3.2.3
Part of Extensions of Mendelian Genetics
Mendel ne socha tha ki har gene sirf do forms mein aati hai (jaise tall vs short). Lekin ek gene DNA ka ek stretch hoti hai — aur DNA kai positions par mutate ho sakta hai. Toh poori population mein, ek hi gene do se zyada alternative forms mein exist kar sakti hai. Inhe hi multiple alleles kehte hain. Woh cheez jo students aksar bhool jaate hain: ek diploid individual ke paas phir bhi sirf do hi alleles hoti hain (har homologous chromosome par ek). "Kai" ki baat population mein hoti hai, kisi ek organism mein nahi.
Definition Multiple allelesEk gene ko multiple alleles wala kaha jaata hai jab usi gene ki teen ya usse zyada alternative forms (alleles) ek population mein same locus par exist karti hain.
Population level: 3, 4, ... kai alleles possible hain.Individual (diploid) level: phir bhi sirf 2 alleles max — har homologous chromosome par ek.Gamete (haploid) level: exactly 1 allele.
Intuition EK PERSON MEIN SIRF DO KYO?Ek diploid organism ke paas har chromosome ki do copies hoti hain → har locus ki do copies → sirf do alleles ki jagah. Possibilities ka pool bahut bada hai, lekin tumhare chromosomes par seats sirf do hain.
Ek "allele" simply ek locus par ek specific DNA sequence hoti hai. Socho: kitni alag-alag sequences possible hain?
Length n A , T , G , C N possible = 4 n Yeh step kyun? n
Chhote se n = 10 4 10 ≈ 1 0 6 biological surprise yeh nahi hai ki multiple alleles exist karti hain — balki yeh hai ki Mendel ka two-allele model kaam kiya hi. Multiple alleles rule hain; do alleles ek special case hain.
Worked example ABO blood group se check karo (k = 3 I A , I B , i G ( 3 ) = 2 3 ⋅ 4 = 6 genotypes Kyun? formula ko real mein dekhne ke liye:
I A I A , I A I B , I A i , I B I B , I B i , ii 6 . ✔
Worked example ABO system — multiple alleles+ codominance + dominance, sab ek saath
Gene I ke human population mein teen alleles hain:
I A A banata hai.I B B banata hai.i koi antigen nahi banata (dono ke liye recessive).
Dominance relations: I A = I B > i I A , I B codominant hain.
Genotype
Phenotype (blood group)
Kyun?
I A I A , I A i A I A i
I B I B , I B i B I B i
I A I B AB I A I B codominant → dono antigens
ii O koi antigen nahi
Result: 6 genotypes → sirf 4 phenotypes (A, B, AB, O).
Worked example Forecast-then-Verify: kya do type-O parents ka bachcha type-A ho sakta hai?Forecast: Nahi.
Verify: Type O = ii i ii O ho sakti hai. ✔ Confirmed impossible — ek real fact jo (historical) paternity disputes mein use hota tha.
Common mistake "Ek individual mein ek gene ki kai alleles ho sakti hain."Yeh sahi kyun lagta hai: Humne abhi kaha ki gene ki bahut saari alleles hain, toh surely ek person sab carry karta hoga.
Fix: Kai alleles population mein exist karti hain. Ek diploid individual ke paas sirf 2 chromosome copies hain → zyada se zyada 2 alleles. "Multiple" ek gene/population property hai, individual ki nahi.
Common mistake "Multiple alleles matlab polygenic inheritance."Yeh sahi kyun lagta hai: Dono mein "multiple" word hai aur dono Mendel ko complicated banate hain.
Fix: Multiple alleles = ek gene, kai versions (ek locus). Polygenic = kai genes jo ek trait ko affect karte hain. Opposite directions! ABO = ek gene, teen alleles. Skin colour = kai genes.
I A I B Yeh sahi kyun lagta hai: Do alag alleles, koi "jeet" nahi raha → blending jaisa lagta hai.
