Sex determination: Father determines sex in humans (XX-XY); birds use ZW-ZZ (female = ZW heterogametic); grasshoppers use XX-XO (male = XO, no Y, 2n-1). Father always gives Y to sons — no X-linked trait can pass from father to son.

X-linked recessive (haemophilia A, colour blindness): Males are hemizygous → any recessive X allele expressed → more common in males. Carrier female × normal male: 50% sons affected, 50% daughters carriers, 0% daughters affected. P(haemophilic first child) = 1/4.

Sickle cell anaemia: HBB gene, chromosome 11; mRNA codon 6 GAG→GUG; Glutamic acid→Valine; HbS polymerizes under low $O_{2}$; autosomal recessive; heterozygotes have partial malaria protection. PKU: PAH gene, chromosome 12; phenylalanine accumulation; treatable by diet. Thalassemia: alpha (chromosome 16) or beta (chromosome 11); quantitative globin reduction.

Chromosomal disorders: Down = 47, +21 (trisomy 21 autosome; both sexes; intellectual disability; epicanthic folds). Turner = 45, XO (FEMALE; no Y; webbed neck; infertile). Klinefelter = 47, XXY (MALE; Y present, SRY gene; gynaecomastia; tall). Super Female = 47, XXX (female; often normal). Barr bodies = X chromosomes − 1.

Pedigree rules: Autosomal dominant = every generation, no skipping, both sexes. Autosomal recessive = skips generations, two carrier parents, both sexes equally. X-linked recessive = more males, no father-to-son transmission. X-linked dominant = affected father → all daughters affected, no sons affected.