Principles of Inheritance and Variation
Why This Chapter Matters
Genetics is ALWAYS in NEET — 8-12 marks. Mendel's laws, dihybrid crosses, codominance, incomplete dominance, linkage, sex-linked traits, and chromosomal disorders are all tested extensively.
Prerequisites
Core Concepts
1. Mendel's Experiments
Mendel used Pisum sativum (garden pea) because:
7 pairs of contrasting characters:
Seed shape: round vs wrinkled
Seed colour: yellow vs green
Pod shape: inflated vs constricted
Pod colour: green vs yellow
Flower colour: violet vs white
Flower position: axial vs terminal
Plant height: tall vs dwarf
2. Mendel's Laws
Law of Dominance: In a cross between true-breeding contrasting characters, only dominant character appears in F₁.
Law of Segregation (Purity of Gametes): Two alleles of a character separate during gamete formation. Each gamete has only ONE allele.
Law of Independent Assortment: Alleles of different characters assort independently during gamete formation (provided genes are on different chromosomes).
3. Monohybrid Cross
TT (Tall) × tt (dwarf)
F₁: All Tt (Tall) — dominance
F₁ × F₁: Tt × Tt
F₂ genotype ratio: 1 TT : 2 Tt : 1 tt = 1:2:1
F₂ phenotype ratio: 3 Tall : 1 dwarf = 3:1
4. Dihybrid Cross
RRYY (Round Yellow) × rryy (Wrinkled Green)
F₁: All RrYy (Round Yellow)
F₁ × F₁: RrYy × RrYy
F₂ phenotype ratio: 9:3:3:1
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
Test cross: F₁ hybrid × homozygous recessive
Monohybrid test cross ratio: 1:1 | Dihybrid: 1:1:1:1
5. Incomplete Dominance
Neither allele is completely dominant. F₁ shows intermediate phenotype.
Example: Red (RR) × White (rr) → F₁ Pink (Rr) flowers in Mirabilis jalapa (4 o'clock plant)
F₁ × F₁: 1 Red : 2 Pink : 1 White (1:2:1 phenotype ratio — same as genotype)
6. Codominance
Both alleles expressed simultaneously in heterozygote.
Example: ABO blood groups
I^A I^A or I^A i → Blood group A
I^B I^B or I^B i → Blood group B
I^A I^B → Blood group AB (CODOMINANCE — both A and B antigens expressed)
ii → Blood group O
7. Multiple Alleles
A gene having more than 2 alleles in a population.
Example: ABO blood groups — I^A, I^B, i (3 alleles for 1 gene)
Hierarchy: I^A = I^B > i
8. Linkage and Crossing Over
Linkage: Genes on the same chromosome tend to be inherited together (do NOT show independent assortment).
Morgan worked on Drosophila melanogaster.
Recombination frequency: % of offspring showing new combinations (recombinant types).
1% recombination = 1 centimorgan (cM) = 1 map unit
Complete linkage: No crossing over → parental type only
Incomplete linkage: Crossing over occurs → parental types + recombinant types
9. Sex Determination
XX-XY system (Humans, Drosophila, most mammals): Females = XX, Males = XY
ZW-ZZ system (Birds, butterflies, some fish): Females = ZW, Males = ZZ
XO system (Grasshopper): Females = XX, Males = XO
10. Sex-Linked Inheritance
Genes on X chromosome (X-linked) show sex-linked inheritance.
Males (XY) have only ONE X — hemizygous, so recessive allele is expressed.
Haemophilia: X-linked recessive. "Royal disease"
Normal female (X^H X^H) | Carrier female (X^H X^h) | Haemophilic female (X^h X^h, rare)
Normal male (X^H Y) | Haemophilic male (X^h Y)
Colour blindness: X-linked recessive. Red-green colour blindness. More common in males.
Trait from father to son: NOT possible for X-linked traits (father gives Y to son)
Trait from father to daughter: Father gives X to daughter (all daughters of colour-blind father are at least carriers)
11. Chromosomal Disorders
Aneuploidy (abnormal chromosome number):
Non-disjunction during meiosis → extra or missing chromosome
Down's Syndrome (Trisomy 21): 47 chromosomes, trisomy of chromosome 21
Features: mental retardation, short stature, abnormal hand palm crease, susceptibility to infections
Maternal age increases risk.
Klinefelter's Syndrome (47, XXY): Male with extra X. Sterile, feminine features (gynaecomastia).
Turner's Syndrome (45, XO or 45, X): Female with one X missing. Short stature, infertile, web neck.
Edwards Syndrome (Trisomy 18): 47 chromosomes, trisomy of chromosome 18.
Polyploidy: Common in plants (not animals). Extra sets of chromosomes.
PYQs (NEET)
NEET 2023: In a test cross of a dihybrid F₁, the expected ratio of offspring is:
1:1:1:1 (if genes are on different chromosomes — independent assortment)
NEET 2022: Turner's syndrome has the genotype:
45, X (or 45, XO) — only one sex chromosome
NEET 2021: In ABO blood groups, codominance is shown by:
Blood group AB (I^A I^B) — both A and B antigens expressed simultaneously
NEET 2020: Red-green colour blindness gene is located on:
X chromosome (X-linked recessive)
MCQ Practice
Q1. Phenotypic ratio in incomplete dominance F₂ cross:
(A) 3:1 (B) 1:2:1 ✓ (C) 9:3:3:1 (D) 1:1
Q2. Which cross verifies genotype of an organism?
(A) Monohybrid cross (B) Dihybrid cross (C) Test cross ✓ (D) Reciprocal cross
Q3. Haemophilia is more common in males because:
(A) Males have less blood (B) Males have only one X chromosome ✓ (C) Gene is on Y (D) Autosomal recessive
Q4 (Hard). If a colour-blind father has a daughter with normal vision, the mother must be:
(A) Colour blind (B) Normal (C) Carrier or normal (if daughter got X^H from mother) (D) Carrier ✓
Q5 (Hard). Frequency of recombinant types in F₂ dihybrid cross (if 0% crossing over occurs):
Two linked genes — no crossing over → only parental types. Recombinant frequency = 0%.