Fix: Yeh codominance hai, incomplete dominance nahi. Dono antigens A aur B fully aur separately appear karte hain (group AB), koi mixed/intermediate product nahi. Koi blending nahi.
Common mistake Genotypes ki sankhya ke liye2 k k 2 Yeh sahi kyun lagta hai: Powers "combinations" ka natural answer lagte hain.
Fix: Do alleles ka order matter nahi karta, toh yeh unordered pairs with repetition hai: 2 k ( k + 1 ) not k 2 I A I B I B I A
Worked example Aur multiple-allele systems
Rabbits mein coat colour: C > c c h > c h > c 4 alleles , ek dominance series .Plants mein self-incompatibility: S 1 , S 2 , S 3 , … S Human HLA genes: hundreds of alleles → isliye organ-transplant matching mushkil hoti hai.
Multiple alleles = population mein ek gene ki 3+ forms ; individual mein max 2 .
Origin = usi locus par mutation .
ABO showcase hai: I A , I B , i I A = I B > i I A / I B ABO ke liye 6 genotypes → 4 phenotypes .
Genotype count: 2 k ( k + 1 )
Recall Khud test karo (answers cover karke)
Ek diploid individual ek locus par kitne alleles carry kar sakta hai? → 2.
"Multiple" alleles actually kahan rehti hain? → population mein.
I A i Group A. I A I B codominant. k = 4 2 4 ⋅ 5 = 10.
Recall Feynman: ek 12-saal ke bachche ko samjhaoImagine karo ek gene ek trading card ki slot hai. Duniya mein bahut saare alag-alag cards ho sakte hain (red, blue, gold...). Lekin tumhare paas sirf do card slots hain — ek mummy se, ek daddy se. Toh chahe duniya mein kitne bhi cards hon, tum sirf ek waqt mein do hi rakh sakte ho. Blood type bilkul aisa hi hai: duniya mein teen "blood cards" hain (I A I B i
"Pool mein bahut, cell mein do, gamete mein ek."
ABO ke liye: "A aur B codominant Bosses hain; chota i Invisible servant hai."
Codominance — I A I B Incomplete Dominance — contrast: blending vs separate expression.Polygenic Inheritance — opposite concept (kai genes, ek trait).Gene Mutation — naye alleles ka source .ABO Blood Group System — canonical worked example.Punnett Square — upar wale crosses ke liye use hone wala tool.Law of Segregation — isliye ek gamete sirf ek allele carry karta hai.
"Multiple alleles" kise define karta hai? Ek population mein same locus par ek gene ke teen ya usse zyada alleles ka exist karna.
Ek single diploid individual ek gene ki kitni alleles carry kar sakta hai? Zyada se zyada do — har homologous chromosome par ek.
Ek locus par ek gamete kitne alleles carry karta hai? Exactly ek (gametes haploid hote hain).
Human ABO gene ki teen alleles kya hain? I A I B i
ABO mein dominance relationship batao. I A = I B > i I A I B i
Blood group A dene wale genotype(s)? I A I A I A i
Blood group AB dene wala genotype aur kyun? I A I B I A I B
Blood group O dene wala genotype? ii
k alleles ke saath genotypes ki sankhya ka formula? G ( k ) = 2 k ( k + 1 )
ABO mein kitne genotypes aur phenotypes hain? 6 genotypes, 4 phenotypes.
Kya do type-O parents ka bachcha non-O ho sakta hai? Nahi; ii × ii ii
Multiple alleles aur polygenic inheritance mein kya fark hai? Multiple alleles = ek gene, kai versions; polygenic = kai genes ek trait ko affect karte hain.
Multiple alleles ki origin kya hai? Population mein time ke saath same gene locus par mutations.
Ek 4-allele multiple-allele system ka naam batao. Rabbit coat colour: C > c c h > c h > c
DNA can mutate at many positions
3+ allele forms at one locus
Diploid individual = 2 alleles max
Two homologous chromosomes
N = 4^n possible sequences
Heterozygotes = k choose 2
ABO blood group = 6 genotypes
Codominance and dominance
