Principles of Inheritance and Variation
'Genetics is the SCIENCE of HEREDITY — it explains both the SIMILARITY and the DIVERSITY among individuals of the same species.'
1. Chapter Overview
This chapter explores the PRINCIPLES by which traits are transmitted from parents to offspring. Topics include: MENDEL'S LAWS OF INHERITANCE (Law of Dominance, Law of Segregation, Law of Independent Assortment), INCOMPLETE DOMINANCE and CODOMINANCE, MULTIPLE ALLELES (ABO blood groups), the CHROMOSOMAL THEORY of inheritance, LINKAGE and CROSSING OVER, SEX DETERMINATION (XX-XY, ZZ-ZW, haplodiploidy), MUTATIONS (gene and chromosomal), and GENETIC DISORDERS (Mendelian disorders like Haemophilia, Sickle cell anaemia, and Chromosomal disorders like Down syndrome, Turner syndrome, Klinefelter syndrome).
2. Mendel's Laws
The Experiments
- Gregor Mendel (1822-1884) worked on PEA PLANTS (Pisum sativum) — choosing SEVEN CONTRASTING TRAITS with DISCRETE variation.
- Key choices: True-breeding lines, discrete characters, large sample sizes, statistical analysis.
Law of Dominance
- 'When two different ALLELES of a gene are present in a HETEROZYGOUS individual, ONLY ONE (the DOMINANT allele) is EXPRESSED in the phenotype. The other (RECESSIVE) allele is masked.'
- Example: Tt — Tall (T dominant), dwarf (t recessive).
Law of Segregation
- 'The TWO ALLELES of a gene SEPARATE during GAMETE FORMATION (meiosis), so that each gamete carries ONLY ONE allele for each gene.'
- Mendelian ratios: Monohybrid cross: F₂ = 3:1 (phenotypic), 1:2:1 (genotypic).
Law of Independent Assortment
- 'When TWO (or more) pairs of alleles are present, they ASSERT INDEPENDENTLY during gamete formation — the inheritance of one trait is INDEPENDENT of another.'
- Dihybrid cross: F₂ = 9:3:3:1 (phenotypic ratio).
- Exception: This law DOES NOT apply to GENES ON THE SAME CHROMOSOME (linked genes).
3. Post-Mendelian Genetics
Incomplete Dominance
- Neither allele is completely dominant — HETEROZYGOTE shows an INTERMEDIATE phenotype.
- Example: Snapdragon (Antirrhinum) flower colour — RR (red) × rr (white) → Rr (PINK).
- 'Pink flowers are NOT blending — the alleles remain SEPARATE. Test cross: Rr × rr → 1 Pink : 1 White.'
Codominance
- BOTH alleles are FULLY expressed in the heterozygote.
- Example: ABO blood group — Iᴬ and Iᴮ are codominant (both expressed). The AB blood group has BOTH A and B antigens on red blood cells.
Multiple Alleles
- More than TWO alleles of a gene exist in a population.
- ABO blood group: THREE alleles — Iᴬ, Iᴮ, i. Iᴬ = Iᴮ (codominant), both dominant over i.
- Genotypes: IᴬIᴬ or Iᴬi (Type A), IᴮIᴮ or Iᴮi (Type B), IᴬIᴮ (Type AB), ii (Type O).
- Rh factor: Rh⁺ (dominant), Rh⁻ (recessive). Rh incompatibility in pregnancy can cause ERYTHROBLASTOSIS FOETALIS.
4. Chromosomal Theory of Inheritance
- 'Sutton and Boveri (1902) proposed that GENES are located on CHROMOSOMES — and chromosomes behave according to Mendel's laws during meiosis.'
- Evidence: Parallel behaviour of genes and chromosomes: (1) Both exist in pairs. (2) They segregate during meiosis. (3) They assort independently (for genes on different chromosomes).
Linkage and Crossing Over
- Linkage: Genes located on the SAME CHROMOSOME tend to be INHERITED TOGETHER — they do NOT assort independently.
- Crossing over: Exchange of segments between HOMOLOGOUS CHROMOSOMES during Prophase I of meiosis — BREAKS linkages.
- Recombination frequency: Depends on the DISTANCE between genes on the chromosome. 'Farther apart → higher recombination frequency. Used to CONSTRUCT linkage maps.'
5. Sex Determination
| System | Mechanism | Examples |
|---|---|---|
| XX-XY (male heterogamety) | Male = XY (heterogametic). Female = XX | Humans, Drosophila |
| ZZ-ZW (female heterogamety) | Female = ZW. Male = ZZ | Birds, snakes, butterflies |
| XO type | Male = XO (only one X). Female = XX | Grasshoppers, cockroaches |
| Haplodiploidy | MALE = HAPLOID (unfertilised eggs). Female = DIPLOID | Honeybees, ants, wasps |
- In humans: The Y chromosome carries the SRY GENE (Sex-determining Region Y) — its presence triggers MASCULINISATION. 'The presence or absence of Y determines sex — NOT the number of X chromosomes.'
6. Mutations
Gene Mutations (Point Mutations)
- Change in the DNA SEQUENCE at a single position.
