GEN-02 HIGH-SPEED REVISION CARD

SEX DETERMINATION

• Father determines sex (XX-XY): X sperm → daughter; Y sperm → son
• Grasshopper: XX-XO (male = XO, no Y, 2n-1; female = XX, 2n)
• Birds: ZW-ZZ (female ZW = heterogametic; male ZZ = homogametic) ← REVERSE of humans

X-LINKED RECESSIVE RULES $\frac{Haemophilia}{Colour Blindness}$

• Males: hemizygous → any recessive X allele expressed → more affected
• Carrier female ($X^A$ $X^a$): normal phenotype, 50% sons affected, 50% daughters carriers
• Affected male ($X^h$ Y): ALL daughters become carriers, ALL sons normal (via non-carrier mother)
• NO father-to-son transmission (father gives Y to sons, NEVER X)
• Affected female: needs $X^a$ from BOTH parents (rare: needs carrier/affected mom + affected dad)

PEDIGREE QUICK-ID

• Every generation affected + at least one affected parent → AUTOSOMAL DOMINANT
• Skips generations + equal sex ratio + unaffected parents → AUTOSOMAL RECESSIVE
• More males + no father-to-son transmission + carrier females → X-LINKED RECESSIVE
• ALL daughters of affected father affected + NO sons → X-LINKED DOMINANT

CHROMOSOMAL DISORDERS

• Turner = FEMALE (no Y); Klinefelter = MALE (Y present, SRY gene)
• Down = ONLY autosomal disorder in this list (chromosome 21 is an autosome)
• Barr bodies = number of X chromosomes − 1

MENDELIAN DISORDERS

KEY PROBABILITIES

• Carrier × Normal male → P(affected child) = 1/4; P(affected son) = 1/2 of sons
• Carrier × Affected male → P(affected child) = 1/2
• Two carriers (autosomal recessive) → P(affected child) = 1/4
• Unaffected sibling of autosomal recessive patient → P(carrier) = 2/3