Why Genetics Matters for NEET
Genetics contributes 6–8 questions in NEET Biology (360 marks total). Monohybrid and dihybrid cross ratios, blood group genetics, sex-linked disorders, and mutations are frequently tested. Master the ratios and cross diagrams first.
1. Mendel's Experiments — The Foundation
Why Mendel Chose Pea Plants (Pisum sativum)
- Short life cycle — multiple generations in one season
- Clear contrasting characters — easy to score
- Naturally self-pollinating — pure lines easy to maintain
- Large number of offspring — statistically reliable
Mendel's Seven Contrasting Characters
| Character | Dominant | Recessive |
|---|---|---|
| Seed shape | Round (R) | Wrinkled (r) |
| Seed colour | Yellow (Y) | Green (y) |
| Pod shape | Inflated (I) | Constricted (i) |
| Pod colour | Green (G) | Yellow (g) |
| Flower colour | Purple (P) | White (p) |
| Flower position | Axial (A) | Terminal (a) |
| Plant height | Tall (T) | Dwarf (t) |
Law 1 — Law of Segregation (Monohybrid Cross)
Statement: During gamete formation, the two alleles of a gene separate (segregate) so that each gamete carries only one allele for each gene.
Cross: Tall × Dwarf pea plant
Key insight: The recessive trait (dwarf) disappears in F1 but reappears in F2 in 25% of plants — this proves alleles segregate and recessive alleles are not lost.
Law 2 — Law of Independent Assortment (Dihybrid Cross)
Statement: Genes for different characters assort independently during gamete formation (applies only to genes on different chromosomes).
Cross: Round Yellow × Wrinkled Green
Memory tip for 9:3:3:1: "Nine three three one — first class gets the most, last class gets the least."
2. Extensions of Mendelian Genetics
Incomplete Dominance
Neither allele is completely dominant. Heterozygote shows intermediate phenotype.
Note: In incomplete dominance, phenotypic ratio = genotypic ratio (both 1:2:1) — unlike Mendel's 3:1.
Codominance
Both alleles are fully expressed in the heterozygote. Neither masks the other.
Example: ABO Blood Groups
| Blood Group | Genotype | Antigen on RBC |
|---|---|---|
| A | I^A I^A or I^A i | A antigen |
| B | I^B I^B or I^B i | B antigen |
| AB | I^A I^B | Both A and B antigens — codominant |
| O | ii | No antigen |
- I^A and I^B are codominant to each other
- Both I^A and I^B are dominant over i (recessive)
Cross: A × B → possible offspring?
Multiple Alleles
A gene with more than two alleles in the population (any individual has only two).
ABO blood group is the classic example — three alleles: I^A, I^B, i.
Pleiotropy
One gene, multiple effects. A single gene influences more than one phenotypic trait.
Example: Sickle Cell Anaemia (HbS gene)
- Abnormal haemoglobin → sickle-shaped RBCs
- This single change causes: anaemia, organ damage, jaundice, poor growth, low immunity
Polygenic Inheritance
Multiple genes, one trait. Several genes act together to produce a single character, giving a continuous range of phenotypes.
Examples: Skin colour (3–6 gene loci), human height, intelligence.
- F2 shows a bell-curve (normal distribution) of phenotypes — no sharp classes.
3. Chromosomal Theory of Inheritance
Sutton and Boveri (1902)
Chromosomes behave exactly like Mendel's factors during meiosis:
- Chromosomes occur in pairs → alleles occur in pairs
- Chromosomes separate at meiosis → alleles segregate
- Chromosome pairs assort independently → genes assort independently
Morgan's Drosophila Experiments
T.H. Morgan worked with Drosophila melanogaster (fruit fly) — chosen because:
- 4 pairs of chromosomes only
- Short generation time (2 weeks)
- Large number of offspring
- Easy to distinguish sex
Sex-linked Inheritance (White eye in Drosophila):
Sex Determination
| System | Species | Females | Males |
|---|---|---|---|
| XX-XY | Humans, Drosophila | XX | XY |
| ZW-ZZ | Birds, butterflies | ZW | ZZ |
| XO | Grasshopper | XX | XO |
In humans: Sex determined by father's sperm (X-bearing or Y-bearing). Y chromosome carries SRY gene → testes development.
