1 Introduction and Learning Objectives

🎥 Watch this before starting!

Learning Objectives.

2 Genetics and Heredity



Human karyotype with 23 chromosome pairs: 22 autosomes and one sex-chromosome pair.
Figure 1. Human karyotype showing 23 pairs of chromosomes, including 22 autosomes and one pair of sex chromosomes. Source: Wikimedia Commons.



DNA (Deoxyribonucleic Acid) is the molecule that carries the genetic instructions essential for life. It contains the information required for an organism’s growth, development, functioning, and reproduction.

DNA is organized into structures called chromosomes. In humans, each cell typically contains 23 pairs of chromosomes (46 total). Genes, which are specific segments of DNA located on chromosomes, carry the instructions for individual traits.

The term genotype refers to an individual’s underlying genetic makeup, while phenotype describes the observable traits that result from the interaction of the genotype with the environment.

Each gene can exist in different versions called alleles, which arise because we inherit one copy of each gene from each parent.

When it comes to gene expression:

For example, a person may have the genotype for brown eyes (Bb), where B is the dominant allele for brown and b is the recessive allele for blue. The phenotype is the actual eye color—brown, in this case—because the dominant gene is expressed, even though the person carries the gene for blue eyes as well.

3 Sex-Linked Traits

♂♀ Did you know? The American Psychological Association defines sex as the characteristics and traits of biological sex (especially physical and biological traits). Gender, by contrast, is defined by the APA as the socially constructed roles, behaviors, activities, and attributes that a given society considers appropriate for different genders. We’ll discuss the topic of gender development in the next lecture.

Some traits are inherited through genes located on the sex chromosomes—the X and Y chromosomes—which together form the 23rd chromosome pair in humans. Many X-linked traits or disorders are more frequently observed in males because the X chromosome carries many more genes than the Y chromosome.

Since males have only one X chromosome (XY), a recessive allele on the X is more likely to be expressed, as there’s no second X that might carry a dominant allele to mask its effect. Females have two X chromosomes (XX), so they typically must inherit two copies of a recessive X-linked allele for the trait to be expressed. This explains why many X-linked recessive disorders are more common in males than females.

🧬 Note: While most individuals have XX or XY sex chromosome pairs, other combinations also occur, such as XXY (Klinefelter syndrome), X0 (Turner syndrome), and XYY (Jacobs syndrome). These variations are known as sex chromosome aneuploidies and are estimated to affect approximately 1 in 400 to 1 in 1,000 live births.

Examples of Sex-Linked Traits:



Ishihara plate showing the number 74, used to screen for red–green color vision deficiency.
Figure 2. An Ishihara test plate used to screen for red–green color vision deficiency (commonly called color blindness). Individuals with typical color vision will see the number 74; those with certain deficiencies may not. Source: Wikimedia Commons.



🔍 Clinical Insight: Y-linked traits exist but are rare and passed only from father to son, as only males have a Y chromosome. An example of this is a form of Y-linked hearing loss caused by deletions in the Y chromosome that is are passed exclusively through the paternal line.

4 Teratogens and Prenatal Development

Infants can be particularly sensitive to environmental agents during critical periods of development.

A teratogen is an environmental agent—such as a drug, medication, or pollutant—that may cause developmental deviations in a growing human organism.



Chart of critical periods of prenatal development showing when organs and systems are most vulnerable to teratogens; darker bars indicate major-structural risk windows and lighter bars indicate periods of elevated physiological/minor anomaly risk.
Figure 3. Critical periods of prenatal development. The chart shows when various organs and systems are most vulnerable to teratogens. Darker bars indicate time frames during which major structural abnormalities are most likely, while lighter bars show periods of heightened risk for physiological defects and minor anomalies. Source: Hill, M.A. (2025) Embryology: Human Abnormal Development.



The effects of a teratogen on the developing fetus depend on both the type of teratogen and the timing of exposure. In general, teratogens tend to have more severe effects earlier in development, particularly during the embryonic period (weeks 3–8) when major organ systems are forming.

Different organ systems and structures develop on distinct timelines. For example, critical aspects of the central nervous system (CNS) begin developing very early, starting around the 3rd week of pregnancy, when the neural plate forms and folds into the neural tube. In contrast, structures like the genitalia start forming later, with differentiation beginning around weeks 7–9. Thus, exposure to a teratogen in the 3rd week is more likely to affect CNS development, whereas exposure around the 8th week or later is more likely to impact genital development.

