State Mendel's law of segregation — Hinglish notes

3.1.4 · Biology › Mendelian Genetics

WHAT is the Law of Segregation?

Kuch key vocabulary jo tumhe yaad rakhni chahiye:

Gene = ek trait ke liye instruction. Allele = us gene ka ek version (e.g. $T$ = tall, $t$ = short).
Diploid (2n) = har gene ki do copies. Haploid (n) = ek copy (gametes).
Homozygous = do same alleles ( $T T$ ya $tt$ ). Heterozygous = do alag ( $T t$ ).

WHY is it true? (First-principles derivation)

Hum famous 3:1 ratio ko segregation se derive karte hain — kabhi bas quote mat karo.

Step 1 — Har parent diploid hai, isliye ek heterozygote $T t$ hota hai. Yeh step kyun? Mendel ne pure tall ( $T T$ ) × pure short ( $tt$ ) cross kiya. Saare F₁ $T t$ hote hain — wo tall dikhte hain kyunki $T$ dominant hai, lekin secretly $t$ carry karte hain.

Step 2 — Segregation: $T t$ parent gametes banata hai. Do alleles alag ho jaate hain. Toh gametes hain half $T$ , half $t$ : $P (T) = \frac{1}{2}, P (t) = \frac{1}{2}$ Yeh step kyun? Yahi toh law hai — ek allele per gamete, aur kyunki ek $T$ aur ek $t$ hai, dono equally likely hain.

Step 3 — F₁ × F₁ self-cross, gametes ko independently combine karo. Har offspring = ek gamete har parent se. Probabilities multiply karo (Punnett square):

♀ \ ♂	$T (½)$	$t (½)$
$T (½)$	$T T (¼)$	$T t (¼)$
$t (½)$	$T t (¼)$	$tt (¼)$

Step 4 — Genotype aur phenotype ratios padho. $Genotype: T T : T t : tt = 1 : 2 : 1$ Kyunki $T$ dominant hai, $T T$ aur $T t$ dono tall dikhte hain: $Phenotype: tall (\frac{1}{4} + \frac{1}{2}) : short \frac{1}{4} = \frac{3}{4} : \frac{1}{4} = 3 : 1$ Yeh step kyun? 3:1 ratio koi axiom nahi hai — yeh automatically nikal aata hai jab alleles ½:½ segregate karte hain aur independently combine hote hain.

Figure — State Mendel's law of segregation

HOW does this happen physically? (The mechanism)

Mendel ko meiosis nahi pata tha — usne numbers se segregation infer kiya. Ab hum jaante hain kyun:

Worked Examples

Common Mistakes (Steel-man + Fix)

Active Recall

Recall Quick self-test (answers chhupa lo!)

Gamete formation ke dauran kya alag hota hai? → gene ke do alleles.
Kis meiotic stage par? → Anaphase I (homologs opposite poles ki taraf).
F₂ genotype ratio? → $1 : 2 : 1$ .
F₂ phenotype ratio (full dominance)? → $3 : 1$ .
Ek gamete mein kitne alleles hote hain? → bilkul ek.

Recall Feynman: ek 12-saal ke baache ko explain karo

Tumhare pocket mein do coloured marbles hain — ek red, ek blue. Jab tum apne dost ko marble dete ho, tum ek baar mein sirf ek de sakte ho, dono nahi. Toh har dost ko bas ek milta hai. Baad mein, do dost ek ek marble laate hain ek naya pair banane ke liye. "Ek baar mein sirf ek marble" ka yeh rule exactly wahi hai jo genes karte hain jab egg aur sperm cells banaate hain: har cell tumhare do copies mein se ek leta hai, ek fair coin flip se decide hota hai.

Flashcards

State Mendel's Law of Segregation.

Ek gene ke do alleles gamete formation ke dauran alag ho jaate hain taaki har gamete sirf ek allele carry kare; pair fertilization par restore ho jaata hai.

Segregation ka physical (cellular) basis kya hai?

Meiosis ke anaphase I ke dauran homologous chromosomes ka opposite poles ki taraf separation.

T t \times T t

cross mein F₂ genotype ratio kya hoga?

1 T T : 2 T t : 1 tt

T t \times T t

cross mein full dominance ke saath F₂ phenotype ratio kya hoga?

3 : 1

(dominant : recessive).

Ek single gamete ek gene ke kitne alleles carry karta hai?

Bilkul ek.

Ek heterozygote (

T t

) kaun se gamete proportions produce karta hai?

Aadhe

T

carry karte hain, aadhe

t

carry karte hain (½ : ½).

Recessive trait F₂ mein dobara kyun appear karta hai jabki F₁ mein gayab ho gaya tha?

Kyunki alleles intact rehte hain (blend nahi hote); recessive allele heterozygotes mein sirf chhupa hota hai aur phir se segregate ho jaata hai.

Test cross kya hai aur yeh kya reveal karta hai?

Ek unknown dominant phenotype ko homozygous recessive (

tt

) se cross karna; offspring ratios reveal karte hain ki unknown homozygous hai (saare dominant) ya heterozygous (1:1).

Kya "dominant" ka matlab hai ki allele population mein zyada common hai?

Nahi — dominance sirf visible phenotype affect karta hai, allele frequency nahi.

Connections

Meiosis — anaphase I segregation ka physical mechanism provide karta hai.
Law of Independent Assortment — Mendel ka 2nd law; segregation ek saath do genes par apply hota hai.
Punnett Square — segregated gametes ko combine karne ka tool.
Dominance and Recessiveness — kyun $3 : 1$ phenotype $1 : 2 : 1$ genotype ko chhupa leta hai.
Test Cross — hidden alleles expose karne ke liye segregation ka use.
Probability in Genetics — ratios ke peeche multiplication & addition rules.