Genetic, Teratogenic, and Low Birth Weight: Key Points for Review

Chromosomal and genetic problems

• Humans have $46$ chromosomes, created by two gametes each with $23$ chromosomes.
• Approximately $92\%$ of people do not develop a serious inherited condition by early adulthood; about $8\%$ do.
• Not exactly $46$: about half of zygotes have more or fewer than $46$ chromosomes due to nondisjunction; many fail to duplicate, divide, differentiate, and implant.
• Trisomies:
  • Trisomy 13 (Ptau): about $1/10000$ births.
  • Trisomy 18 (Edwards): about $1/5000$ births.
  • Trisomy 21 (Down syndrome): about $1/700$ births; characteristics often present and cognitive development typically slower.
• Sex chromosome abnormalities:
  • Turner syndrome (XO): one sex chromosome in females; notable features include short stature and other developmental differences.
  • Other anomalies (e.g., XXX, XYY, XXY/Klinefelter): specific patterns in cognition, fertility, and maturation.
  • Frequency examples: Turner $1/300$; Klinefelter $1/700$ in males; XYY $1/1000$; XXX $1/1000$ in females.
• Single-gene disorders:
  • Dominant: many are mild; some are not evident until adulthood (e.g., certain forms of Alzheimer's disease, Huntington's chorea, Marfan syndrome).
  • Recessive: more numerous; carriers are typically asymptomatic.
  • X-linked: hemophilia, Duchenne muscular dystrophy, fragile X syndrome (CGG repeats >200).
  • Common autosomal recessives in populations: ~1 in $10$ North American adults carries an allele for cystic fibrosis, thalassemia, or sickle cell disease.
  • Sickle cell: carriers can have malaria protection; multiple alleles exist; prevalence varies by ancestry (e.g., ~$8\%$ in Americans with African ancestry).
• Population genetics & polygenic traits:
  • Most health-related traits arise from many genes; Neanderthal ancestry contributes to some protections and susceptibilities (e.g., skin traits, certain diseases).
  • Genes can have both beneficial and harmful effects; interactions with environment shape outcomes (e.g., anxiety may be protective in some contexts and harmful in others).
• Gene–environment interactions & nurture:
  • Innate susceptibilities interact with prenatal and postnatal environments; nurture often modulates genetic risk.
• Teratogens, timing, and vulnerability:
  • Teratogens can cause visible birth defects or brain/behavioral issues; timing (critical/sensitive periods) is crucial.
  • Fathers and other relatives influence risk via stress or support; pregnancy itself is a critical window.
  • Some teratogens damage the brain at any time; alcohol is a key example with dose-related effects.
  • Male fetuses have higher vulnerability to teratogens than female fetuses (hazard rate varies by teratogen).
  • Folic acid: maternal folate status affects neural tube defects; folic acid supplementation reduces risk significantly.
• Folic acid study example:
  • Supplemented mothers: neural tube defect rate $rac{1}{250}$
  • Non-supplemented mothers: $rac{13}{300}$; supplementation markedly lowers risk, though some genetically at risk may still have defects.
• Innate vulnerability and nurture interplay:
  • Genes set tendencies for moods, imagination, empathy; experiences shape how those tendencies express themselves.

Teratogens and prenatal development

• Critical periods: damage can occur during specific gestational windows; some insults occur before pregnancy is known.
• Timing matters for behavioral teratogens: second half of pregnancy can be especially sensitive to binge alcohol exposure.
• Dose thresholds: many teratogens have a level above which damage occurs; no universally safe dose for psychoactive substances.
• Alcohol as teratogen:
  • Fetal alcohol syndrome (FAS): facial distortions and growth issues when exposure is heavy early.
  • Fetal alcohol effects (FAD): behavioral and developmental effects later; no safe level established.
• Genetic susceptibility to teratogens:
  • Male fetuses may be more vulnerable to some teratogens.
  • Maternal alleles affecting folic acid metabolism can increase neural tube defect risk in offspring; supplementation mitigates risk.
• Prevention during pregnancy:
  • Avoid teratogens, ensure good nutrition, folic acid supplementation, up-to-date immunizations, appropriate weight gain.

Low birth weight (LBW) and outcomes

• Definitions:
  • LBW: <2500\text{ g}
  • Very low birth weight (VLBW): <1500\text{ g}
  • Extremely low birth weight (ELBW): <1000\text{ g}
  • Normal birth weight: between $2500\text{ g}$ and about $4000\text{ g}$.
• Prevalence:
  • US LBW: $\approx 0.08 = 8\%$ of births.
  • Lowest rates: some high-income nations (e.g., Sweden) < $4\%$; higher rates in others (varies by year and country).
• Causes:
  • Maternal malnutrition, undernutrition, and poor diet; weight gain of less than about $1.3\text{ kg}$ per month in the last 6 months is linked to LBW.
  • Psychoactive drug use (including cigarettes and alcohol).
  • Multiple births (twins/triplets) and associated slower weight gain.
• Consequences across development:
  • Every milestone (smiling, feeding, walking, talking) can be delayed in LBW infants.
  • Higher risk of cognitive, visual, and hearing impairments; later life risks include diabetes, obesity, heart disease, depression.
  • SGA (small for gestational age) includes full-term babies who are small; LBW can be due to prematurity or growth restriction.
• Long-term plasticity:
  • Some LBW individuals catch up in brain development by age ~4 if no major health issues; adult outcomes vary.
• Immigrant paradox:
  • Babies of immigrant mothers often heavier and healthier than native-born peers of similar SES; hypotheses include strong social support and healthier behaviors.
• Trends and context (US):
  • LBW rates rose to around $0.0828 = 8.28\%$ by 2018 after prior declines; multiple factors proposed (nutrition, stress, access to care, unintended pregnancies, obesity/diabetes in mothers).
  • Fewer multiple births due to assisted reproduction, yet LBW rates rise, suggesting other factors (nutrition, healthcare access) are at play.
• Prevention and public health implications:
  • Improve nutrition, reduce drug use, enhance prenatal care, and support for expectant families.
  • Macro- and microsystem factors (socioeconomic status, immigration status) influence LBW risk; immigrant paradox suggests social support buffers risk.
  • Global patterns: LBW declines with better prenatal care in some countries (e.g., Cuba, Chile, China); rises in others due to hunger, disease, conflict.
• Takeaway:
  • LBW is a key predictor of lifelong health; preventing LBW through nutrition, health care, and social support yields broad benefits.