GEN-02 Bulleted NEET Facts

Father determines sex in XX-XY system; mother always contributes X. Father contributes X (daughter) or Y (son).
Birds use ZW-ZZ: female ZW is heterogametic (produces Z and W eggs). Male ZZ is homogametic. This is the REVERSE of humans.
Grasshoppers use XX-XO: male XO (2n-1; no Y); female XX (2n). Males produce X and O sperm equally.
Hemizygosity in males (one X, no backup): any allele on X is expressed → X-linked recessive conditions more common in males.
Haemophilia A: X-linked recessive; F8 gene, X chromosome; deficiency of clotting factor VIII; prolonged bleeding.
Colour blindness: X-linked recessive; photopigment genes on X chromosome; inability to distinguish red from green.
Key inheritance rule: Father passes Y to ALL sons, X to ALL daughters. NO father-to-son X-linked transmission.
Carrier female cross ( $X^H$ $X^h$ × $X^H$ Y): 50% sons affected, 50% daughters carriers, no daughters affected.
Sickle cell mutation: HBB gene, chromosome 11; mRNA codon 6 GAG→GUG; glutamic acid→valine; autosomal recessive.
HbS polymerizes under low $O_{2}$ → sickle RBCs → vaso-occlusion + haemolytic anaemia.
PKU: PAH gene, chromosome 12; phenylalanine hydroxylase deficiency; phenylalanine accumulates; autosomal recessive; treatable by diet.
Alpha-thalassemia: HBA genes, chromosome 16; reduced alpha-globin synthesis; autosomal recessive.
Beta-thalassemia: HBB gene, chromosome 11; reduced beta-globin synthesis; autosomal recessive.
Down syndrome: 47, +21 (trisomy 21 = autosome); either sex; intellectual disability, epicanthic folds, simian crease; risk increases with maternal age.
Turner syndrome: 45, XO (monosomy X); FEMALE; short stature, webbed neck, shield chest, infertility; 0 Barr bodies.
Klinefelter syndrome: 47, XXY; MALE (SRY gene on Y); gynaecomastia, tall, small testes; 1 Barr body.
Super Female: 47, XXX; female; often phenotypically normal; 2 Barr bodies.
Barr body rule: number of Barr bodies = total X chromosomes − 1.
Pedigree rules: Autosomal dominant = every generation, both sexes; Autosomal recessive = skips generations, both sexes; X-linked recessive = more males, no father-to-son; X-linked dominant = all daughters of affected father are affected.
Conditional probability: unaffected sibling of autosomal recessive patient has 2/3 probability of being a carrier.