Top NEET Facts — Mendelian Genetics Ratios and Definitions — Summary

Mendel used Pisum sativum — self-pollinating, 7 contrasting traits, 7 chromosome pairs, short generation time
7 dominant traits: Tall stem, Violet flower, Axial position, Round seed, Yellow seed colour, Inflated pod, Green pod colour
TRAP: Yellow seeds (dominant) BUT Green pods (dominant) — do not generalise
Law of Segregation = First Law: alleles separate during meiosis; one allele per gamete
Law of Independent Assortment = Second Law: different genes on different chromosomes assort independently in meiosis
Monohybrid F2: Phenotypic = 3:1 | Genotypic = 1:2:1 (complete dominance)
Dihybrid F2: Phenotypic = 9:3:3:1 (complete dominance, different chromosomes)
Test cross: Unknown × tt → all dominant = TT; 1:1 ratio = Tt
Dihybrid test cross (TtRr × ttrr) = 1:1:1:1 (four equal classes with independent assortment)
Incomplete dominance: F1 = intermediate phenotype; F2 phenotypic = genotypic = 1:2:1 (e.g., pink snapdragons)
Co-dominance: Both alleles fully expressed in F1; F2 = 1:2:1 (e.g., AB blood group)
ABO: 3 alleles ( $I^A$ , $I^B$ , i), 6 genotypes, 4 phenotypes; $I^A$ = $I^B$ > i
Universal Donor = O (no antigens); Universal Recipient = AB (no antibodies)
Pleiotropy: 1 gene → many traits (HbS = sickle cell anemia)
Polygenic: Many genes → 1 trait, continuous variation (skin colour, height)
Chromosomal Theory: Sutton + Boveri (1902) — genes on chromosomes
Linkage: Morgan + Drosophila — genes on same chromosome inherited together
Recombination frequency = % recombinant offspring = genetic distance in centimorgans (cM)
Independent Assortment does NOT apply to linked genes — major exception
Modified dihybrid F2 ratios (all sum to 16): 9:7 (complementary), 9:3:4 (recessive epistasis), 12:3:1 (dominant epistasis), 15:1 (duplicate), 13:3 (inhibitory)
Number of gamete types = 2^n (n = heterozygous gene pairs); genotypes in F2 = 3^n; phenotypes = 2^n