Class 10 Science · Chapter 8
Short answer:
NCERT Class 10 Science Chapter 8 — Heredity ka complete solution set. Isme 6 in-text questions (page 129 par 2 + page 133 par 4) aur 4 exercise questions — sab NCERT ke exact order me, Hinglish explanation ke saath. Saath me Mendel ke monohybrid (3:1) aur dihybrid (9:3:3:1) crosses ka ready-reference table, dono crosses ke proper Punnett squares, 6 common exam mistakes, board-style practice questions aur 6 FAQs. Coverage: variation ka accumulation, inherited traits, Mendel ke rules, dominant/recessive, genotype vs phenotype, genes se traits kaise express hote hain, chromosomes ka role, aur human sex determination. Note: Rationalised NCERT syllabus me is chapter ka naam sirf “Heredity” hai — evolution wala portion (Darwin, fossils, speciation, homologous/analogous organs) hata diya gaya hai, isliye yahan bhi wo cover nahi kiya gaya.
Heredity ka matlab hai — ek generation se doosri generation me traits ka transfer, aur is transfer ke apne fixed rules hain jo Mendel ne matar ke paudhon se nikale the. Reproduction wale chapters me humne dekha ki new individuals parents jaise hote hain, par bilkul same nahi. Ye chapter wahi se aage badhta hai: aakhir similarity aati kahan se hai, aur variation banta kaise hai?
Mendel ne matar ke paudhe (Pisum sativum) isliye chune kyunki unme clear-cut contrasting traits the (lamba/bauna, gol/jhurridar beej, violet/safed phool), self-pollination naturally hoti hai isliye control aasan tha, aur ek generation jaldi aa jati thi. Sabse badi baat — Mendel pehla insaan tha jisne har generation me har trait wale plants ko gin kar record kiya. Yahi counting usse laws of inheritance tak le gayi.
Is chapter me aap teen cheezein pakka samajh lo: (1) dominant aur recessive trait ka farak, (2) genotype ratio aur phenotype ratio alag-alag hote hain, aur (3) human beings me bachche ka sex father ke chromosome se decide hota hai, mother ke nahi. Exam me sabse zyada marks isi teen cheezon par kate jaate hain.
Chapter 8 Summary — 5 Minute Revision
1. Variation reproduction ke dauraan kaise accumulate hoti hai
- Har generation ko previous generation se basic body design bhi milta hai aur usme subtle changes bhi.
- Asexual reproduction: ek bacterium divide hoke do banta hai, phir chaar — ye chaaron lagbhag identical hote hain. Sirf DNA copying ki chhoti inaccuracies se hi minor differences aate hain.
- Sexual reproduction: do parents ka genetic material milta hai, isliye kahin zyada diversity generate hoti hai.
- Sab variations ke survive karne ke chances barabar nahi hote. Environment ke hisaab se kuch variations fayda dete hain — jaise garmi sehne wale bacteria heat wave me behtar survive karte hain. Environmental factors dwara variants ka selection hi evolutionary processes ka base banta hai.
2. Inherited traits — earlobe wali activity
- Free earlobe aur attached earlobe — human population me do variants milte hain (Fig. 8.2).
- Class ke students aur unke parents ka data correlate karke inheritance ka rule guess kiya jaata hai.
- Point ye hai: bachcha ek human hi hota hai (similarity), par exactly parents jaisa nahi dikhta (variation).
3. Mendel ke rules — monohybrid cross (ek trait)
- Mendel ne garden pea ke contrasting characters liye: gol/jhurridar beej, lamba/bauna plant, safed/violet phool, waghera.
- Tall × Short cross → F1 me saare plants tall. Koi “medium-height” plant nahi — matlab traits mix nahi hote (no blending).
- F1 tall plants ko self-pollinate karo → F2 me 1/4 plants short nikal aate hain.
- Iska matlab: F1 me tallness aur shortness dono traits inherit hue the, par sirf tallness express hui.