In addition to teratogens, other factors such as diet and stress can interfere with fetal development and cause abnormalities.

🔎 Explore This: Curious about how a single cell becomes a baby? Follow this to an interactive tool from Kaiser Permanente that walks you through the stages of prenatal development—month by month. Follow along to see how key organs, senses, and physical features emerge during each stage of pregnancy.

5 Brain Development in Childhood



Prenatal brain development timeline indicating neuronal proliferation, migration, differentiation, synaptogenesis, pruning, and myelination.
Figure 4. Timeline of brain development before birth, highlighting key neurodevelopmental processes—neuronal proliferation, migration, differentiation, synapse formation, pruning, and myelination—which begin early in gestation and extend into infancy. Source: ResearchGate.





The vast majority of neurons are formed prior to birth; however, some neurogenesis—particularly in the hippocampus—continues into adulthood. Even still, postnatal increases in brain size are mostly due to:

Over time, however, reductions in the number of neural connections also occur. Synaptic pruning is an important developmental process that eliminates unnecessary or weak synaptic connections to strengthen more frequently used pathways. The timing of synaptogenesis and pruning varies across brain regions: sensory and motor cortices develop first, followed by association cortices and the corpus callosum, and, finally, prefrontal regions that support higher-order cognitive functions.

Interestingly, even as cortical thinning occurs, the overall weight of the brain continues to increase throughout adolescence. This is because myelination, synaptogenesis, and glial cell proliferation outpace the loss in brain size and weight caused by pruning.

🧠 Clinical Insight: Some neurodevelopmental disorders, such as Autism Spectrum Disorder (ASD), may involve atypical patterns of synaptic pruning. This may reflect an overabundance of synaptic connections, which can interfere with efficient neural processing and contribute to sensory overload or atypical information integration.

6 Sensory and Cognitive Development in Infancy

Vision: Newborns cannot see very well and are very nearsighted. It is not until 1-2 years of age that they see as well as adults. Newborns are attracted to faces, especially the contrasts and edges of facial features.

Hearing: Infants are initially startled by loud noises and will turn their head toward sound. However, this behavior seems to be more of a reflex—it temporarily disappears and then reappears around 3 or 4 months of age. Infants can distinguish between speech sounds like “Pa” and “Ba” after just a month. Interestingly, research suggests they may actually distinguish some sounds better than adults. For example, Japanese infants can distinguish between “La” and “Ra”, while many Japanese adults cannot (called perceptual narrowing).

Taste and Smell: Infants can discriminate between different tastes shortly after birth. They will turn their heads toward sweet smells and away from noxious odors like rotten eggs.

Learning and Memory: Evidence suggests that infants can learn and do have memory. For example, they can learn to turn right or left when a buzzer sounds.

👶 Fun fact: Even before birth, learning and memory are evident: infants have been shown to prefer the sound of their mother’s voice and even familiar stories that were read to them while in the womb!

7 Reflexes

Newborns are born with several reflexes that are automatic, unlearned responses to specific types of stimulation. These reflexes are important indicators of neurological development and help infants interact with their environment. These include:

🎥 Watch This: Follow this to a Khan Academy video to learn more about key neonatal reflexes.

8 Motor and Language Development



Graphic of motor development milestones from 0–12 months: lying, rolling, sitting, crawling, standing, and walking.
Figure 5. Illustration of major motor development milestones from birth to one year (lying, rolling, sitting, crawling, standing, walking). Source: Wikimedia Commons.



Motor development in infancy follows a predictable sequence of milestones. Most babies first learn to lift their heads, then roll over, followed by crawling, and eventually walking. These physical achievements reflect the growing coordination between the brain and body as the nervous system matures.

Language development in infancy and toddlerhood also follows a consistent progression. In the early months, infants begin by responding to voices and vocalizing in response to social stimulation. Around 4 to 7 months, they begin cooing and babbling, eventually producing repetitive syllables such as “ma-ma” or “da-da.” Comprehension begins to deepen as infants recognize tones, gestures, and simple commands. By the end of the first year, many children say their first word and can understand basic prohibitions and questions. As they grow into the second year, vocabulary expands rapidly. By 18 to 24 months, toddlers typically use two words in combination and begin forming simple sentences with pronouns and phrases (although this milestone is highly variable). Comprehension also continues to improve, including the understanding of prepositions and more complex language patterns.

