Subtopic 1: Sex Determination Systems (~120 words)

Humans use the XX-XY system; males (XY) are heterogametic and determine offspring sex by contributing either X or Y sperm to the X-carrying egg. Females (XX) are homogametic. Grasshoppers use the XX-XO system — males are XO (one fewer chromosome than female XX), producing X and O sperm in equal proportions; no Y chromosome exists. Birds, butterflies and some fish use the ZW-ZZ system, the reverse of the human system — females are ZW (heterogametic) and males are ZZ (homogametic). In ZW-ZZ organisms, the mother determines offspring sex. Drosophila (fruit fly) also uses XX-XY with male heterogamety.

Subtopic 2: Sex-Linked Inheritance (~130 words)

X-linked recessive disorders (haemophilia A, colour blindness) affect males predominantly because males are hemizygous — their single X chromosome has no masking allele on the Y. Carrier females ($X^A$ $X^a$) are phenotypically normal and pass the recessive allele to 50% of offspring. The fundamental rule is that fathers pass their Y chromosome to sons; therefore no X-linked trait passes from father to son. In the standard cross (carrier female × normal male): 50% of sons are affected, 50% of daughters are carriers, no daughters are affected. An affected female ($X^a$ $X^a$) requires an affected father ($X^a$ Y) and carrier/affected mother. Criss-cross inheritance describes the pattern where the trait alternates between sexes across generations through carrier females.

Subtopic 3: Pedigree Analysis (~100 words)

Autosomal dominant: appears in every generation, both sexes equally affected, at least one affected parent always present. Autosomal recessive: may skip generations, both sexes affected equally, affected individuals typically have two carrier parents. X-linked recessive: predominantly males affected, no male-to-male transmission, carrier females link affected generations. X-linked dominant: affected father passes to ALL daughters and NO sons; affected heterozygous mother passes to 50% of all children. The "no generation skipping" of dominant vs. "skipping" of recessive and the sex ratio together enable reliable pedigree pattern identification.

Subtopic 4: Mendelian Disorders (~120 words)

Sickle cell anaemia: point mutation (GAG→GUG) in HBB gene on chromosome 11 changes glutamic acid (position 6) to valine in beta-globin. HbS polymerizes under low $O_{2}$ causing sickle-shaped RBCs, vaso-occlusion, and haemolytic anaemia. Autosomal recessive; heterozygotes $\frac{HbA}{HbS}$ have partial malaria protection. PKU: deficiency of phenylalanine hydroxylase (PAH gene, chromosome 12) causes phenylalanine accumulation; autosomal recessive; treatable by early dietary restriction. Thalassemia: reduced alpha-globin (HBA genes, chromosome 16) or beta-globin (HBB, chromosome 11) synthesis; autosomal recessive; quantitative (not structural) defect. Haemophilia A: clotting factor VIII deficiency (F8 gene, X chromosome); X-linked recessive; prolonged bleeding and joint swelling.

Subtopic 5: Chromosomal Disorders (~130 words)

All four standard chromosomal disorders arise from non-disjunction during meiosis. Down syndrome (47, +21): trisomy 21 (autosome), affects both sexes, intellectual disability, epicanthic folds, short stature; risk increases with maternal age. Turner syndrome (45, XO): monosomy X, always female (no Y), short stature, webbed neck, shield chest, infertility (streak gonads), 0 Barr bodies. Klinefelter syndrome (47, XXY): always male (Y present, SRY triggers male development), gynaecomastia, tall stature, small testes, reduced fertility, 1 Barr body. Super Female (47, XXX): always female, 2 Barr bodies, often phenotypically normal, may have reduced fertility. Key rule: presence of Y → male; absence of Y → female, regardless of X chromosome number.