Solve dihybrid crosses — Hinglish notes

3.1.7 · Biology › Mendelian Genetics

Dihybrid cross KYA hota hai?

9:3:3:1 ratio KYUN aata hai (scratch se derivation)

Chaliye famous ratio ko banate hain, ratton ki jagah samajhte hain.

Step 1 — Parents. $R R Y Y (round yellow) \times r r y y (wrinkled green)$ Yeh step kyun? True-breeding parents sirf ek tarah ke gamete banate hain, isliye F1 uniform hota hai.

Step 2 — F1 gametes. $R R Y Y$ sirf $R Y$ gametes banata hai; $r r y y$ sirf $r y$ gametes banata hai. Toh har F1 hai: $R r Y y (round yellow, lekin andar chhuppe recessives ke saath)$

Step 3 — F1 × F1 gametes. Ek heterozygote $R r Y y$ chaar equally likely gametes banata hai. Chaar kyun? Har gene ek allele independently contribute karta hai: ${R, r} \times {Y, y} = {R Y, R y, r Y, r y} each = \frac{1}{4}$ Yeh step kyun? Independent assortment kehta hai ki R/r ka choice Y/y ke choice ko affect nahi karta, isliye hum saare $2 \times 2$ combinations lete hain.

Step 4 — Har gene ko alag treat karo, phir multiply karo. Ek gene $R r \times R r$ ke liye: offspring $\frac{3}{4}$ round (R_) aur $\frac{1}{4}$ wrinkled (rr) hain. Waise hi $Y y \times Y y$ ke liye: $\frac{3}{4}$ yellow, $\frac{1}{4}$ green.

Ab phenotype probabilities ko multiply karo:

Kisi bhi dihybrid cross ko HOW solve karein — do methods

Method A: 4×4 Punnett square

Har parent ke 4 gametes upar aur side mein likho, 16 boxes bharo, phenotypes count karo. Reliable hai par slow hai.

Method B: Forked-line / multiply method (80/20 power tool)

Dihybrid ko do monohybrid crosses mein tod do.
Har ek ko alag solve karo (aasaan 3:1, 1:1, ya all-dominant).
Jo combo chahiye uske liye probabilities multiply karo.

Worked examples

Forecast-then-Verify drill

Common mistakes (Steel-manned)

Flashcards

Dihybrid cross kya hota hai?

Ek cross jo do genes/traits ko ek saath track karta hai, jaise

R r Y y \times R r Y y

Dihybrid cross mein hum single-gene probabilities kyun multiply kar sakte hain?

Kyunki Law of Independent Assortment ki wajah se — do genes independently assort karte hain, isliye events independent hain (AND → multiply).

R r Y y \times R r Y y

ka phenotypic ratio kya hota hai?

9 : 3 : 3 : 1

(16 mein se).

R r Y y

kitne gamete types produce karta hai aur woh kya hain?

Chaar —

R Y, R y, r Y, r y

, har ek ki probability

1/4

R r Y y \times R r Y y

se P(wrinkled AND green)?

\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}

Dihybrid test cross

R r Y y \times r r y y

ka ratio kya hota hai?

1 : 1 : 1 : 1

Probabilities kab ADD karte hain vs MULTIPLY?

Mutually exclusive "OR" outcomes ke liye add karo; independent "AND" outcomes ke liye multiply karo.

R r Y y \times R r Y y

se P(at least one dominant trait)?

1 - \frac{1}{16} = \frac{15}{16}

9+3+3+1 = 16 kyun hai?

Yeh

4 \times 4

Punnett-square boxes ke barabar hai (har parent se 4 gametes).

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho tum do sikke ek saath uchhaal rahe ho: ek sikka decide karta hai ki matar round hai ya wrinkled, doosra decide karta hai yellow hai ya green. Round aur yellow "lucky" sides hain jo 4 mein se 3 baar aate hain. Kyunki dono sikke ek doosre se baat nahi karte, dono lucky sides paane ke liye multiply karo: $\frac{3}{4} \times \frac{3}{4}$ . Yeh chaar combos ke liye karo aur tumhein famous $9 : 3 : 3 : 1$ milega — yeh bas do coin games ek saath khele ja rahe hain.

Connections

Monohybrid cross — woh 3:1 building block jise hum multiply karte hain.
Law of Independent Assortment — multiply karne ka justification.
Probability rules in genetics — product rule vs sum rule.
Test cross — gametes padhne ke liye $r r y y$ use karna; $1 : 1 : 1 : 1$ deta hai.
Gene linkage — jo cheez 9:3:3:1 ko tod deti hai.
Punnett square — visual tally method.