9 Piaget’s Theory of Cognitive Development

For most of human history, society was largely agrarian, and children typically lived and worked on farms alongside their families. In the 19th century, with the rise of industrialization, many children moved into factory work and were expected to learn the same skills as adults. However, over the past century, the role of education—particularly secondary education—has become increasingly recognized as critical to healthy cognitive and social development. Today, formal schooling is viewed as essential for preparing children for complex modern societies.

Part of the rationale for moving children into the classroom, rather than relying solely on learning through work, is the recognition that children think differently than adults. It’s not just that they have fewer skills or less knowledge—children literally reason and process information in ways that are qualitatively different from adults.

The idea that children are not simply little adults was emphasized by Swiss psychologist Jean Piaget (1896–1980). Piaget began his career working in Alfred Binet’s laboratory (whom we’ll learn more about when we cover the topic of intelligence). Piaget became fascinated by the mistakes children made—not just the answers, but the reasoning behind them. From this interest, he developed one of the most influential theories in developmental psychology.

Piaget viewed children as little scientists who develop mental frameworks, or schemas, to understand the world. A schema is an organized pattern of thought or behavior that helps a child interpret and respond to new experiences. These schemas are constantly tested and refined through experience. When new information fits an existing schema, the child assimilates it—integrating the information into their current understanding. When the schema doesn’t work, the child must accommodate by modifying the schema to incorporate the new information.

Importantly, children don’t have access to the same cognitive tools at every stage of development.

Piaget argued that biological maturation enables new cognitive abilities over time, but that active exploration and learning are essential for those abilities to develop. In other words, both nature and nurture play critical roles. You can think of it like a video game: as players (children) progress through the levels (developmental stages), new tools and abilities become available, but they still have to learn how to use them effectively.

Specifically, Piaget identified four stages of cognitive development:

  1. Sensorimotor Stage (Birth–2 years): Infants explore the world through sensory experience and motor actions. They learn that their actions affect the world around them. A key milestone in this stage is the development of object permanence—understanding that objects continue to exist even when they are not visible.

  2. Preoperational Stage (2–7 years): Children begin to use symbols, such as words and images, to represent objects. However, they lack logical reasoning and struggle with concepts like conservation (understanding that quantity remains the same despite changes in shape) and egocentrism (difficulty seeing perspectives other than their own).

  3. Concrete Operational Stage (7–11 years): Logical thinking develops, but it is still tied to concrete objects and real-world situations. Children gain a better understanding of conservation and begin to consider others’ perspectives. Abstract or hypothetical reasoning remains limited.

  4. Formal Operational Stage (11 years and up): Adolescents develop the ability to think logically about abstract ideas and hypothetical situations. They can reason about concepts like justice, freedom, and algebraic variables, and they approach problems more systematically.

🎥 Watch This: Follow this link to a YouTube video that demonstrates how a child in the Preoperational stage struggles with the concept of conservation. Note how transformations of objects (e.g., spreading out coins or pouring liquid into a different container) affect the child’s perception of quantity or amount.

🎥 Watch this before continuing!

Piaget in context.

10 Attachment

Much work has been done to study what is generally referred to as attachment: an infant’s tendency to seek closeness to particular people and to feel more secure in their presence.

Attachment theory was developed by the British psychoanalyst John Bowlby (1907–1990) and significantly expanded by Mary Ainsworth (1913–1990), who created the Strange Situation paradigm for studying attachment.

The Strange Situation involves the following sequence:

  1. A mother and child enter a room.
  2. A stranger enters.
  3. The mother leaves.
  4. The mother returns.

The infant’s behavior during this sequence is thought to indicate the type of attachment the child has with the mother. There are four types:

Some research suggests that early attachment styles can predict future behavior—particularly in areas such as emotional regulation, relationships, and social functioning. Secure attachment is often linked to sensitive and responsive caregiving. Insecure styles may result from inconsistent, neglectful, or intrusive caregiving.

11 Summary

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Summary.