- Isse Mendel ne propose kiya: sexually reproducing organism me har trait ke liye factor (ab gene) ki do copies hoti hain. Ye do copies identical ho sakti hain ya alag.
- TT aur Tt dono tall hain; sirf tt short hai. Yaani ek hi ‘T’ kaafi hai plant ko tall banane ke liye, jabki short hone ke liye dono copies ‘t’ honi zaroori hain.
- ‘T’ jaise traits = dominant; ‘t’ jaise = recessive.
Punnett square — F1 (Tt) × F1 (Tt):
| Gametes | T | t |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
↔ Table ko side me swipe karein
- Phenotype ratio (jo dikhta hai): 3 tall : 1 short
- Genotype ratio (andar ka gene combination): 1 TT : 2 Tt : 1 tt
4. Dihybrid cross — do traits ek saath (independent inheritance)
- Tall + round seed wale plant ko short + wrinkled seed wale se cross karo → F1 saare tall aur round. Matlab tallness aur round seed dono dominant hain.
- F1 ko self-pollinate karo → F2 me parental combinations (tall-round, short-wrinkled) ke saath naye combinations bhi aate hain: tall-wrinkled aur short-round.
- Naye combinations ka aana hi sabit karta hai ki dono traits independently inherit hote hain.
- NCERT Fig. 8.5 me seed shape aur seed colour ka cross diya hai: RRyy (round, green) × rrYY (wrinkled, yellow) → F1 saare RrYy (round, yellow).
- F1 selfing se F2 me Mendel ko 556 seeds me mile: 315 round-yellow, 108 round-green, 101 wrinkled-yellow, 32 wrinkled-green — yaani lagbhag 9 : 3 : 3 : 1.
Punnett square — RrYy × RrYy (16 boxes):
| Gametes | RY | Ry | rY | ry |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
↔ Table ko side me swipe karein
- 9 round-yellow | 3 round-green | 3 wrinkled-yellow | 1 wrinkled-green
5. Traits express kaise hote hain? (gene → protein → trait)
- Cellular DNA cell me protein banane ki information ka source hai. DNA ka jo hissa ek protein ki information deta hai, use us protein ka gene kehte hain.
- NCERT ka example — plant height: plants me growth trigger karne wala hormone hota hai.
Hormone kitna banega ye us enzyme ki efficiency par depend karta hai jo usse banata hai.
- Enzyme efficiently kaam kare → zyada hormone → plant tall.
- Gene me alteration ho aur enzyme kam efficient ho jaye → kam hormone → plant short.
- Isliye kehte hain: genes traits ko control karte hain.
6. Chromosomes — independent inheritance ka mechanism
- Dono parents progeny ke DNA me barabar contribution dete hain, isliye har pea plant me har gene ke do sets hote hain — ek har parent se.
- Ye tabhi chalega jab har germ cell (gamete) me gene ka sirf ek set ho.
- Agar gene set ek lambi DNA thread ki tarah poora ka poora inherit hota, to ‘R’ aur ‘y’ hamesha linked rehte aur Fig. 8.5 wala independent inheritance possible hi nahi hota.
- Iska jawaab: gene set ek single thread nahi, balki alag-alag independent pieces me hota hai — har piece ko chromosome kehte hain.
- Har cell me har chromosome ki do copies (ek male parent se, ek female parent se). Har germ cell har pair me se ek chromosome leti hai — wo maternal ya paternal, koi bhi ho sakta hai.
- Do germ cells milte hain to progeny me normal chromosome number restore ho jaata hai — isse species ke DNA ki stability bani rehti hai.
7. Sex determination
- Alag-alag species alag strategy use karti hain:
- Kuch reptiles: fertilised eggs jis temperature par rakhe jaate hain, wahi decide karta hai male banega ya female — yaani environmental cue.
- Snails: individual apna sex badal sakta hai — matlab sex genetically determined nahi hai.
- Human beings: sex bade paimane par genetically determined hai.
