Glossary — Key Terms with NEET Definitions — Notes

Term	Definition	NEET Context
Allele	Alternative forms of a gene at the same chromosomal locus	"Mendel's factors" — each gene has 2 alleles per individual
Dominant allele	Allele that is expressed in both homozygous and heterozygous conditions	Represented by uppercase letter (T, R, A)
Recessive allele	Allele expressed only in homozygous condition; masked by dominant allele	Represented by lowercase letter (t, r, a)
Homozygous	Both alleles at a locus are identical (TT or tt)	Also called "pure-breeding" or "true-breeding"
Heterozygous	Two different alleles at a locus (Tt)	Also called "hybrid" or "carrier"
Phenotype	Observable physical expression of a trait	Depends on genotype + environment
Genotype	Genetic constitution — which alleles are present	Not always visible; needs test cross or molecular analysis
Monohybrid cross	Cross involving one pair of contrasting traits	Produces 3:1 (F2) or 1:1 (test cross) ratios
Dihybrid cross	Cross involving two pairs of contrasting traits	Produces 9:3:3:1 (F2) or 1:1:1:1 (test cross)
Test cross	Crossing unknown dominant phenotype with homozygous recessive to determine genotype	1:1 → heterozygous; all dominant → homozygous dominant
Back cross	F1 offspring crossed with a parent; includes test cross	Broader term; test cross is a specific back cross
F1 generation	First filial generation — offspring of P (parental) cross	All Tt in Mendel's monohybrid crosses
F2 generation	Second filial generation — offspring of F1 × F1 cross	Shows classical Mendelian ratios
Incomplete dominance	Heterozygote shows intermediate/blended phenotype	Pink snapdragons (Rr); F2 = 1:2:1
Co-dominance	Both alleles fully expressed simultaneously in heterozygote	Blood group AB ( $I^A$ $I^B$ )
Multiple alleles	More than 2 alleles for one gene existing in a population	ABO: $I^A$ , $I^B$ , i (3 alleles in population)
Pleiotropy	One gene affecting multiple unrelated traits	HbS allele → sickle cell anemia affecting multiple systems
Polygenic inheritance	Multiple genes with additive effects controlling one trait	Skin colour, height — continuous variation
Linkage	Genes on same chromosome tend to be inherited together	Exception to Law of Independent Assortment
Recombination	New allele combinations created by crossing over during meiosis I	Produces recombinant offspring in test crosses
Epistasis	One gene masks or modifies expression of another gene	Produces modified dihybrid ratios (9:7, 12:3:1, etc.)
Centimorgan (cM)	Unit of genetic distance = 1% recombination frequency	1 cM = 1 map unit; named after T.H. Morgan
Geitonogamy	Pollination between different flowers on the same plant	Genetically equivalent to self-pollination in homozygous plants
Emasculation	Removal of stamens to prevent self-pollination	Mendel's technique for controlled cross-pollination
Chiasma	X-shaped structure where crossing over occurs between non-sister chromatids	Forms during pachytene of Prophase I; plural = chiasmata