent.

• Egg and Sperm: Human eggs and sperm each contain 23 chromosomes, selected from the parent's 46. Upon fertilization, they unite to form a zygote with 46 chromosomes.

• Autosomes: The first 22 pairs of chromosomes, which are not involved in determining sex.

• Sex Chromosomes: The 23rd pair determines biological sex. Males have XY (X from mother, Y from father); females have XX (X from both parents).

DNA and Genes

DNA is the molecular basis of heredity, encoding instructions for cellular function and development.

• DNA: Each chromosome consists of a single DNA molecule, which is a double helix composed of four chemical bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

• Gene: A segment of DNA that provides instructions for a specific biochemical function or trait.

• Organization: The order and number of A, T, G, and C bases create the genetic code.

Genotype and Phenotype

Genotype and phenotype describe the genetic makeup and observable traits of an individual.

• Genotype: The complete set of genes inherited from both parents.

• Phenotype: The observable traits resulting from the interaction of genotype and environmental influences. These traits can be physical, behavioral, or psychological.

Alleles and Genetic Variation

Alleles are different forms of a gene that contribute to genetic diversity and trait variation.

• Alleles: Each gene can exist in multiple forms (alleles), each carrying instructions for a specific variation of a trait.

• Homozygous: Both alleles for a trait are the same (e.g., both parents contribute the allele for brown eyes).

• Heterozygous: The alleles for a trait are different (e.g., one parent contributes the allele for blue eyes, the other for brown eyes).

Dominant and Recessive Alleles

Some alleles are dominant, while others are recessive, affecting how traits are expressed.

• Dominant Allele: Its instructions are followed even if only one copy is present.

• Recessive Allele: Its instructions are followed only if both alleles are recessive (homozygous recessive).

• Incomplete Dominance: Sometimes, the phenotype is intermediate between dominant and recessive traits (e.g., mild sickle-cell trait).

Single-Gene and Chromosomal Disorders

Genetic disorders can result from mutations in single genes or abnormalities in chromosomes.

• Inherited Disorders: Often involve two recessive alleles (e.g., sickle-cell disease, PKU), but some involve dominant alleles (e.g., Huntington's disease).

• Chromosomal Abnormalities: Extra, missing, or damaged chromosomes can lead to abnormal development (e.g., Down syndrome, Turner's syndrome, Klinefelter's syndrome).

Heredity, Environment, and Development

Behavioral genetics studies how heredity and environment interact to shape behavioral and psychological traits.

• Polygenetic Inheritance: Most traits are influenced by multiple genes, with dominant alleles leading to stronger expression.

• Twin Studies: Compare monozygotic (identical) and dizygotic (fraternal) twins to measure genetic influence.

• Adoption Studies: Compare adopted children to biological and adoptive parents to separate genetic and environmental effects.

• DNA Sampling: Used to link genotypes to behavioral traits (e.g., reading disabilities).

Gene-Environment Interaction

Genes and environment interact dynamically throughout development.

• Heritability Coefficient: A statistical estimate of the proportion of trait variation due to genetic factors.

• Niche Picking: Individuals may select environments that complement their genetic tendencies.

• Nonshared Environmental Influences: Environmental factors that make children within a family different from each other.

Periods of Prenatal Development

Prenatal development is divided into three periods, each characterized by specific developmental milestones.

• Period of the Zygote (Weeks 1-2): Fertilized egg travels down the fallopian tube and implants in the uterine wall. The germ disc forms the baby; other cells form the placenta.

• Period of the Embryo (Weeks 3-8): Body structures and internal organs develop. Three layers form: Ectoderm (outer, becomes skin and nervous system), Mesoderm (middle, becomes muscles and bones), Endoderm (inner, becomes digestive system and lungs). The amniotic sac and umbilical cord support development. Growth follows the cephalocaudal principle (head develops first) and proximodistal principle (center develops before extremities).

• Period of the Fetus (Weeks 9-38): Rapid growth and maturation of body systems. Brain development accelerates, and senses become active. Age of viability is reached at 22-28 weeks.

Influences on Prenatal Development

Several factors can affect prenatal development, including maternal age, nutrition, stress, and exposure to teratogens.

• Nutrition: Increased caloric and nutrient intake is essential for healthy development.

• Stress: High stress, especially early in pregnancy, can negatively impact development.

• Maternal Age: Optimal age for pregnancy is between 20 and 35 years.

Teratogens: Drugs, Diseases, and Environmental Hazards

Teratogens are agents that cause abnormal prenatal development.

• Drugs: Alcohol, nicotine, caffeine, and aspirin can cause birth defects (e.g., Fetal Alcohol Spectrum Disorder).

• Diseases: AIDS, cytomegalovirus, rubella, syphilis, and herpes can harm the fetus directly or during birth.

• Environmental Hazards: Exposure to air pollution, lead, PCBs, and X-rays can affect physical and mental development and increase risk of health issues.

Principles of Teratogen Effects

The impact of teratogens depends on several factors.

• Genotype: The organism's genetic makeup influences susceptibility.

• Timing: The stage of prenatal development affects the impact.

• Specificity: Each teratogen affects particular aspects of development.

• Dosage: The amount of exposure matters.

• Delayed Effects: Damage may not be evident at birth.

Prenatal Diagnosis and Treatment

Various methods are used to monitor and treat prenatal development.

• Genetic Counseling: Assesses risk of inherited disorders.

• Prenatal Diagnosis: Includes ultrasound, amniocentesis, chorionic villus sampling, and non-invasive prenatal testing (NIPT).

• Fetal Medicine: Includes administering medicine, fetal surgery, and experimental genetic engineering (e.g., CRISPR).

Labor and Delivery

Labor and delivery occur in three stages, each with distinct physiological processes.

• Stage 1 (12-24 hours before birth): Contractions intensify; cervix dilates to about 10 cm.

• Stage 2 (<1 hour before birth): Baby passes through cervix and vagina; crowning and birth occur.

• Stage 3 (minutes after birth): Placenta is expelled.

Approaches to Childbirth

Prepared childbirth and home births are alternatives to traditional hospital delivery.

• Prepared Childbirth: Involves classes, relaxation, and coaching techniques to manage pain without anesthetics.

• Home Births: Account for about 1% of births; safety depends on risk factors and available medical support.

Adjustment to Parenthood

Parents experience significant psychological and physiological changes after birth.

• Postpartum Adjustment: 50% experience irritability and stress in the first 1-2 weeks; 10-15% experience postpartum depression, which can affect the baby.

• Risk Factors: High hormonal levels, pre-pregnancy depression, stress, and lack of support increase risk; breast-feeding reduces risk.

Birth Complications

Complications during birth can affect infant health and survival.

• Hypoxia: Inadequate blood and oxygen supply to the baby.

• Cesarean Section (C-section): May be necessary for certain complications.

• Premature Birth: Birth before 36 weeks.

• Low Birth Weight: Less than 5.5 pounds; very low is less than 3.3 pounds; extremely low is less than 2.2 pounds.

Infant Mortality

Infant mortality rates reflect the percentage of infants who die before age one and vary by country.

• U.S. Rate: Just under 1% (7 of 1,000 live births).

• International Comparison: 15 industrialized nations have lower rates than the U.S.

• Contributing Factors: Low birth weight due to lack of prenatal care, fewer paid leaves for pregnant women.