- Silent mutation: No change in amino acid (wobble). Missense: Changes one amino acid (e.g., Sickle cell anaemia — GAG → GTG, Glu → Val). Nonsense: Introduces PREMATURE STOP CODON.
Chromosomal Mutations
- Deletion: Loss of a segment. Duplication: Repetition of a segment. Inversion: Reversal of a segment. Translocation: Segment moves to a DIFFERENT chromosome.
7. Genetic Disorders
Mendelian Disorders (Single Gene)
| Disorder | Gene | Chromosome | Inheritance | Defect |
|---|---|---|---|---|
| Haemophilia | Factor VIII/IX | X | X-LINKED RECESSIVE | Blood clotting DEFECT |
| Sickle cell anaemia | β-globin | 11 | AUTOSOMAL RECESSIVE | HbS — sickle-shaped RBCs |
| Cystic fibrosis | CFTR | 7 | AUTOSOMAL RECESSIVE | Thick mucus in lungs/digestive system |
| Phenylketonuria (PKU) | PAH | 12 | AUTOSOMAL RECESSIVE | Cannot metabolise phenylalanine |
| Colour blindness | Opsin genes | X | X-LINKED RECESSIVE | Cannot distinguish colours (red-green) |
Chromosomal Disorders
| Syndrome | Karyotype | Features |
|---|---|---|
| Down syndrome | TRISOMY 21 (47, +21) | Intellectual disability, characteristic facial features, heart defects |
| Turner syndrome | MONOSOMY X (45, XO) | Short stature, webbed neck, sterile (ovarian failure) |
| Klinefelter syndrome | 47, XXY | Male, tall, reduced testosterone, sterile |
| Cri-du-chat | DELETION on chromosome 5 | Cat-like cry, intellectual disability |
8. Pedigree Analysis
- 'A PEDIGREE is a FAMILY TREE showing the inheritance pattern of a trait. It helps determine: (1) Is the trait DOMINANT or RECESSIVE? (2) Is it AUTOSOMAL or SEX-LINKED?'
- Key patterns: Autosomal dominant → appears in EVERY generation, affects males and females equally. Autosomal recessive → SKIPS generations, appears when both parents are carriers. X-linked recessive → MORE males affected, carrier females pass to sons.
9. Common Mistakes
- Incomplete dominance is NOT blending: The alleles REMAIN separate and can segregate in subsequent generations — unlike blending inheritance which was DISPROVED by Mendel.
- Linked genes do NOT follow independent assortment: The 9:3:3:1 ratio ONLY applies to genes on DIFFERENT chromosomes.
- Sex of a child is determined by the SPERM: The ovum contributes one X chromosome. The sperm contributes either X or Y. 'The father DETERMINES the sex of the child.'
- Haemophilia is X-linked recessive: More common in MALES (only one X chromosome). Females need two mutated alleles to be affected (rare).
10. CBSE Exam Focus
- Mendel's laws — monohybrid and dihybrid crosses, ratios
- Incomplete dominance and codominance — examples, test cross
- ABO blood group — multiple alleles, genotypes and phenotypes
- Linkage and crossing over — concept, recombination frequency
- Sex determination — different systems, SRY gene
- Genetic disorders — Mendelian (haemophilia, sickle cell anaemia) and chromosomal (Down, Turner, Klinefelter)
11. Self-Test
Q1: A cross between a tall plant (Tt) and a dwarf plant (tt) produced 200 offspring. How many would be tall? A1: Tt × tt → Tt (tall) and tt (dwarf) in 1:1 ratio. Expected tall = 100.
Q2: What would be the blood group of a child if the father is IᴬIᴮ and the mother is Iᴬi? A2: Father's alleles: Iᴬ, Iᴮ. Mother's alleles: Iᴬ, i. Possible genotypes: IᴬIᴬ (A), Iᴬi (A), IᴮIᴬ (AB), Iᴮi (B). Possible blood groups: A, AB, B.
Q3: Why do more males than females have colour blindness? A3: Colour blindness is X-LINKED RECESSIVE. Males have only ONE X chromosome — if they inherit the mutated allele, they are affected. Females need TWO mutated alleles (one from each parent) to be affected — extremely RARE.
Q4: Differentiate between Turner syndrome and Klinefelter syndrome. A4: Turner: 45, XO — FEMALE, short stature, sterile, webbed neck. Klinefelter: 47, XXY — MALE, tall, reduced testosterone, sterile.
Q5: In a dihybrid cross (YyRr × YyRr), what is the probability of a YYRR offspring? A5: Probability of YY = 1/4. Probability of RR = 1/4. Combined (independent assortment): 1/4 × 1/4 = 1/16.
12. Conclusion
The principles of inheritance are the FOUNDATION of genetics:
- MENDEL: 'His laws — dominance, segregation, independent assortment — are the CORNERSTONES of classical genetics.'
- LINKAGE: 'Mendel's third law has EXCEPTIONS — genes on the same chromosome are LINKED and travel together.'
- 'From Mendel's pea plants to modern genomic medicine — the principles of inheritance remain the SAME, but our understanding has DEEPENED enormously.'
