Genetics, Prenatal Development, and the Newborn

Development does not start at birth; it starts at conception, and the board expects you to know that timeline cold. Items in this area test whether you can trace a trait from gene to chromosome to visible outcome, place a prenatal milestone in the correct period, name the exposure that produces a specific birth defect, and read a newborn's first score of life. Get the sequence and the vocabulary right and these items become some of the easiest points on the Developmental Psychology subject.

Genes, chromosomes, and how traits are expressed

Every cell (except mature sex cells) carries 46 chromosomes arranged in 23 pairs, threadlike structures of tightly coiled DNA. A gene is a segment of DNA that is the basic unit of hereditary instruction, and the complete set of genetic instructions a person inherits is the genotype. The phenotype is what actually shows up: the observable physical, behavioral, and physiological traits that result from the genotype interacting with the environment. The board loves this distinction because two people can share a genotype for a trait yet display different phenotypes depending on nutrition, illness, or experience.

Single-gene traits follow patterns first described by Gregor Mendel. A dominant allele expresses its trait whenever present, even paired with a different allele, while a recessive allele expresses its trait only when paired with another recessive allele (a homozygous recessive genotype). A person who carries one dominant and one recessive allele is a carrier: heterozygous, phenotypically normal, but able to pass the recessive allele to offspring. This is exactly how PKU and sickle cell trait quietly pass through families before appearing in a child of two unaffected carrier parents. Sex-linked traits follow a related but distinct rule: because the gene sits on the X chromosome, conditions like red-green color blindness and hemophilia appear far more often in males, who have only one X to draw from.

Chromosomal and genetic disorders the board tests

Disorder	Genetic basis	Key features
Down syndrome (Trisomy 21)	Three copies of chromosome 21	Intellectual disability, distinct facial features, heart defects; risk rises sharply with maternal age
Turner syndrome	A female with only one X chromosome (45,X)	Short stature, infertility, webbed neck; normal intelligence in most cases
Klinefelter syndrome	A male with an extra X chromosome (XXY)	Reduced testosterone, infertility, tall stature, learning difficulties
Phenylketonuria (PKU)	Autosomal recessive single-gene disorder	Body cannot metabolize phenylalanine; untreated, causes intellectual disability; managed by strict diet
Fragile X syndrome	Mutation on the X chromosome	Most common inherited cause of intellectual disability; more severe in males

Down syndrome and Turner syndrome are chromosomal disorders (an error in chromosome number), while PKU is a classic single-gene, autosomal recessive disorder, a distinction the board tests directly by asking you to sort conditions into the correct category. In the Philippines, RA 9288, the Newborn Screening Act of 2004, mandates screening every infant for PKU, congenital hypothyroidism, and other metabolic disorders within the first 24 to 72 hours of life, because early detection lets a diet-controlled condition like PKU be managed before permanent damage occurs.

Conception and the three prenatal periods

Conception occurs when a sperm fertilizes an ovum, forming a single-celled zygote that carries a full set of 46 chromosomes. From that moment, prenatal development unfolds across three periods.

• Germinal period (weeks 1-2): the zygote divides rapidly, travels down the fallopian tube, and implants in the uterine wall. This period produces the blastocyst, and it ends at successful implantation. A large proportion of zygotes never survive this stage.
• Embryonic period (weeks 3-8): now called an embryo, the organism forms all major organ systems and body structures in a process called organogenesis. The three layers differentiate: the ectoderm (skin, nervous system), mesoderm (muscle, skeleton, circulatory system), and endoderm (digestive and respiratory linings). Because every major structure is being laid down, this is the period of greatest vulnerability to harmful exposures.
• Fetal period (week 9 to birth): the organism, now a fetus, undergoes rapid growth and the fine-tuning of organ systems already formed. Movement becomes detectable around 16 to 20 weeks (quickening), and the age of viability, the point at which a fetus born prematurely has a reasonable chance of survival with medical support, falls around 22 to 24 weeks, once the lungs and brain are sufficiently developed.

Teratogens and the principles that govern their harm

A teratogen is any external agent, a drug, virus, chemical, or other environmental factor, that can cause abnormal prenatal development or birth defects. Common examples the board cites include alcohol (producing fetal alcohol spectrum disorders), nicotine, certain medications, radiation, and infections such as rubella and Zika. Teratogenic risk follows several well-established principles.

• Timing is critical. The embryonic period is the most sensitive window because organs are actively forming; damage during this stage is more likely to be structural and severe, while fetal-period exposure is more likely to stunt growth or impair function.
• Susceptibility is not uniform. Genotype, maternal health, and dose all modify how much harm a given exposure causes; the same exposure can affect two fetuses differently.
• Effects are dose-related. Greater amount, frequency, or duration of exposure generally produces greater harm.
• A teratogen's effect is often organ-specific. Each teratogen tends to target the structures forming at the time of exposure rather than causing uniform, whole-body damage.

The newborn: reflexes, states, and assessment

A healthy newborn arrives with a set of innate reflexes, automatic, involuntary responses that signal an intact nervous system. The rooting reflex turns the head toward a touch on the cheek; the sucking reflex draws in anything placed at the lips; the Moro (startle) reflex produces an arm-flinging, embracing motion in response to a sudden loss of support or loud noise; the Babinski reflex fans the toes when the sole of the foot is stroked; and the grasping (palmar) reflex closes the fingers around an object pressed into the palm. Most of these reflexes are present at birth and disappear on a predictable schedule as the cortex matures; a reflex that persists past its expected window, or is absent when it should be present, is itself a diagnostic red flag.

Newborns also cycle through distinct states of arousal, ranging from regular (quiet) sleep and irregular (active) sleep, through drowsiness and quiet alertness, to active alertness and crying. Quiet alertness is the state in which infants are most receptive to stimulation and social interaction, which is why it is prized in both caregiving and infant-research settings.

The standard first assessment of newborn health is the APGAR score, developed by anesthesiologist Virginia Apgar, administered at 1 minute and again at 5 minutes after birth. It rates five signs, each scored 0 to 2, for a total of 0 to 10:

• Appearance (skin color)
• Pulse (heart rate)
• Grimace (reflex irritability)
• Activity (muscle tone)
• Respiration (breathing effort)

A score of 7 to 10 indicates a newborn in good condition; lower scores flag the need for immediate medical intervention. Because the 5-minute score better predicts short-term outcome than the 1-minute score, the board frequently asks which of the two carries more clinical weight.

Board traps and takeaways

• Do not confuse genotype (the inherited instructions) with phenotype (the expressed, observable trait); environment can shift phenotype without changing genotype.
• Sort disorders correctly: Down and Turner syndromes are chromosomal (wrong number of chromosomes), while PKU is a single-gene, autosomal recessive disorder.
• Keep the three prenatal periods in strict order and match the marker term: germinal ends at implantation, embryonic covers organogenesis (highest vulnerability), and fetal covers growth and viability.
• Teratogen questions hinge on timing: embryonic-period exposure tends to cause structural defects, fetal-period exposure tends to impair growth or function.
• Remember the 5-minute APGAR is the better predictor of outcome, not the 1-minute score, and that APGAR has five components, not four.
• Link policy to content: RA 9288 mandates newborn screening for PKU and similar disorders, reflecting why early metabolic detection matters so much for lifelong phenotype.