Exam 4 · Week 7 · BIO 280 Pathophysiology
M7 · Genetics
The vocabulary, the four inheritance patterns and how to spot each one on a pedigree, and what teratogens do and when.
▸M7GeneticsWeek 7
💡 The one idea
The pattern tells you the risk. Once you can name the inheritance pattern, the recurrence percentage falls straight out of it — you do not have to memorize a number per disease.
Dominant needs one copy. Recessive needs two. X-linked recessive hits boys.
📚 The vocabulary
- Genotype = the genes present. Phenotype = what you can actually observe.
- Homozygous = two identical alleles. Heterozygous = two different ones.
- Carrier = heterozygous for a recessive condition — has the allele, does not have the disease.
- Penetrance = what proportion of people with the genotype show the phenotype. Expressivity = how severely they show it.
- Humans have 46 chromosomes — 22 autosomal pairs plus one sex pair.
🧬 The four patterns
| Pattern | Recognize it by | Risk per pregnancy | Examples |
|---|---|---|---|
| Autosomal dominant | Appears in every generation; affected parent, affected child; males and females equally | 50% if one parent affected | Huntington, Marfan, familial hypercholesterolemia, polycystic kidney disease |
| Autosomal recessive | Skips generations; parents are unaffected carriers | 25% affected, 50% carrier, 25% clear | Cystic fibrosis, sickle cell, PKU, Tay–Sachs |
| X-linked recessive | Males affected, females carry. No male-to-male transmission. Passed from carrier mother to son. | 50% of sons affected, 50% of daughters carriers | Hemophilia A, Duchenne muscular dystrophy, red–green color blindness |
| Multifactorial | Genes plus environment; clusters in families without a clean pattern | Not a fixed number | Neural tube defects, cleft lip/palate, hypertension, type 2 diabetes |
Male-to-male transmission rules out X-linked immediately — a father gives his son a Y, never his X. That single fact answers most pedigree questions.
🧺 Chromosomal disorders
| Disorder | Chromosomes | Features |
|---|---|---|
| Down syndrome | Trisomy 21 | Intellectual disability, characteristic facies, single palmar crease, hypotonia; congenital heart defects. Risk rises with maternal age. |
| Edwards | Trisomy 18 | Severe; most die in infancy |
| Patau | Trisomy 13 | Severe; cleft, polydactyly |
| Turner | 45, XO — female | Short stature, webbed neck, no spontaneous puberty, infertility, coarctation |
| Klinefelter | 47, XXY — male | Tall, small testes, gynaecomastia, infertility |
Nondisjunction — chromosomes failing to separate during meiosis — is the mechanism behind all of the trisomies and monosomies above.
🚨 Teratogens — timing is everything
Weeks 3 to 8 of gestation is organogenesis and the period of greatest vulnerability. Many women do not yet know they are pregnant.
- Before implantation (~2 weeks) — all-or-nothing: either loss or no effect.
- Weeks 3–8 — structural malformation.
- After week 8 — growth restriction and functional problems, especially the CNS, which keeps developing to term.
Classic teratogens: alcohol (fetal alcohol spectrum disorder — no safe amount, no safe time), isotretinoin, warfarin, ACE inhibitors, phenytoin and valproate, lithium, tetracycline, live vaccines, rubella, cytomegalovirus, toxoplasmosis, radiation.
Folic acid 400–800 mcg daily before conception prevents
neural tube defects — the tube closes by week 4, so starting after a positive test is
already late.
⭐ Pharmacogenetics
Genes change how a drug behaves in a given person. The commonest route is the cytochrome P450 enzymes: a poor metabolizer clears a drug slowly and reaches toxic levels on a normal dose, while an ultra-rapid metabolizer clears it before it can work.
This is why two patients on an identical dose can have opposite outcomes, and why the answer to "the dose is right but the patient is toxic" is not always an error.
🎯 Module quiz
Questions for this module.
Nothing here yet — drop it in when you have it