Principles of Inheritance and VariationNEET Botany · Class 12 · NCERT Chapter 2

Principles of Inheritance and Variation is a Very High Weightage chapter with 6 to 8 questions in most NEET exams. Questions focus on Mendel's laws, monohybrid (3:1) and dihybrid (9:3:3:1) crosses, test cross, incomplete dominance vs codominance, ABO blood groups (multiple alleles), pleiotropy, polygenic inheritance, sex determination types (XY, XO, ZW, ZO), sex-linked inheritance (haemophilia, colour blindness), linkage and recombination (Morgan's work), chromosomal disorders (Down, Klinefelter, Turner), and pedigree analysis. This chapter is one of the highest-yield chapters in NEET Biology.

Mendel chose garden pea because: (1) Many TRUE-BREEDING varieties existed (homozygous lines that always produced the same trait). (2) The plant has SHARP CONTRASTING TRAITS (e.g., tall vs short, round vs wrinkled, yellow vs green). (3) Pea flowers are normally SELF-POLLINATED (autogamous) due to cleistogamous structure, so Mendel could prevent unwanted cross-pollination. He could also do controlled CROSS-POLLINATION by removing anthers (emasculation) and dusting pollen by hand. (4) Pea has a SHORT LIFE CYCLE (a few months), allowing many generations in a few years. (5) It produces LARGE NUMBERS of offspring per cross, allowing reliable statistical analysis. Mendel studied 7 contrasting characters: seed shape, seed colour, flower colour, pod shape, pod colour, flower position, plant height.

Both are exceptions to complete dominance, but they differ in how the heterozygote looks: (1) INCOMPLETE DOMINANCE: the heterozygote shows an INTERMEDIATE / BLENDED phenotype - neither dominant allele fully expressed. Example: Mirabilis jalapa flower colour: RR red x rr white → Rr PINK (intermediate). F2 ratio: 1 red : 2 pink : 1 white (= 1:2:1 phenotypic = genotypic). (2) CODOMINANCE: the heterozygote shows BOTH parental phenotypes EQUALLY and INDEPENDENTLY (no blending). Example: ABO blood group: IA + IB = AB (both A and B antigens present). Roan coat in cattle: red x white = roan (mixture of red and white hairs, both visible). NEET trap: pink (incomplete) is a NEW colour; AB (codominance) is BOTH alleles expressed.

The ABO blood group is controlled by THREE alleles at one gene (ABO gene on chromosome 9): IA, IB, and i. (1) IA and IB are CODOMINANT to each other (both express their antigens when together). (2) Both IA and IB are DOMINANT over i (recessive). The four blood groups: A (IA IA or IA i), B (IB IB or IB i), AB (IA IB - codominance, both antigens), O (ii - no antigens). NEET facts: this is an example of MULTIPLE ALLELES (more than 2 alleles in the population). The IA and IB alleles produce antigens A and B respectively; the i allele produces no antigen. The ABO gene encodes a glycosyltransferase enzyme that adds sugar groups to the H antigen. People with O blood are universal donors (no A or B antigens to react). People with AB are universal recipients.

Three main systems for NEET: (1) XY (humans, Drosophila): males are XY (heterogametic - produce 2 types of gametes: X and Y), females are XX (homogametic). The Y chromosome carries male-determining genes (SRY in humans). (2) XO (grasshopper): males are XO (have only ONE X, no Y), females are XX. Males are heterogametic (X or no chromosome). (3) ZW (birds, butterflies): FEMALES are ZW (heterogametic), males are ZZ (homogametic). The reverse of human XY system. (4) ZO (some moths, fishes): females are ZO, males are ZZ. NEET trap: in birds, the FEMALE is the heterogametic sex (ZW), opposite to humans where male (XY) is heterogametic. Sex chromosomes in humans: 22 pairs of autosomes + 1 pair of sex chromosomes (XX in female, XY in male).

LINKAGE: when two or more genes are located on the SAME CHROMOSOME, they tend to be inherited TOGETHER (they do not segregate independently as Mendel's 3rd law would predict). INDEPENDENT ASSORTMENT happens only when genes are on DIFFERENT chromosomes. Discovered by T.H. MORGAN in Drosophila experiments. Linked genes: alleles on the same chromosome stay together unless crossing over separates them. Recombination frequency: the percentage of offspring that show new (non-parental) gene combinations = (number of recombinants / total offspring) x 100. The MAP DISTANCE between two genes on a chromosome is measured in CENTIMORGANS (cM); 1 cM = 1% recombination frequency. Genes very close together: low recombination, tightly linked. Genes far apart: high recombination, almost independent. Morgan's work showed that Mendel's 3rd law (independent assortment) only works for genes on different chromosomes. He won the Nobel Prize in 1933 for this work.

All three are CHROMOSOMAL DISORDERS caused by ANEUPLOIDY (abnormal chromosome number from non-disjunction during meiosis). (1) DOWN'S SYNDROME: trisomy 21 (extra copy of chromosome 21). Total chromosomes: 47 (22 pairs of autosomes + chromosome 21 has 3 copies + XX or XY). Symptoms: short stature, characteristic facial features, intellectual disability, heart defects. AUTOSOMAL aneuploidy. (2) KLINEFELTER'S SYNDROME: XXY (extra X chromosome in a male). Total chromosomes: 47 (22 pairs of autosomes + XXY). Symptoms: tall stature, sterile, develops some female features (gynecomastia). SEX CHROMOSOMAL aneuploidy. (3) TURNER'S SYNDROME: XO (missing X chromosome in a female). Total chromosomes: 45 (22 pairs of autosomes + only one X). Symptoms: short stature, sterile, lack of secondary sexual characteristics, webbed neck. SEX CHROMOSOMAL aneuploidy. All three are caused by NON-DISJUNCTION (failure of chromosomes to separate) during meiosis I or II.