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শিক্ষামূলক নোট: এই পৃষ্ঠা একাডেমিক জীববিজ্ঞান শেখা ও পরীক্ষার প্রস্তুতির সহায়ক।

Genetics Lecture 05: Dihybrid Cross

Concept Overview

Dihybrid cross হলো এমন genetic cross যেখানে একসাথে দুটি contrasting character অনুসরণ করা হয়। Mendel pea plant-এ seed shape and seed colour একসাথে অনুসরণ করে দেখিয়েছিলেন যে এক gene pair-এর allele অন্য gene pair-এর allele থেকে independently gamete-এ যেতে পারে, যদি genes different chromosomes-এ থাকে বা sufficiently far apart থাকে।

Classic example:

Round yellow seed × Wrinkled green seed
RRYY × rryy
  ↓
F1: all RrYy = round yellow
  ↓
F1 selfing
RrYy × RrYy
  ↓
F2 phenotypic ratio: 9 : 3 : 3 : 1

Why This Matters

Dihybrid cross learner-কে একসাথে probability, gamete formation, allele assortment and phenotype ratio বুঝতে শেখায়। Monohybrid cross একটি character শেখায়; dihybrid cross শেখায় inheritance is combinational. এটি Mendel’s second law বা law of independent assortment বোঝার foundation.

Use the central framework pages below for the full method. This page keeps only the topic-specific learning path so learners do not meet the same boilerplate repeatedly.

LOLO / LALA Objectives, outcomes, activities, and applications. Bloom Taxonomy Remember, understand, apply, analyze, evaluate, and create. CQ Studio Board-style creative question structure and answer logic. Assessment Rubric MCQ, short answer, CQ, diagram, practical, and reflection quality. Practical Framework Observation, evidence, life application, and reflective responsibility.

Dihybrid Learning Focus

এই lecture central LBFL framework-কে two-character inheritance-এ প্রয়োগ করে। Learner-এর focus হবে dihybrid cross, independent assortment, gamete types, F1 genotype, F2 Punnett square logic, 9:3:3:1 ratio, and biological meaning.

Example Characters

Character 1

Seed shape: Round (R) is dominant over wrinkled (r).

Character 2

Seed colour: Yellow (Y) is dominant over green (y).

Pure dominant parent

RRYY = round yellow.

Pure recessive parent

rryy = wrinkled green.

Parental Cross and F1

P generation:
RRYY × rryy

Gametes:
RY × ry

F1 generation:
All RrYy

F1 phenotype:
All round yellow

F1 shows both dominant traits because R masks r for seed shape and Y masks y for seed colour.

Gamete Formation in F1

A dihybrid individual RrYy can produce four types of gametes.

RrYy
  ↓ independent assortment
RY, Ry, rY, ry

Each gamete receives one allele from each gene pair.

Law of Independent Assortment

Law of independent assortment states that alleles of different gene pairs assort independently into gametes during gamete formation, when the genes are on different chromosomes or sufficiently far apart.

R/r pair separates
  +
Y/y pair separates
  ↓
Alleles combine into four gamete types
RY, Ry, rY, ry

F2 Combination Logic

F1 selfing:

RrYy × RrYy

Each parent produces:

RY, Ry, rY, ry

A 4 × 4 Punnett square gives 16 possible zygotic combinations.

F2 Phenotypic Ratio

F2 phenotype Genotype pattern Ratio part
Round yellow R_Y_ 9
Round green R_yy 3
Wrinkled yellow rrY_ 3
Wrinkled green rryy 1

Final phenotypic ratio:

9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Why 9:3:3:1 Appears

Each character separately follows a 3:1 phenotypic ratio.

Seed shape: 3 round : 1 wrinkled
Seed colour: 3 yellow : 1 green

When both independent character ratios combine:

(3 : 1) × (3 : 1)
  ↓
9 : 3 : 3 : 1

Dihybrid Cross Flowchart

Pure parents: RRYY × rryy
  ↓
F1: all RrYy
  ↓
Four gamete types: RY, Ry, rY, ry
  ↓
F1 selfing: RrYy × RrYy
  ↓
16 combinations
  ↓
F2 phenotype ratio 9:3:3:1

Important Limitation

Independent assortment is not universal. If two genes are very close on the same chromosome, they may be linked and may not assort independently.

Genes on different chromosomes
  ↓
Independent assortment likely

Genes close on same chromosome
  ↓
Linkage likely
  ↓
9:3:3:1 may not appear

Common Mistakes to Avoid

Mistake 1

Writing only two gametes for RrYy. A dihybrid can form four gamete types: RY, Ry, rY, ry.

Mistake 2

Confusing law of segregation with law of independent assortment. Segregation separates alleles of one gene pair; independent assortment combines different gene pairs.

Mistake 3

Assuming every two-character cross must show 9:3:3:1. Linkage or gene interaction can modify ratios.

Mistake 4

Memorizing 9:3:3:1 without understanding the 4 gametes and 16 combinations.

Synaptic Bridge

Dihybrid cross teaches combination thinking. Outcomes are not always controlled by one factor. In biology and life, multiple variables can assort, combine and produce new patterns. A disciplined learner asks: which factors are independent, which are linked, and which are interacting?

Critical Thinking Questions

  1. Why does RrYy produce four types of gametes?
  2. How does independent assortment differ from segregation?
  3. Why does F2 have 16 possible combinations?
  4. Why does 9:3:3:1 ratio depend on independent assortment?
  5. When might a dihybrid cross fail to show 9:3:3:1?

References

  • Standard HSC Biology Genetics notes.
  • Integrated Genetics references on dihybrid cross and independent assortment.
  • NCERT Biology: Principles of Inheritance and Variation.