CBSE · Class 10 · 🔬 Science · Chapter 8
Heredity
VariationHeredityMendel's LawsDominant and Recessive TraitsGenes and ChromosomesSex Determination
Chapter 8, Heredity, explores the fundamental principles by which traits are passed from one generation to the next. It covers the accumulation of variations during reproduction, Mendel's laws of inheritance, the role of genes and chromosomes in trait expression, and the mechanism of sex determination in human beings. Understanding heredity is crucial for grasping evolution and the diversity of life.
Variation ka Accumulation during Reproduction
Jab organisms reproduce karte hain, toh unke offsprings apne parents jaise hi hote hain, but unmein thode differences (variations) bhi hote hain.
Variations ka Importance:
- Survival Advantage: Kuch variations organisms ko changing environment mein survive karne mein help karte hain. Jaise, heat-resistant bacteria heatwave mein survive kar jaate hain.
- Evolution ka Base: Ye variations hi evolution ka foundation banate hain. Natural selection ke through, beneficial variations wale individuals zyada survive karte hain aur reproduce karte hain.
Asexual vs Sexual Reproduction mein Variation:
- Asexual Reproduction:
- DNA copying mein minor inaccuracies ki wajah se variations aate hain.
- Offsprings generally genetically identical hote hain.
- Example: Bacteria ka divide hona.
- Sexual Reproduction:
- Greater diversity generate hoti hai.
- Parents se genetic material ka combination hota hai.
- Crossing over aur independent assortment jaise processes bhi variations add karte hain.
Accumulation of Variations:
- Variations generation after generation accumulate hote rehte hain.
- Ek generation mein jo variation aata hai, woh next generation mein inherit hota hai, aur usmein naye variations add ho jaate hain.
- Ye process diversity ko badhata hai species ke andar.
Example: Agar ek trait 'A' 10% population mein hai aur trait 'B' 60% mein, toh 'A' naya variation ho sakta hai aur 'B' purana, jo zyada spread ho chuka hai.
Important
Variations species ke survival ke liye crucial hain, especially jab environment change hota hai.
Tip
Asexual vs Sexual reproduction mein variations ke differences ko yaad rakho. Sexual reproduction mein zyada variations hote hain.
Heredity aur Mendel ke Laws
Heredity kya hai?
- Heredity woh process hai jismein traits parents se offspring mein pass hote hain.
- Isse offsprings mein basic body design similar rehta hai, but kuch differences bhi hote hain.
Inherited Traits:
- Traits woh characteristics hain jo parents se offspring mein aate hain.
- Example: Eye colour, hair colour, earlobe type (free ya attached).
- Har trait ke liye, offspring ko do copies of genes milte hain, ek father se aur ek mother se.
Gregor Mendel – Father of Genetics:
- Mendel ne garden pea plants (_Pisum sativum_) par experiments kiye.
- Unhone seven pairs of contrasting characters ko study kiya (e.g., Tall/Dwarf, Round/Wrinkled seeds, Violet/White flowers).
- Mendel ne monohybrid cross (ek trait ki inheritance) aur dihybrid cross (do traits ki inheritance) perform kiye.
Mendel ke Laws of Inheritance:
1. Law of Dominance:
- Jab contrasting traits wale parents ko cross kiya jaata hai, toh F1 (first filial) generation mein sirf dominant trait hi express hota hai.
- Recessive trait F1 generation mein suppress ho jaata hai, but F2 generation mein reappear hota hai.
- Example: Tall (T) x Dwarf (t) $\rightarrow$ F1 generation mein sab Tall (Tt).
2. Law of Segregation (Purity of Gametes):
- Har trait ke liye, do alleles hote hain, aur gamete formation ke time ye alleles separate (segregate) ho jaate hain.
- Har gamete mein sirf ek allele jaata hai.
- Ye segregation random hota hai, isliye gametes mein equal chances hote hain har allele ke.
- F1 generation ke self-pollination se F2 generation mein 3:1 phenotypic ratio (dominant:recessive) aur 1:2:1 genotypic ratio (homozygous dominant:heterozygous:homozygous recessive) milta hai.
3. Law of Independent Assortment:
- Jab do ya do se zyada different traits ki inheritance ko consider kiya jaata hai, toh har trait ke alleles independently assort hote hain during gamete formation.
- Ek trait ke alleles ka segregation doosre trait ke alleles ke segregation ko affect nahi karta.
- Example: Seed shape (Round/Wrinkled) aur Seed colour (Yellow/Green) ka inheritance.
- Dihybrid cross mein F2 generation ka phenotypic ratio 9:3:3:1 hota hai.
Monohybrid Cross (Example: Tall vs Dwarf Pea Plants):
- Parents (P): Tall (TT) x Dwarf (tt)
- Gametes: T, t
- F1 Generation: Tt (All Tall)
- F1 Self-pollination: Tt x Tt
- F2 Generation: TT, Tt, Tt, tt
- Phenotypic Ratio: 3 Tall : 1 Dwarf
- Genotypic Ratio: 1 TT : 2 Tt : 1 tt
Dihybrid Cross (Example: Round Yellow vs Wrinkled Green Seeds):
- Parents (P): Round Yellow (RRYY) x Wrinkled Green (rryy)
- Gametes: RY, ry
- F1 Generation: RrYy (All Round Yellow)
- F1 Self-pollination: RrYy x RrYy
- F2 Generation:
- Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
- Genotypic Ratio: Complex, but based on independent assortment.