- Human beings me 22 pairs chromosomes aise hain jinki maternal aur paternal copy proper pair banati hai. Ek pair — sex chromosomes — hamesha perfect pair nahi hota.
- Female = XX (perfect pair). Male = XY (normal-sized X + chhota Y).
- Har bachche ko maa se hamesha X milta hai — chahe wo ladka ho ya ladki. Isliye sex ka faisla wahi karta hai jo father se milta hai: father se X → ladki, father se Y → ladka.
- Result: lagbhag aadhe bachche ladke, aadhe ladkiyan.
Punnett square — human sex determination:
| Mother's gametes → Father's gametes ↓ | X | X |
|---|---|---|
| X | XX — girl | XX — girl |
| Y | XY — boy | XY — boy |
↔ Table ko side me swipe karein
8. What you have learnt (NCERT ke apne points)
- Reproduction ke dauraan banne wali variations inherit ho sakti hain.
- Ye variations individuals ke survival ko badha sakti hain.
- Sexually reproducing individuals me ek hi trait ke gene ki do copies hoti hain. Agar copies identical na hon, to jo trait express hota hai wo dominant, doosra recessive kehlata hai.
- Ek hi individual ke traits alag-alag inherit ho sakte hain, jisse sexual reproduction ki offspring me naye combinations bante hain.
- Alag species me sex alag factors se decide hota hai. Human beings me bachche ka sex is baat par depend karta hai ki paternal chromosome X hai (ladki) ya Y (ladka).
In-Text Questions — Solutions
Q1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Trait B pehle aaya hoga.
Asexually reproducing species me variation sirf DNA copying ki chhoti-chhoti inaccuracies se banti hai, aur phir wo dhire-dhire generation-dar-generation aage badhti hai. Yaani ek naya trait jitna purana hoga, utni zyada generations use population me phailne ka mauka mila hoga.
Trait B population ke 60% me hai jabki trait A sirf 10% me. Itni zyada frequency tabhi banti hai jab trait ko phailne ka lamba time mila ho. Isliye B purana (earlier arisen) hai aur A haal hi me (recently) aaya hoga.
Q2. How does the creation of variations in a species promote survival?
Environment kabhi ek jaisa nahi rehta — temperature badalta hai, water supply badalti hai, naye predators ya diseases aa jaate hain. Agar species ke saare individuals bilkul ek jaise hon, to koi ek pratikool (drastic) change poori population ko ek saath khatam kar sakta hai.
Variations hone se alag-alag individuals ke paas alag-alag advantages hote hain. Jab environment badalta hai, to kam se kam kuch individuals aise nikal aate hain jinme us naye condition ko sehne ki capacity hoti hai. Wo survive karte hain, reproduce karte hain aur species bach jaati hai.
NCERT ka example: agar heat wave aaye, to jo bacteria garmi jhel sakte hain wahi behtar survive karenge. Environmental factors dwara aise variants ka selection hi evolutionary processes ka base hai.
Dhyan do: har variation fayda nahi deti — variation sirf survival ka chance badhati hai, guarantee nahi deti.
Q3. How do Mendel's experiments show that traits may be dominant or recessive?
Mendel ne ek tall pea plant ko short pea plant se cross kiya.
- F1 generation: saare plants tall nikle. Ek bhi medium-height plant nahi mila — matlab traits aapas me mix (blend) nahi hote, parents me se sirf ek trait dikhta hai.
- F2 generation: F1 tall plants ko self-pollinate kiya to progeny me 1/4 plants short nikal aaye — ratio 3 tall : 1 short.
Yahi decisive proof hai. Shortness wala trait F1 me gayab ho gaya tha par khatam nahi hua tha — kyunki wo F2 me wapas aa gaya. Iska matlab F1 plants me dono traits (tallness + shortness) maujood the, par sirf tallness express hui.
