Glossary Note

Term	Definition	NEET Context
Aneuploidy	Abnormal chromosome number (not an exact multiple of the haploid set)	Cause of chromosomal disorders: monosomy (2n-1), trisomy (2n+1)
Autosome	Any chromosome other than the sex chromosomes (chromosomes 1–22 in humans)	Down syndrome involves autosome 21; Mendelian disorders involve autosomes
Carrier	Heterozygous individual carrying a recessive allele but not expressing the disorder	Applies to females for X-linked recessive; applies to both sexes for autosomal recessive
Hemizygous	Having only one allele for a gene (no second copy); characteristic of X-linked genes in males	Explains why X-linked recessive conditions are more common in males
Heterogametic	Producing two types of gametes with respect to sex chromosomes	Male in XX-XY and XX-XO; Female in ZW-ZZ
Homogametic	Producing only one type of gamete with respect to sex chromosomes	Female in XX-XY and XX-XO; Male in ZW-ZZ
Karyotype	The complete set of chromosomes of an organism, arranged by size, morphology and banding pattern	Used to diagnose chromosomal disorders (Down: 47,+21; Turner: 45,XO; Klinefelter: 47,XXY)
Monosomy	Presence of only one chromosome of a particular type (2n-1)	Turner syndrome: 45, XO
Non-disjunction	Failure of chromosomes or chromatids to separate during meiosis, producing aneuploid gametes	Cause of all four NEET chromosomal disorders
Obligate carrier	Individual who must be a carrier based on pedigree evidence, regardless of their own phenotype	Daughter of an affected father in X-linked recessive is an obligate carrier
Pedigree	A chart showing the genetic relationships and traits within a family over multiple generations	Used in NEET to identify inheritance patterns
SRY gene	Sex-determining region Y; gene on the Y chromosome that triggers testicular development and male phenotype	Explains why Klinefelter (XXY) is male and Turner (XO) is female
Trisomy	Presence of three copies of a chromosome (2n+1)	Down syndrome: trisomy 21 (47, +21)
X-inactivation	Random silencing of one X chromosome in each somatic cell of a female	Barr body = inactive X; number of Barr bodies = X chromosomes - 1
Criss-cross inheritance	Pattern in X-linked inheritance where the trait appears to cross between sexes in successive generations	X-linked recessive: maternal grandfather → carrier daughter → affected grandson
Point mutation	Change in a single nucleotide base pair	Sickle cell anaemia: GAG→GUG (codon 6 of HBB gene)
Haemophilia A	X-linked recessive disorder due to deficiency of clotting factor VIII (F8 gene, X chromosome)	Prolonged bleeding, joint swelling; more common in males
HbS	Sickle haemoglobin; structurally abnormal beta-globin with valine at position 6 instead of glutamic acid	Polymerizes under low $O_{2}$ ; causes sickle-shaped RBCs and vaso-occlusion