Chapter Three: Developmental Psychology

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Conceiving New Life

Introduction to Human Development

Human development begins before birth, starting with the union of sperm and egg. This process is influenced by genetic factors and environmental conditions, with both risks and resilience present from the earliest stages.

Development starts at conception, not birth.
Both genes and environment shape outcomes.
Risks include genetic disorders, lack of prenatal care, and environmental stressors.

Cultural Beliefs About Conception

Different cultures have unique beliefs about how life begins, reflecting their values and worldviews.

Many traditional societies have folk explanations for conception, such as spirits or cosmic forces.
Beliefs about the timing of conception (e.g., during or after menstruation) vary widely.
Astrological and spiritual factors are often considered influential in conception and the child's future.

Fertilization and Assisted Reproductive Technologies

Fertilization

Fertilization is the process by which male and female gametes (sperm and ovum) unite to form a zygote, which then divides and develops into a new individual.

Ova are present in females from birth; sperm are produced continuously in males after puberty.
Ovulation occurs about every 28 days in women until menopause.
Fertilization typically occurs in the fallopian tube.
If fertilization does not occur, the ovum and sperm are eliminated from the body.

Assisted Reproductive Technologies (ART)

ART provides alternative methods for conception for couples experiencing infertility.

Artificial insemination: Sperm is injected into the female reproductive tract.
In vitro fertilization (IVF): Ova are fertilized outside the body and implanted in the uterus.
Cryopreservation: Eggs are frozen for future use.
Multiple embryo transfer increases the risk of multiple births and associated complications.

Multiple Births

Multiple births can result from natural or assisted conception, with varying risks and genetic implications.

Dizygotic (fraternal) twins: Two separate eggs fertilized by two sperm; genetically like siblings.
Monozygotic (identical) twins: One fertilized egg splits; genetically identical.
Multiple births are more common with ART and in certain populations.
Risks increase with the number of fetuses (e.g., preterm delivery, low birth weight).

Mechanisms of Heredity

The Genetic Code

Genetics is the study of heredity, focusing on how traits are passed from parents to offspring through DNA.

DNA is composed of four bases: adenine (A), thymine (T), cytosine (C), guanine (G).
Genes are segments of DNA located on chromosomes; humans have 23 pairs of chromosomes.
Each gene's sequence determines the production of specific proteins.
The genome is the complete set of genes in an organism.
Gene expression is influenced by environmental factors (epigenetics).

Sex Determination

Sex is determined by the combination of sex chromosomes inherited from each parent.

Females: XX; Males: XY.
The SRY gene on the Y chromosome triggers male development.
Hormonal influences and genetic variants further shape sexual differentiation.

Patterns of Genetic Transmission

Traits are inherited according to principles first described by Gregor Mendel.

Dominant traits are expressed if at least one dominant allele is present.
Recessive traits are expressed only if both alleles are recessive.
Alleles are alternative forms of a gene; individuals can be homozygous or heterozygous for a trait.
Most traits are polygenic (influenced by multiple genes) and multifactorial (influenced by genes and environment).

Epigenetic Influences

Epigenetics refers to changes in gene expression that do not alter the DNA sequence but affect how genes are read.

Environmental factors (nutrition, stress, toxins) can add chemical tags to DNA, turning genes on or off.
Epigenetic changes can explain differences between identical twins and may be heritable.

Genetic and Chromosomal Abnormalities

Types and Transmission

Genetic and chromosomal abnormalities can result from mutations, dominant or recessive inheritance, or errors in cell division.

Dominant disorders: Only one copy of the abnormal gene is needed (e.g., Huntington's disease).
Recessive disorders: Both copies must be abnormal (e.g., cystic fibrosis, Tay-Sachs).
Sex-linked disorders: Genes on the X chromosome affect males more often (e.g., hemophilia, color blindness).
Chromosomal abnormalities: Extra or missing chromosomes (e.g., Down syndrome, Klinefelter syndrome, Turner syndrome).

Table: Common Chromosomal Disorders

Disorder	Chromosomal Pattern	Main Features
Down syndrome (Trisomy 21)	Extra 21st chromosome	Intellectual disability, characteristic facial features
Klinefelter syndrome	XXY	Male with some female characteristics, infertility
Turner syndrome	XO	Female, short stature, infertility
Trisomy-13	Extra 13th chromosome	Severe intellectual and physical disabilities

Genetic Counseling and Testing

Genetic counseling helps prospective parents assess risks for inherited disorders using family history, physical exams, and laboratory tests (e.g., karyotyping).

Screening can reduce the incidence of certain genetic diseases in high-risk populations.

Studying the Influences of Heredity and Environment

Measuring Heritability

Heritability estimates the proportion of variation in a trait due to genetic factors within a population.

Methods include family, twin, and adoption studies.
Heritability is expressed as a value between 0.0 and 1.0 (or 0% to 100%).
Heritability does not apply to individuals, only to populations.