Jo trait do alag copies hone par bhi express ho jaye use dominant (T) kehte hain; jo trait sirf tab dikhe jab uski dono copies ho (tt) use recessive kehte hain. Isliye TT aur Tt dono tall hain, sirf tt short hai.
Q4. How do Mendel's experiments show that traits are inherited independently?
Mendel ne ek saath do traits wala cross (dihybrid cross) kiya — tall + round-seed wale plant ko short + wrinkled-seed wale plant se.
- F1: saare plants tall aur round-seeded — yaani tallness aur round seed dono dominant traits hain.
- F2: F1 ke self-pollination se kuch plants parents jaise mile (tall-round, short-wrinkled), lekin kuch bilkul naye combinations bhi mile — tall-wrinkled aur short-round.
Agar dono traits ek doosre se bandhe (linked) hote, to F2 me sirf parental combinations hi aate. Naye combinations ka aana sabit karta hai ki seed shape ka factor aur plant height ka factor ek doosre se independent hoke gametes me jaate hain aur zygote banate waqt swatantra roop se recombine ho jaate hain.
NCERT Fig. 8.5 wale cross me ye ratio numbers se dikhta hai: RRyy (round, green) × rrYY (wrinkled, yellow) → F1 saare RrYy (round, yellow). F1 selfing se 556 seeds me mile — 315 round-yellow, 108 round-green, 101 wrinkled-yellow, 32 wrinkled-green, yaani lagbhag 9 : 3 : 3 : 1.
F2 dihybrid phenotype ratio = 9 : 3 : 3 : 1
Q3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?
Nahi, ye information kaafi nahi hai.
Dominance decide karne ke liye sirf itna jaanna kaafi nahi ki bachche me kaunsa trait dikha. Yahan sirf ek bachche ka data hai aur dono parents ke traits alag-alag hain. Beti me O group aaya — par isse ye pata nahi chalta ki O dominant hai ya recessive, kyunki dono hi situation me ye result possible ho sakta hai depending on parents ke gene combinations.
Dominance tabhi confirm hota hai jab hum generations ka data dekhein — khaas taur par F1 progeny. Jaise Mendel ne kiya: agar A-group aur O-group parents ke saare bachche ek hi group ke nikal aayein, to wahi trait dominant hai. Ek akela bachcha kuch prove nahi karta.
Extra (chapter ke bahar ka fact): asal me blood group A, O par dominant hota hai — us aadmi ka genotype IAi raha hoga aur beti ii. Par ye baat is question ke data se nahi nikaali ja sakti, isliye exam me jawab “information not sufficient” hi likhna hai.
Q4. How is the sex of the child determined in human beings?
Human beings me sex genetically determined hota hai — sex chromosomes se.
- Human cells me chromosomes ke 22 pairs aise hote hain jinki maternal aur paternal copy proper pair banati hai. Ek pair — sex chromosomes — hamesha perfect pair nahi hota.
- Female = XX (dono X, perfect pair). Male = XY (ek normal-sized X aur ek chhota Y).
- Isliye maa ke saare eggs me X hi hota hai, jabki father ke sperms do type ke hote hain — aadhe X wale, aadhe Y wale.
- Fertilisation ke waqt agar X wala sperm milta hai → XX → ladki. Agar Y wala sperm milta hai → XY → ladka.
| Mother (XX) ke gametes → Father (XY) ke gametes ↓ | X | X |
|---|---|---|
| X | XX — girl | XX — girl |
| Y | XY — boy | XY — boy |
↔ Table ko side me swipe karein
Har bachche ko maa se to X milna hi milna hai. Isliye bachche ka sex poori tarah is baat par depend karta hai ki father se X aaya ya Y. Nateeja: lagbhag aadhe bachche ladke aur aadhe ladkiyan.
Social point (exam me bonus): isliye ladki paida hone ke liye maa ko blame karna scientifically bilkul galat hai.
Poore Class 10 Science ke handwritten colour notes
IITian & district toppers ke banaye short notes — revision-ready, diagram ke saath. Board se pehle poora syllabus 3 din me revise.
