Fundamental Genetic Concepts
Chromosomes are composed of DNA, the molecule that stores genetic instructions. Each human gamete contains 23 chromosomes because meiosis halves the diploid number. The sex chromosomes determine biological sex: XX yields a female, while XY yields a male. Therefore, the statement that a 23rd chromosome of XY indicates a female is false.
In heterozygous individuals, the dominant allele is expressed while the recessive allele is masked. This principle explains why many traits appear to follow simple Mendelian patterns.
Genetic variation also influences complex traits. In Western societies, the estimated heritability of intelligence is about 0.5, meaning roughly half of the observed variance is attributable to genetic factors.
Gene‑environment interaction refers to differences in reactivity to environmental qualities due to genetics. In contrast, epigenetics studies how the environment can modify gene expression without altering the DNA sequence.
Chromosomal Disorders and Their Impact
A well‑known chromosomal disorder is Trisomy 21, commonly called Down syndrome, which results from an extra copy of chromosome 21.
Bronfenbrenner’s Ecological Model
The microsystem includes the child’s immediate surroundings such as family and friends. Beyond the microsystem, the exosystem comprises settings that indirectly affect the child, for example:
- Neighborhood
- School
- Religious community
Socio‑economic status (SES) is strongly linked to academic outcomes; children from higher‑SES families tend to achieve better school performance.
Gene‑Environment Correlation
Gene‑environment correlation means that genes influence the environments we experience. Because twins share both genetics and many aspects of their environment, twin studies may overestimate heritability; shared environments can confound genetic estimates.
Prenatal Development
Human development before birth is divided into three periods:
- Germinal (0‑2 weeks) – fertilization to implantation.
- Embryonic (2‑8 weeks) – organogenesis; this stage is the most sensitive window for teratogens.
- Fetal (9 weeks to birth) – growth and maturation of established structures.
A teratogen is any environmental agent that causes prenatal damage. Examples include alcohol, certain medications, and infections. Prenatal alcohol exposure can lead to irreversible fetal damage, known as fetal alcohol spectrum disorders.
Newborn Assessment and Reflexes
The APGAR score, used to evaluate a newborn’s health immediately after birth, ranges up to 10. Among the reflexes observed in newborns, the rooting reflex causes the infant to turn its head toward a cheek touch, helping locate the breast for feeding.
Sleep and SIDS Risk
Newborn sleep cycles include both NREM and REM stages. In NREM sleep, the infant is still with no eye movement, whereas REM sleep features movements and occasional eye rolls. Sleeping on the stomach (prone position) increases the risk of sudden infant death syndrome (SIDS).
Hormonal Changes in New Mothers
Oxytocin levels rise in mothers after childbirth, promoting calmness, responsiveness, and bonding with the infant. This “love hormone” supports lactation and attachment.
Postpartum Depression
Postpartum depression affects about 15‑20% of women and roughly 8% of fathers, highlighting the need for awareness and support for both parents.
Distinguishing Gene‑Environment Interaction from Epigenetics
While gene‑environment interaction focuses on how genetic differences affect an individual’s response to environmental factors, epigenetics examines how environmental exposures can alter gene activity at the molecular level.
Summary of Key Points
- DNA composes chromosomes; each gamete carries 23 chromosomes.
- Sex is determined by X and Y chromosomes; XY indicates male.
- Dominant alleles are expressed in heterozygotes; recessive alleles are masked.
- Trisomy 21 causes Down syndrome.
- Microsystem = immediate environment; exosystem = indirect influences.
- Higher SES correlates with better academic outcomes.
- Gene‑environment correlation shapes the environments we encounter.
- Prenatal periods: germinal → embryonic → fetal; early weeks are most vulnerable to teratogens.
- APGAR score maxes at 10; rooting reflex aids feeding.
- Prone sleeping raises SIDS risk; NREM is quiet, REM is active.
- Oxytocin promotes maternal bonding; postpartum depression is common in both mothers and fathers.
- Interaction vs. epigenetics: response vs. expression changes.
