Trap 1: Klinefelter is Male, Not Female

Wrong thinking: "47, XXY has two X chromosomes → must be female." Correct: The Y chromosome carries SRY gene → male. Two X chromosomes do NOT make an individual female if Y is present. NEET answer: Klinefelter (47, XXY) = male.

Trap 2: Turner is Female, Not Male

Wrong thinking: "45, XO has only one X chromosome → could be male." Correct: No Y chromosome present → no SRY → default female development. NEET answer: Turner (45, XO) = female.

Trap 3: Father Does NOT Pass X-Linked Trait to Son

Wrong thinking: "Haemophilic father passes haemophilia to his sons." Correct: Father passes Y to sons (not X). Haemophilic father ($X^h$ Y) passes $X^h$ to ALL daughters and Y to ALL sons. Sons' haemophilia comes from the MOTHER, not the father. Rule: "Father to daughter via X; never father to son via X."

Trap 4: Probability of Haemophilic SON vs. Haemophilic CHILD

Wrong thinking: In cross $X^H$ $X^h$ × $X^H$ Y, probability of haemophilic child = 1/2. Correct: P(haemophilic son) = 1/2 of sons, but P(haemophilic child among ALL children) = 1/4 (because 1/2 of children are sons and 1/2 of those sons are haemophilic = 1/4 of all children).

Trap 5: Down Syndrome is Autosomal Trisomy, Not Sex Chromosome

Wrong thinking: Down syndrome affects only one sex because it involves chromosome 21 and sex chromosomes. Correct: Chromosome 21 is an AUTOSOME. Down syndrome affects both males and females equally. Turner, Klinefelter, and Super Female involve sex chromosomes. Down syndrome is the only autosomal aneuploidy in the standard NEET list.

Trap 6: Sickle Cell Mutation Direction

Wrong thinking: "Valine at position 6 is replaced by glutamic acid in sickle cell." Correct: GLUTAMIC ACID (normal, position 6) is replaced by VALINE (mutant, position 6). The mutation goes Glu → Val (NOT Val → Glu). Mnemonic: "Normal G-six has Glu; Sick cell swaps Glu for Val."

Trap 7: Alpha vs. Beta Thalassemia Chromosomes

Wrong thinking: Both alpha and beta thalassemia involve chromosome 11. Correct: ALPHA-thalassemia = chromosome 16 (HBA genes). BETA-thalassemia = chromosome 11 (HBB gene, same gene as sickle cell). Memory trick: "Alpha 16 — A comes before B just as 16 comes after 11 but Alpha-16 is the pair to remember."

Trap 8: Carrier Concept Applies Only to Females for X-Linked Disorders

Wrong thinking: A normal-looking male can be a "carrier" for an X-linked recessive disorder. Correct: Males CANNOT be carriers for X-linked recessive disorders. They are hemizygous — one X, no second copy. A male is either AFFECTED ($X^a$ Y) or NORMAL ($X^A$ Y). The carrier state requires two X chromosomes with one being recessive ($X^A$ $X^a$). Males lack this heterozygous option.

Trap 9: Autosomal Dominant Requires an Affected Parent

Wrong thinking: Two normal parents can produce a child with an autosomal dominant disorder (like X-linked or autosomal recessive). Correct: Autosomal DOMINANT requires at least one parent to carry the dominant allele and therefore be affected. Two normal parents → no dominant allele to pass → no affected children (except de novo mutation). Two normal parents + affected child = autosomal RECESSIVE (both parents are carriers).

Trap 10: ZW-ZZ Female is Heterogametic (Not Male)

Wrong thinking: "Like humans, the male is always heterogametic in all sex determination systems." Correct: In the ZW-ZZ system (birds, butterflies), the FEMALE (ZW) is heterogametic (produces Z and W eggs). The MALE (ZZ) is homogametic. This is the reverse of the human XX-XY system. NEET exploits this contrast repeatedly.