Exercise Questions — Solutions (Q1–Q4)
Q1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as (a) TTWW (b) TTww (c) TtWW (d) TtWw
Answer: (c) TtWW
Reasoning — do parts me todo:
Flower colour: saari progeny violet hai. Short parent white hai, yaani uska genotype ww hai (recessive trait tabhi dikhta hai jab dono copies recessive ho). Agar tall parent Ww hota to aadhi progeny white nikalti. Kyunki ek bhi white nahi aayi, tall parent WW hi hona chahiye.
| w | w | |
|---|---|---|
| W | Ww (violet) | Ww (violet) |
| W | Ww (violet) | Ww (violet) |
↔ Table ko side me swipe karein
Height: lagbhag aadhi progeny short hai. Short parent tt hai. Agar tall parent TT hota to koi bhi short progeny nahi aati (saare Tt = tall). 50% short tabhi milta hai jab tall parent Tt ho — ye ek test cross hai, jiska ratio 1 tall : 1 short hota hai.
| t | t | |
|---|---|---|
| T | Tt (tall) | Tt (tall) |
| t | tt (short) | tt (short) |
↔ Table ko side me swipe karein
Dono milakar tall parent ka genotype = TtWW.
Q2. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Nahi, sirf isi observation se dominant ya recessive decide nahi kiya ja sakta.
Ye study sirf itna batati hai ki eye colour ek inherited trait hai — parents se bachchon me jaata hai. Par inheritance ka hona alag baat hai aur dominance ka pata lagana alag.
Yahan dono parents ka trait ek jaisa (light) hai. Aise case me:
- Agar light colour recessive hai, to dono parents ka genotype homozygous recessive hoga aur saare bachche light-eyed honge.
- Agar light colour dominant hai, tab bhi light-eyed parents ke bachche light-eyed ho sakte hain.
Dono explanations same observation fit karti hain, isliye data decisive nahi hai.
Kya chahiye tha: Mendel wala approach — ek contrasting cross ka data. Yaani light-eyed parent aur dark-eyed parent ke bachche (F1) dekho. Jo trait F1 ke saare bachchon me dikhe wo dominant, aur jo gayab hoke agli generation me wapas aaye wo recessive. Bina F1–F2 data ke dominance ka dawa karna galat hai.
Q3. Outline a project which aims to find the dominant coat colour in dogs.
Idea wahi hai jo Mendel ne matar par lagaya tha — sirf organism badal gaya hai.
Step 1 — Pure-breeding parents lo. Do aise dogs chuno jinka coat colour clearly contrasting ho, maan lo black aur white. Dono pure-breeding (homozygous) hone chahiye — yaani unke breeding records dikhate hon ki kai generations se unki aulad hamesha same colour ki hi aayi hai. Ye step chhoot gaya to poora project bekaar ho jaayega.
Step 2 — Cross karo (parent generation). Ek variety ka male aur doosri variety ki female lo, aur cross karao. (Reciprocal cross bhi karo — black male × white female aur white male × black female — taaki confirm ho jaye ki result kis parent se colour aaya us par depend nahi karta.)
Step 3 — F1 puppies ka coat colour record karo. Har puppy ka colour likho aur gino. Agar saare F1 puppies black hain → black dominant, white recessive. Agar saare white hain → white dominant.
Step 4 — F1 × F1 cross karke confirm karo. F1 ke do dogs ko aapas me cross karao aur F2 puppies gino. Agar dominance sahi samjha tha, to F2 me lagbhag 3 : 1 ratio milega (3 dominant colour : 1 recessive colour), aur jo colour F1 me gayab tha wo F2 me wapas dikhega.