Human X-linked Recessive Conditions
| Condition | Gene Location | Males Affected | Females Affected |
|---|---|---|---|
| Colour blindness | X chromosome | X^b Y | X^b X^b only |
| Haemophilia A | X chromosome | X^h Y | X^h X^h only |
Why males are more commonly affected: Males have only one X chromosome — one recessive allele is enough. Females need two copies of the recessive allele.
Carrier female: X^B X^b — normal phenotype but 50% chance of passing recessive allele to sons.
4. Linkage and Crossing Over
Linkage
Genes located on the same chromosome tend to be inherited together — they do not independently assort.
- Linked genes give ratios different from 9:3:3:1
- Morgan first demonstrated linkage in Drosophila
Complete linkage: Linked genes always transmitted together — only parental combinations in offspring.
Incomplete linkage: Crossing over occasionally separates linked genes — some recombinant types appear.
Crossing Over
Exchange of chromosome segments between homologous chromosomes during prophase I of meiosis I.
Recombination frequency: Percentage of recombinant offspring. Used to calculate map distance.
- 1% recombination frequency = 1 centimorgan (cM) = 1 map unit
- Genes far apart on chromosome → more crossing over → higher recombination frequency
5. Mutations
Gene Mutations (Point Mutations)
Changes in DNA base sequence:
| Type | Description | Example |
|---|---|---|
| Transition | Purine → Purine or Pyrimidine → Pyrimidine | A→G or C→T |
| Transversion | Purine → Pyrimidine or vice versa | A→C or G→T |
| Insertion | Addition of a base pair | — |
| Deletion | Removal of a base pair | — |
Frameshift mutations: Insertions and deletions shift the reading frame → all downstream codons change → usually non-functional protein.
Chromosomal Mutations
Change in number (Aneuploidies):
| Condition | Chromosome Count | Description |
|---|---|---|
| Down syndrome | 47 (trisomy 21) | Three copies of chromosome 21 |
| Turner syndrome | 45 (45,X) | Monosomy — only one X |
| Klinefelter syndrome | 47 (XXY) | Extra X in male |
| Patau syndrome | 47 (trisomy 13) | Severe intellectual disability |
Change in structure:
- Deletion: segment lost
- Duplication: segment repeated
- Inversion: segment reversed
- Translocation: segment moves to non-homologous chromosome
Polyploidy: Entire genome duplicated. Common in plants (wheat: hexaploid 6n=42). Induced by colchicine (inhibits spindle formation).
Mutagens
| Type | Example | Mechanism |
|---|---|---|
| Physical | UV radiation | Thymine dimers |
| Chemical | Colchicine | Disrupts spindle → polyploidy |
| Chemical | Alkylating agents | Add alkyl groups to bases |
| Biological | Transposons | Jump into genes |
Board Examination Questions
1-Mark Questions
Q1: What is the genotypic ratio of F2 in a monohybrid cross? 1:2:1 (AA : Aa : aa)
Q2: Which enzyme does colchicine inhibit? Colchicine does not inhibit an enzyme — it inhibits spindle fibre formation by binding tubulin, preventing chromosome separation during cell division.
Q3: Name the scientist who demonstrated semiconservative replication. Meselson and Stahl (1958).
Q4: What is a carrier in genetics? A carrier is a heterozygous individual (e.g., X^B X^b) who has one copy of a recessive allele but shows no symptoms of the associated disease.
Q5: Define pleiotropy. Pleiotropy is the phenomenon where a single gene controls or influences multiple phenotypic traits. Example: sickle cell anaemia gene affects RBC shape, oxygen transport, and many organ systems.
2-Mark Questions
5-Mark Questions
NEET Focus Points
2. Blood groups: Know all 6 genotypes, universal donor (O/ii), universal recipient (AB/I^A I^B) 3. Sex-linked: Colour blindness and haemophilia — X-linked recessive, why males affected more 4. Chromosomal disorders: Down (trisomy 21), Turner (45,X), Klinefelter (47,XXY) — chromosome numbers 5. Mutation types: Point mutation (sickle cell = one amino acid change), frameshift, chromosomal
| 1. Ratios to memorise: Monohybrid F2 = 3:1 phenotypic, 1:2:1 genotypic | Dihybrid F2 = 9:3:3:1 | Incomplete dominance F2 = 1:2:1 |
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