How Heredity and Environment Work Together

Reaction range: The range of possible phenotypes for a given genotype, depending on environmental conditions.
Canalization: Some traits are highly canalized (e.g., motor development), meaning they are strongly programmed by genes and less influenced by environment.
Genotype-environment interaction: The same environment can have different effects on individuals with different genotypes.
Genotype-environment correlation: Genetic tendencies influence the environments individuals experience.

Types of Genotype-Environment Correlations

Passive: Parents provide both genes and environment (e.g., musical parents create a musical home).
Reactive/Evocative: A child's genetic traits evoke responses from others (e.g., a sociable child receives more social opportunities).
Active (niche-picking): Individuals seek out environments that fit their genetic tendencies (e.g., an athletic child joins sports teams).

Nonshared Environmental Influences

Even siblings in the same family experience unique environments, contributing to differences in personality and development.

Nonshared experiences include different treatment by parents, unique events, and individual choices.

Characteristics Influenced by Heredity and Environment

Physical health: Many diseases and life span are influenced by genetics and environment.
Obesity: Heritability is high, but environment (diet, exercise, culture) also plays a major role.
Intelligence: Strong genetic influence, but environment (especially in early life) is important.
Temperament and personality: Largely inborn, but shaped by experience and environment.
Psychopathology: Disorders like schizophrenia, autism, and depression have genetic and environmental components.

Prenatal Development

Cultural Beliefs About Prenatal Development

Beliefs about pregnancy and prenatal care vary widely across cultures, influencing dietary practices, behaviors, and attitudes toward medical care.

Stages of Prenatal Development

Germinal stage (0-2 weeks): Zygote divides and implants in the uterus; formation of blastocyst and embryonic disk.
Embryonic stage (2-8 weeks): Major organs and body systems form (organogenesis); most vulnerable to teratogens.
Fetal stage (8 weeks-birth): Rapid growth, organ refinement, and development of sensory and cognitive abilities.

Principles of Development

Cephalocaudal: Development proceeds from head to tail.
Proximodistal: Development proceeds from the center outward.

Perceptual and Cognitive Development in Fetuses

Taste and smell develop by 20 weeks; exposure to flavors in utero influences later preferences.
Fetuses respond to sound and vibration by 26 weeks; recognize mother's voice.
Fetal memory begins around 30 weeks gestation.

Environmental Influences on Prenatal Development

Maternal Factors

Nutrition and weight: Adequate nutrition and appropriate weight gain are crucial; deficiencies can cause birth defects and long-term health issues.
Physical activity: Moderate exercise is beneficial; strenuous work may increase risk of complications.
Drug intake: Many prescription and recreational drugs are teratogenic (e.g., alcohol, nicotine, opioids, cocaine, methamphetamine).
Maternal illness: Infections (e.g., HIV, rubella, toxoplasmosis, COVID-19) can harm the fetus.
Emotional state: Chronic stress and depression can negatively affect fetal development.
Maternal age: Both very young and older mothers face increased risks for complications and birth defects.
Environmental hazards: Exposure to pollution, chemicals, radiation, and toxins can cause developmental problems.

Paternal Factors

Paternal age, lifestyle, and exposure to toxins can affect sperm quality and increase risk of birth defects.
Secondhand smoke from fathers can harm fetal development.

Monitoring and Promoting Prenatal Development

Prenatal Assessment and Care

Noninvasive screening: Ultrasound, maternal blood tests, and cell-free DNA scans detect potential problems.
Invasive procedures: Amniocentesis and chorionic villus sampling provide definitive genetic diagnoses.
Prenatal care: Early and high-quality care improves outcomes; disparities in access affect risk.

Key Terms and Concepts

Zygote: The single cell formed at conception.
Teratogen: Any environmental agent that can cause harm during prenatal development.
Phenotype: Observable characteristics resulting from genotype and environment.
Genotype: The genetic makeup of an individual.
Heritability: The proportion of variation in a trait due to genetic factors.
Canalization: The degree to which a trait is genetically programmed and resistant to environmental influence.
Reaction range: The range of possible phenotypes for a given genotype under different environmental conditions.

Sample Equations and Diagrams

Heritability estimate: where is heritability, is genetic variance, and is total phenotypic variance.
Genotype combinations for a single-gene trait (e.g., hair color): Possible genotypes: (homozygous dominant), (heterozygous), (homozygous recessive)

Summary Table: Stages of Prenatal Development

Stage	Time Frame	Main Events
Germinal	0-2 weeks	Fertilization, cell division, implantation
Embryonic	2-8 weeks	Organogenesis, major body systems form, high vulnerability
Fetal	8 weeks-birth	Growth, organ refinement, sensory and cognitive development

Checkpoint Questions

Explain and give at least one example of reaction range, canalization, and each of the three genotype-environment correlations.
Differentiate the three types of genotype-environment correlation.
List three kinds of influences that contribute to nonshared environmental effects.

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