Step 5 — Data analyse karo. Zyada se zyada litters ka data lo, kyunki ek litter me puppies kam hote hain aur chhote sample me ratio saaf nahi dikhta. Numbers ko table me likhkar ratio nikaalo — Mendel ki asli taakat counting hi thi.
| Generation | Cross | Expected result agar black dominant hai |
|---|---|---|
| P | pure black (BB) × pure white (bb) | — |
| F1 | — | saare Bb — 100% black |
| F2 | Bb × Bb | 3 black : 1 white |
↔ Table ko side me swipe karein
Practical note: dogs me coat colour ek se zyada gene series se control hota hai aur generation time lamba hai, isliye ye project theoretically design karna aasan hai par practically kaafi lamba chalta hai. Exam me steps aur expected ratios likhna kaafi hai.
Q4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Ye chromosomes ke behaviour se ensure hota hai, sirf sanyog se nahi.
- Sexually reproducing organism ki har body cell me har chromosome ki do copies hoti hain — ek male parent se aur ek female parent se aayi hui. Isliye har gene ke bhi do sets hote hain.
- Germ cells (gametes) banate waqt har pair me se sirf ek chromosome gamete me jaata hai. Isliye har sperm aur har egg me chromosomes ka aadha (single) set hota hai.
- Ye ek chromosome maternal origin ka ho sakta hai ya paternal — par sankhya hamesha aadhi hi rehti hai.
- Fertilisation par sperm aur egg fuse hote hain, aur zygote me chromosome ka poora normal number restore ho jaata hai — jisme aadhe chromosomes father se aur aadhe mother se hote hain.
Isi wajah se bachche ko dono parents se practically barabar genetic material milta hai, aur har trait par paternal aur maternal DNA dono ka asar ho sakta hai. Saath hi generation-dar-generation species ke chromosome number ki stability bhi bani rehti hai — warna har generation me chromosome number double hota chala jaata.
Important Equations — Ek Nazar Me
Mendel's crosses — ratios ready reference
| Cross (parents) | F1 result | F2 ratio (F1 selfing se) | Kya prove karta hai |
|---|---|---|---|
| Monohybrid Tall (TT) × Short (tt) |
Saare Tt — tall (100%) | Phenotype 3 : 1 (3 tall : 1 short) Genotype 1 : 2 : 1 (1 TT : 2 Tt : 1 tt) |
Ek trait dominant hai, doosra recessive; traits blend nahi hote; recessive trait F1 me chhupa rehta hai aur F2 me wapas aata hai |
| Dihybrid RRyy (round, green) × rrYY (wrinkled, yellow) |
Saare RrYy — round, yellow | Phenotype 9 : 3 : 3 : 1 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green |
Do traits independently inherit hote hain — F2 me parental ke alawa naye combinations bhi bante hain |
| Test cross Tall (Tt) × Short (tt) |
— (direct progeny hi dekhi jaati hai) | 1 : 1 (1 tall : 1 short) | Kisi dominant-dikhne wale plant ka genotype TT hai ya Tt — ye pata karne ka tareeka |
| Human sex determination Mother (XX) × Father (XY) |
XX (girl) aur XY (boy) | 1 : 1 (~50% ladke : ~50% ladkiyan) | Bachche ka sex father ke chromosome (X ya Y) se decide hota hai, mother se nahi |
↔ Table ko side me swipe karein
Zaroori terms ek jagah
| Term | Matlab | Example |
|---|---|---|
| Gene / factor | DNA ka wo hissa jo ek protein ki information deta hai aur trait control karta hai | T, t |
| Dominant trait | Ek hi copy se bhi express ho jaata hai | Tallness (T) |
| Recessive trait | Sirf tab express hota hai jab dono copies recessive hon | Shortness (t) |
| Genotype | Gene combination (jo andar hai) | TT, Tt, tt |
| Phenotype | Jo trait dikhta hai | Tall, Short |
| Homozygous | Dono copies same | TT ya tt |
| Heterozygous | Dono copies alag | Tt |
| Chromosome | Gene set ka ek independent piece; har cell me pairs me | Human: 22 pairs + 1 sex pair |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Genotype ratio aur phenotype ratio ko mila dena. Monohybrid F2 ka phenotype ratio 3 : 1 hai (3 tall : 1 short) aur genotype ratio 1 : 2 : 1 (1 TT : 2 Tt : 1 tt). Question me “ratio of tall to short plants” poocha ho to 3 : 1 likho; “ratio of TT, Tt, tt” poocha ho to 1 : 2 : 1. Ulta likhoge to marks seedha kat jaayega.
