Human Development: A Life-Span View - Chapter 2: Biological Foundations: Heredity, Prenatal Development, and Birth

Chapter 2: Biological Foundations: Heredity, Prenatal Development, and Birth

Introduction and Chapter Objectives

Icebreaker Activity: Students discussed scenarios related to birth plans, including advice on what to include, who to have in the room, epidural considerations, and recording the birth.
Chapter Objectives: By the end of this chapter, students should be able to:
- Describe human chromosomes, genetic disorders, and the interactions between heredity and environment.
- Describe the periods of the zygote, embryo, and fetus.
- Describe how general risk factors and teratogens affect prenatal development.
- Describe the three stages of childbirth, birth complications, and infant mortality.

2.1: In the Beginning: 23 Pairs of Chromosomes

Key Questions:
- What are chromosomes and genes? How do they carry hereditary information?
- What are common chromosome problems and their consequences?
- How is children's heredity influenced by their environment?
Mechanisms of Heredity: Chromosomes
- Human eggs contain $23$ chromosomes from the female parent's $46$ .
- Human sperm contain $23$ chromosomes from the male parent's $46$ .
- Analogy: Like mixing two hands of cards (one from each parent), shuffling, and then dealing a hand of $23$ pairs of chromosomes.
Mechanisms of Heredity: Egg and Sperm United
- The union of egg and sperm forms a zygote, which has a complete set of $23$ paired chromosomes (totalling $46$ ).
- These $46$ chromosomes contain approximately $25,000$ genes.
- Autosomes: The first $22$ pairs of chromosomes.
- Sex Chromosomes: The $23^{rd}$ pair that determines the child's sex assigned at birth.
  - Males: XY chromosomes (X from mother, Y from father).
  - Females: XX chromosomes (X from mother, X from father).
Mechanisms of Heredity: Genotype and Phenotype
- Genotype: The complete set of genes an individual has inherited.
- Phenotype: The observable expression of traits, resulting from the combination of genotype and environmental influences. This includes physical, behavioral, and psychological traits.
Mechanisms of Heredity: Alleles
- Traits are produced by the interaction of genes on each pair of chromosomes.
- Alleles: Different forms in which genes come, each carrying instructions for specific variations of a trait.
- Homozygous Alleles: When alleles are identical, meaning both parents contributed similar genes for a particular trait (e.g., both contribute for blue eyes).
- Heterozygous Alleles: When parents contribute different versions of a trait (e.g., one for blue eyes, one for brown eyes).
Mechanisms of Heredity: Dominant vs. Recessive Alleles
- Dominant Allele: Its instructions are followed, overriding the recessive allele.
- Recessive Allele: Its instructions are followed only if both alleles for the trait are recessive (a homozygous condition).
- Incomplete Dominance: A condition where the phenotype falls between the expressions of the dominant and recessive alleles. Example: A mild form of the sickle-cell trait that manifests under vigorous physical exertion.
Single-Gene Inheritance: Refers to traits determined by a single gene.
Two Types of Genetic Disorders
- Inherited Disorders: Usually involve two recessive alleles (e.g., sickle-cell disease, PKU), but some can involve a dominant allele (e.g., Huntington's disease).
- Abnormal Chromosomes: Occur due to extra, missing, or damaged chromosomes, leading to abnormal development. Examples: Down syndrome, Turner's syndrome, Klinefelter's syndrome.
Heredity, Environment, and Development
- Behavioral Genetics: The study of the inheritance of behavioral and psychological traits. This is complex as most traits are polygenetic, involving many genes.
- Polygenetic Inheritance: Occurs when multiple genes influence the phenotype of a physical, psychological, or behavioral trait. Expression is stronger with more dominant alleles and weaker with more recessive alleles in the combination.
Behavioral Genetics: Mechanisms and Methods
- Dizygotic (Fraternal) Twins: Result from two separate eggs fertilized by two separate sperm. They are no more genetically similar than other siblings.
- Monozygotic (Identical) Twins: Result from one egg and one sperm that splits into two soon after conception, making them genetically identical.
- Both types of twins often share similar experiences and environments. Scientists compare identical and fraternal twins to assess the influence of heredity on traits.
Paths from Genes to Behavior
- Heredity and environment constantly interact throughout development.
- Heritability Coefficients: Measures of the extent to which differences in a trait are due to genetic factors.
- Genes can influence the environments an individual experiences, a concept known as niche-picking.
- Nonshared Environmental Influences: Environmental factors that make children within the same family different from one another.
Think, Pair, Share Activity: An exercise to identify characteristics as phenotype, genotype, or environmentally determined (e.g., eye color, height, autosomes, vocabulary, a heterozygous recessive allele).