Key Terms:
- Gene: DNA ka segment jo ek specific protein banane ki information carry karta hai.
- Allele: Ek gene ke alternative forms (e.g., T aur t).
- Homozygous: Jab ek organism mein ek trait ke liye identical alleles hon (TT ya tt).
- Heterozygous: Jab ek organism mein ek trait ke liye different alleles hon (Tt).
- Phenotype: Organism ka observable characteristic (e.g., Tall, Dwarf).
- Genotype: Organism ka genetic makeup (e.g., TT, Tt, tt).
Formula
Monohybrid Cross F2 Phenotypic Ratio: 3:1 Monohybrid Cross F2 Genotypic Ratio: 1:2:1 Dihybrid Cross F2 Phenotypic Ratio: 9:3:3:1
Misconception
Students often confuse phenotype aur genotype. Phenotype = jo dikhta hai (physical appearance), Genotype = genetic makeup.
Traits ka Expression: Genes aur Chromosomes
Genes kaise Traits ko Control karte hain?
- DNA cells mein information ka source hai proteins banane ke liye.
- DNA ka ek segment jo ek specific protein banane ki information deta hai, use gene kehte hain.
- Proteins hi body ke functions aur characteristics ko control karte hain.
- Example: Plant ki height. Agar ek gene efficient enzyme banata hai jo growth hormone produce karta hai, toh plant tall hoga. Agar gene mein alteration hai aur enzyme less efficient hai, toh hormone kam banega aur plant short hoga.
- Is tarah, genes traits ko control karte hain.
Chromosomes aur Inheritance:
- Har cell mein do sets of genes hote hain, ek mother se aur ek father se.
- Ye genes chromosomes par located hote hain.
- Har species mein fixed number of chromosomes hote hain.
- Human beings mein 23 pairs of chromosomes hote hain (total 46).
- 22 pairs autosomes (body characteristics control karte hain).
- 1 pair sex chromosomes (sex determination karte hain).
Germ Cells (Gametes) ka Role:
- Normal body cells (somatic cells) mein diploid (2n) number of chromosomes hote hain (do copies of each chromosome).
- Gametes (sperm aur egg) mein haploid (n) number of chromosomes hote hain (ek copy of each chromosome).
- Ye haploid condition meiosis cell division se achieve hoti hai.
- Jab male aur female gametes fuse karte hain (fertilization), toh zygote mein diploid number of chromosomes restore ho jaata hai.
- Isse DNA ki stability maintain rehti hai species mein.
Independent Inheritance ka Basis:
- Mendel ke experiments se pata chala ki traits independently inherit hote hain.
- Iska reason hai ki genes separate independent pieces (chromosomes) par hote hain.
- Gamete formation ke time, har pair se ek chromosome randomly gamete mein jaata hai.
- Isliye, ek chromosome par present gene ka inheritance doosre chromosome par present gene ke inheritance ko affect nahi karta.
DNA copying inaccuracies:
- DNA replication ke time minor errors ho sakte hain.
- Ye errors bhi variations create karte hain, especially asexual reproduction mein.
Definition
Gene: DNA ka functional unit jo ek specific protein ya RNA molecule banane ki information carry karta hai.
Important
Har trait ke liye do alleles hote hain, jo homologous chromosomes par same locus par present hote hain.
Sex Determination
Sex determination ka mechanism different species mein different hota hai.
Strategies for Sex Determination:
- Environmental Cues: Kuch animals (jaise kuch reptiles) mein, egg incubation temperature decide karta hai ki offspring male hoga ya female.
- Sex Change: Kuch animals (jaise snails) sex change kar sakte hain, indicating ki sex genetically determined nahi hai.
- Genetically Determined: Humans mein, sex genetically determined hota hai chromosomes ke through.
Human Beings mein Sex Determination:
- Humans mein 23 pairs of chromosomes hote hain.
- Pehle 22 pairs autosomes hote hain, jo body ke non-sexual characteristics control karte hain.
- 23rd pair sex chromosomes hota hai, jo sex determine karta hai.
- Females: Do X chromosomes (XX) hote hain.
- Males: Ek X chromosome aur ek Y chromosome (XY) hota hai.
Inheritance Pattern:
- Mother (XX): Sirf X chromosome produce kar sakti hai (all eggs carry X).
- Father (XY): Do types ke sperm produce kar sakta hai – 50% X-carrying aur 50% Y-carrying.
| Parents | Gametes | Offspring Genotype | Offspring Phenotype | |---|---|---|---| | Female (XX) | X | XX | Girl | | Male (XY) | X or Y | XY | Boy |
- Agar X-carrying sperm egg ko fertilize karta hai, toh zygote XX hoga (girl).
- Agar Y-carrying sperm egg ko fertilize karta hai, toh zygote XY hoga (boy).
- Iska matlab hai ki child ka sex father ke sperm par depend karta hai.
- Probability: Har pregnancy mein 50% chance hota hai boy hone ka aur 50% chance hota hai girl hone ka.
Sex-linked Inheritance:
- Kuch traits sex chromosomes par present genes se inherit hote hain.
- Example: Colour blindness, Haemophilia (zyadatar X-linked recessive disorders hain).
Important
Human beings mein father hi child ka sex determine karta hai, not the mother.
Misconception
Log aksar mother ko blame karte hain for child's sex, which is biologically incorrect. Father ke sperm mein hi X ya Y chromosome hota hai jo sex decide karta hai.