- Bachche ka sex maa se batana. Maa XX hai, isliye uske har egg me X hi jaayega — uske paas koi choice hi nahi hai. Father XY hai, uske aadhe sperm X wale aur aadhe Y wale hote hain. Isliye bachcha ladka hoga ya ladki ye hamesha father ke chromosome se decide hota hai.
- “Dominant” ka matlab “zyada common” samajh lena. Dominant ka matlab sirf itna hai ki heterozygous (Tt) condition me wo trait express ho jaata hai. Population me wo trait kam bhi ho sakta hai. Isi tarah recessive trait ka matlab “kamzor” ya “durlabh” nahi hota — wo bas Tt me chhup jaata hai.
- Gamete me dono alleles likh dena. Punnett square banate waqt Tt parent ke gametes T aur t hote hain — “Tt” nahi. Gamete me har gene ki sirf ek copy jaati hai. Dihybrid me RrYy ke gametes RY, Ry, rY, ry (chaar type) hote hain. Ye galti karte hi poora square aur ratio galat ho jaata hai.
- Ek bachche ya ek observation se dominance decide kar dena. Blood group wale aur light-eye wale dono questions ka NCERT jawab yahi hai ki information sufficient nahi hai. Dominance sirf tab prove hota hai jab contrasting parents ka F1 aur F2 data ho. Bina data ke “A dominant hai” likh dena galat approach hai.
- F1 aur F2 ke results aapas me badal dena. Ratio (3:1 ya 9:3:3:1) F2 me milta hai, F1 me nahi — F1 me to saare plants ek jaise (dominant phenotype) hote hain. Aur ye F2 ratio tabhi banta hai jab F1 ko self-pollinate kiya jaye, parent plants ko nahi.
Board-Style Important Questions
- 1 mark: A pea plant with genotype Tt is crossed with a plant of genotype tt. Write the phenotypic ratio of the progeny obtained.
- 1 mark: Name the type of chromosome pair present in a human male and in a human female, and state which parent's gamete decides the sex of the child.
- 2 marks: Differentiate between dominant and recessive traits with one example of each from Mendel's experiments on pea plants.
- 3 marks: A tall pea plant with round seeds is crossed with a short pea plant with wrinkled seeds. State the phenotype of the F1 progeny. When the F1 progeny are self-pollinated, which new combinations of traits appear in the F2 generation, and what does their appearance prove?
- 5 marks: (a) Explain with the help of a cross how Mendel showed that the trait for shortness in pea plants was present in the F1 generation but was not expressed. (b) Write the genotypic and phenotypic ratios obtained in the F2 generation. (c) Draw a Punnett square to show how the sex of a child is determined in human beings.
Quick Quiz — Score Check Karein
Q1. Mendel ne apne famous experiments ke liye matar ka paudha (Pisum sativum) kyun chuna?
Q2. Tall (TT) pea plant ko Short (tt) plant se cross karne par F1 generation me kya milta hai?
Q3. F1 (Tt) tall plants ko self-pollinate karne par F2 me phenotype ratio kya hota hai?
Q4. Ek Tt genotype wale plant ko tt genotype wale plant se cross kiya jaye (test cross), to progeny ka phenotype ratio kya hoga?
Q5. RRyy (round, green) ko rrYY (wrinkled, yellow) se cross karne par F1 generation ka phenotype kya hoga?
Q6. Dihybrid cross ke F2 generation me naye combinations (tall-wrinkled, short-round) ka aana kya prove karta hai?
Q7. Human beings me bachche ka sex kis se decide hota hai?