2.2: From Conception to Birth

Key Questions:
- What happens to a fertilized egg in the first two weeks?
- When do body structures and internal organs emerge?
- When do body systems begin to function well enough to support life?
Periods of Prenatal Development
- Prenatal development averages $38$ weeks, measured from the date of conception (approx. two weeks after the last menstrual period).
- It is divided into three periods:
  - Period of the Zygote (weeks $1-2$ )
  - Period of the Embryo (weeks $3-8$ )
  - Period of the Fetus (weeks $9-38$ )
Periods of Prenatal Development: The Zygote (Weeks 1–2)
- Following fertilization (natural or in vitro), the zygote travels down the fallopian tube.
- It then implants in the uterine wall, which triggers hormonal changes to prevent menstruation.
- The germ disc in the center of the zygote comprises cells that will develop into the baby.
- Other cells form the placenta, which is crucial for supporting the baby's development.
Periods of Prenatal Development: The Embryo (Weeks 3–8)
- During this period, body structures, internal organs, and three distinct layers of the embryo develop:
  - Ectoderm (Outer Layer): Forms hair, skin, and the nervous system.
  - Mesoderm (Middle Layer): Develops into muscles, bones, and the circulatory system.
  - Endoderm (Inner Layer): Becomes the digestive system and lungs.
- The amniotic sac fills with fluid, and the umbilical cord connects the embryo to the placenta.
- The umbilical cord and placenta facilitate the exchange of nutrients, oxygen, vitamins, and waste products between the birthing parent and the embryo.
- Embryonic growth follows two key principles:
  - Cephalocaudal Principle: Development proceeds from head to the rest of the body (e.g., head develops first).
  - Proximodistal Principle: Growth occurs from parts near the center of the body outward to more distant parts (e.g., trunk before limbs).
Periods of Prenatal Development: The Fetus (Weeks 9–38)
- This period starts at week $9$ and concludes at birth.
- The fetus grows significantly in size, and bodily systems begin to function.
- Many systems essential for human life undergo final development.
- All brain regions grow, with particular development in the cerebral cortex.
- Week 9: Differentiation of ovaries and testes occurs.
- Week 12: The circulatory system begins to function.
- Week 16: Movements of the fetus can be felt by the mother.
- Week 20: Eyebrows, eyelashes, and scalp hair appear. The skin thickens and is covered by vernix, a protective greasy substance.
- Weeks 22–28: This is considered the age of viability, meaning the fetus might survive if born prematurely.
- Weeks 22 and beyond: The senses become active; the fetus can remember voices, music, and tastes.
Think, Pair, Share Activity: A discussion among classmates about which stage of prenatal development (zygote, embryo, or fetus) is most important and why, based on varying standards.

2.3: Influences on Prenatal Development

Key Questions:
- How is prenatal development influenced by a pregnant person’s age, nutrition, and stress?
- How do diseases, drugs, and environmental hazards affect prenatal development?
- What general principles govern how prenatal development can be harmed?
- How can prenatal development be monitored, and can abnormal development be corrected?
Teratogens: Drugs, Diseases, and Environmental Hazards
- Teratogens: Agents (drugs, diseases, environmental hazards) that cause abnormal prenatal development.
- Drugs:
  - Examples: alcohol, aspirin, caffeine, nicotine.
  - Fetal Alcohol Spectrum Disorder (FASD): Caused by alcohol consumption during pregnancy.
  - Nicotine use increases risks of miscarriage, low birth weight, and birth defects.
- Diseases:
  - Examples: AIDS, cytomegalovirus, genital herpes, rubella (German measles), syphilis.
  - Some, like cytomegalovirus, rubella, and syphilis, can directly attack the fetus via the placenta.
  - Others, like AIDS and genital herpes, can attack the fetus during birth.
  - Medications for the pregnant parent after illness do not prevent existing damage to the fetus.
- Environmental Hazards:
  - Examples: air pollution, lead, PCBs, X-rays.
  - Exposure to these hazards can affect both physical and mental development prenatally and postnatally.
  - Can also increase the child's risk of health issues like leukemia after birth.
How Teratogens Influence Prenatal Development (Five Principles)
1. Genotype Dependency: The effect of a teratogen varies based on the organism's genotype.
2. Developmental Timing: The impact changes depending on the developmental stage during exposure.
3. Specific Effects: Each teratogen tends to affect a particular aspect of prenatal development.
4. Dosage Dependency: The impact is proportional to the dosage of the teratogen.
5. Delayed Impact: Damage from teratogens is not always evident at birth and may appear later.
Discussion Activity: Teratogens: Listing consequences of exposure to alcohol, nicotine, lead, and rubella during prenatal development.
Prenatal Diagnosis and Treatment
- Genetic Counseling: Provides information about risks of genetic disorders.
- Prenatal Diagnosis Methods:
  - Ultrasound: Uses sound waves to create images of the fetus.
  - Amniocentesis: Sampling amniotic fluid to detect chromosomal abnormalities.
  - Chorionic Villus Sampling (CVS): Sampling placental tissue for genetic testing.
  - Non-Invasive Prenatal Testing (NIPT): Blood test screening for certain chromosomal conditions.
- Fetal Therapy: Directly treating the fetus.
  - Administering medicine to the fetus.
  - Fetal surgery to correct conditions like spina bifida and circulatory problems.
- Genetic Engineering: Involves replacing defective genes with synthetic, normal genes.

2.4: Labor and Delivery

Key Questions:
- What are the phases of labor and delivery?
- What are