Principles of Inheritance and Variation: Class 12 Biology Chapter 5 Comprehensive Guide
Genetics, Biology ki wo branch hai jo Inheritance (virsat) aur Variation (badlav) ke principles ko samajhati hai. Kya aapne kabhi socha hai ki ek hathi ka bacha hathi hi kyun hota hai, ya aap apne parents se milte-julte kyun dikhte hain, lekin bilkul unke jaise nahi? In sab sawalon ke jawab humein is chapter mein milte hain.
1. Important Terminology (Zaroori Shabd)
Chapter shuru karne se pehle, in terms ko samajhna bahut zaroori hai:
Genetics: Study of heredity and variation.
Heredity/Inheritance: Parents se offspring (bachon) tak characters ka transfer hona.
Variation: Parents aur offspring ke beech ka antar.
Gene: Inheritance ki unit (Mendel ne ise 'Factor' kaha tha).
Allele: Gene ke alag-alag roop (e.g., Tallness ke liye T aur dwarfness ke liye t).
Phenotype: Jo bahar se dikhta hai (e.g., Tall ya Dwarf).
Genotype: Genetic makeup (e.g., TT, Tt, ya tt).
2. Mendel’s Laws of Inheritance
Gregor Mendel ko "Father of Genetics" kaha jata hai. Unhone Garden Pea (Pisum sativum) par 7 saal (1856-1863) tak experiments kiye.
Mendel ne Pea Plant hi kyun chuna?
Ismein clear contrasting characters the (jaise lamba/bona, gol/jhurridar beej).
Inka lifespan chhota hota hai.
Inmein self-pollination aur cross-pollination dono karwaya ja sakta hai.
[Image 1: Mendel’s Seven Pairs of Contrasting Traits in Pea Plant]
(Description: A clear, colorful educational chart showing the seven traits Mendel studied: Seed shape, seed color, flower color, pod shape, pod color, flower position, and stem height. Each trait is split into 'Dominant' and 'Recessive' columns with illustrative icons.)
Mendel ke Experiments:
Mendel ne do main tarah ke crosses kiye:
A. Monohybrid Cross (Ek gene ka inheritance)
Mendel ne ek tall (TT) aur ek dwarf (tt) plant ko cross karwaya.
F1 Generation: Saare plants Tall (Tt) the.
F2 Generation: Selfing ke baad, plants 3:1 ke ratio mein mile (3 Tall, 1 Dwarf).
Phenotypic Ratio: 3:1
Genotypic Ratio: 1(TT) : 2(Tt) : 1(tt)
[Image 2: Monohybrid Cross Punnett Square]
(Description: A classic Punnett square diagram showing the cross between a homozygous Tall plant (TT) and a homozygous Dwarf plant (tt). It details the F1 generation (all Tt) and the selfing of F1 to produce the F2 generation, clearly indicating the 3:1 phenotypic and 1:2:1 genotypic ratios.)
B. Dihybrid Cross (Do genes ka inheritance)
Mendel ne do characters ko ek saath liya (jaise Round Yellow seed vs Wrinkled Green seed).
F2 Phenotypic Ratio: 9:3:3:1 (Yeh ratio bahut important hai).
Mendel ke Laws (Inferences):
Law of Dominance: F1 generation mein sirf dominant character hi dikhta hai.
Law of Segregation: Gametes bante waqt alleles ek dusre se alag (segregate) ho jate hain. (Isse 'Law of Purity of Gametes' bhi kehte hain).
Law of Independent Assortment: Dihybrid cross mein, ek character ka inheritance dusre se bilkul independent hota hai.
3. Deviations from Mendelism (Mendel ke niyamon se hatkar)
Har jagah Mendel ke rules apply nahi hote:
Incomplete Dominance: F1 generation parents se nahi milti, balki dono ka mix hoti hai (e.g., Mirabilis jalapa / 4 o'clock plant mein Red aur White flower ko cross karne par Pink flower milte hain).
[Image 3: Incomplete Dominance in Snapdragon Flower]
(Description: A genetic cross diagram showing a Red (RR) Snapdragon flower crossed with a White (rr) flower. The F1 generation consists of all Pink (Rr) flowers, demonstrating an intermediate phenotype. The F2 generation resulting from F1 selfing shows a 1:2:1 ratio of Red:Pink:White.)
Co-dominance: F1 generation mein dono parents ke characters dikhte hain (e.g., Humans mein ABO Blood Grouping). Blood group I gene se control hota hai, jiske teen alleles hain: $I^A$, $I^B$, aur $i$.
4. Chromosomal Theory of Inheritance
Sutton aur Boveri ne kaha ki genes chromosomes par hote hain. Chromosomes ka behavior aur genes ka behavior bilkul same hota hai (segregation aur independent assortment).
Linkage aur Recombination
T.H. Morgan ne Drosophila melanogaster (fruit fly) par kaam kiya aur dekha ki kuch genes ek hi chromosome par paas-paas hote hain aur saath hi inherit hote hain. Is phenomenon ko Linkage kehte hain. Door hone par genes recombine karte hain.
5. Sex Determination (Ling Nirdharan)
Alag-alag organisms mein sex determine karne ke tareeke alag hain:
XO Type: Grasshopper mein (Males mein sirf ek X chromosome hota hai).
XY Type: Humans aur Drosophila mein (Males mein XY, Females mein XX).
ZW Type: Birds mein (Females mein ZW, Males mein ZZ).
Humans mein, sex hamesha male (pita) decide karta hai kyunki unke paas Y chromosome hota hai.
6. Mutations aur Genetic Disorders
DNA sequence mein achanak badlav ko Mutation kehte hain. Isse genetic disorders ho sakte hain.
A. Mendelian Disorders (Single gene ki vajah se)
Haemophilia: Sex-linked recessive. Blood clot nahi hota.
Sickle-cell Anaemia: Autosomal recessive. RBC ka shape sickle (hansiya) jaisa ho jata hai.
Phenylketonuria: Inborn error of metabolism.
B. Chromosomal Disorders (Chromosome number mein badlav)
Down’s Syndrome: Trisomy of chromosome 21 (Ek extra 21st chromosome).
[Image 4: Karyotype of Down's Syndrome]
(Description: A human karyotype chart showing all 23 pairs of chromosomes. A circle highlights position 21, showing three chromosomes (trisomy) instead of the normal pair, indicating Down's syndrome.)
Klinefelter’s Syndrome: Males mein ek extra X (XXY condition).
Turner’s Syndrome: Females mein ek X kam (XO condition).
Conclusion
Chapter 5 Biology ka base hai. Agar aapne Punnett square banana aur ratios nikalna seekh liya, toh Genetics aapke liye bahut easy ho jayegi. NCERT ke tables aur diagrams ko dhyan se padhein.
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