Q8. Human male aur female me sex chromosomes ka genotype kya hota hai?
Q9. Asexually reproducing species me trait A sirf 10% population me hai jabki trait B 60% population me hai. Konsa trait pehle arise hua hoga?
Q10. Ek aadmi (blood group A) ki shaadi ek aurat (blood group O) se hoti hai aur unki beti ka blood group O hai. Kya is se pata chalta hai ki A ya O dominant hai?
Aksar Poochhe Jaane Wale Sawaal
Mendel ne apne experiments ke liye matar ka paudha hi kyun chuna?
Kyunki matar (Pisum sativum) me kai clear-cut contrasting traits milte hain jaise lamba/bauna, gol/jhurridar beej, violet/safed phool — inme koi beech ki halat nahi hoti isliye ginna aasan hai. Iske alawa matar me self-pollination naturally hoti hai, jisse Mendel pure-breeding lines bana paya aur cross ko control kar paya. Plant chhota hai, ugana sasta hai aur ek generation jaldi aa jati hai, isliye kai generations ka data kam samay me mil gaya.
Genotype aur phenotype me exact farak kya hai?
Genotype yaani gene ka combination jo andar maujood hai — jaise TT, Tt ya tt. Phenotype yaani jo trait bahar dikhta hai — jaise tall ya short. Do plants ka phenotype same ho sakta hai par genotype alag: TT aur Tt dono tall dikhte hain, lekin unke gene combinations alag hain. Isi wajah se Tt plant ki progeny me short plants nikal aate hain, jabki TT plant ki progeny me nahi.
F1 generation me short plants gayab kyun ho gaye the, aur F2 me wapas kaise aa gaye?
F1 plants ka genotype Tt tha — unme tallness aur shortness dono ke factors maujood the. Par shortness recessive hai, isliye T ki maujoodgi me wo express nahi hua. Trait chhupa tha, khatam nahi hua tha. Jab F1 (Tt) ko self-pollinate kiya gaya, to kuch progeny me dono t copies ek saath aa gayin (tt), aur wahi plants short nikle — lagbhag ek chauthai. Yahi Mendel ka sabse bada proof tha.
9:3:3:1 ratio yaad rakhne ka koi aasan tareeka hai?
Ise 3:1 ka do baar istemal samjho. Har trait alag-alag dekho to har ek ka apna 3:1 ratio banta hai, aur kyunki traits independently inherit hote hain, dono ratios multiply ho jaate hain: (3:1) x (3:1) = 9 : 3 : 3 : 1. Yaani 9 me dono dominant traits, 3 me pehla dominant doosra recessive, 3 me pehla recessive doosra dominant, aur 1 me dono recessive. Total 16 boxes ka Punnett square banta hai.
Kya sirf tallness aur roundness hi dominant traits hote hain?
Nahi. Dominant hona trait ki apni koi khoobi nahi hai — ye sirf ye batata hai ki heterozygous condition me kaunsa trait express hota hai. Mendel ke pea experiments me tallness, round seed, yellow seed aur violet flower dominant nikle, par NCERT ke Fig. 8.5 wale cross me green seed colour recessive hai. Kaunsa trait dominant hai ye hamesha experiment ke data (F1 aur F2) se decide hota hai, guess se nahi.
Agar sex determination 50-50 hai to kuch families me saare ladke ya saari ladkiyan kyun hote hain?
Kyunki har fertilisation ek alag, independent event hai. 50-50 ka matlab ye nahi ki har family me barabar batwara hoga — ye bade population ka average hai. Jaise sikka uchhalne par har baar 50% chance hota hai, par lagataar chaar baar heads bhi aa sakte hain. Chhote family size me ratio se hatna bilkul normal hai, aur isme maa ka koi role nahi hota.
Class 10 Science — Saare Chapters
Board exam tak sirf revision karna hai?
Class 10 Science ke saare chapters ke colour handwritten short notes — diagrams, formulas aur important points ek jagah.
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