Biology - Botany - How to Solve: Principles of Inheritance and Variation (Monohybrid/Dihybrid, Pedigree, Linkage, Sex Determination) – NEET UG Guide

How to Solve: Principles of Inheritance and Variation (Monohybrid/Dihybrid, Pedigree, Linkage, Sex Determination) – NEET UG Guide

Introduction

"Mastering inheritance patterns doesn’t just explain why you have your dad’s nose—it unlocks 8-10 high-weightage NEET questions worth 32-40 marks. Miss this, and you’re leaving easy marks on the table."

WHAT YOU NEED TO KNOW FIRST

1. Basic genetics vocabulary – Gene, allele, dominant, recessive, homozygous, heterozygous.
2. Mendel’s laws – Law of segregation, law of independent assortment.
3. Chromosome basics – Autosomes vs. sex chromosomes, gamete formation.

(If you’re shaky on these, pause and review before proceeding.)

KEY TERMS & FORMULAS

1. Monohybrid Cross

• Phenotypic ratio (F₂): 3 : 1 (dominant : recessive)
• Genotypic ratio (F₂): 1 : 2 : 1 (homozygous dominant : heterozygous : homozygous recessive)
• Test cross ratio: 1 : 1 (if heterozygous parent) or 100% dominant (if homozygous parent)

2. Dihybrid Cross

• Phenotypic ratio (F₂): 9 : 3 : 3 : 1 (both dominant : first dominant, second recessive : first recessive, second dominant : both recessive)
• Formula for gamete combinations: 2ⁿ (where n = number of heterozygous gene pairs)
• Example: AaBb → 2² = 4 gametes (AB, Ab, aB, ab)

3. Pedigree Analysis

• Symbols:
• Square = Male
• Circle = Female
• Shaded = Affected
• Half-shaded = Carrier (for recessive traits)
• Line through = Deceased
• Key rules:
• Autosomal dominant: Affected individuals in every generation, no skipping.
• Autosomal recessive: Skips generations, unaffected parents can have affected children.
• X-linked recessive: More males affected, carrier mothers pass to sons.

4. Linkage & Recombination

• Complete linkage: Genes inherited together → no crossing over → parental combinations only.
• Incomplete linkage: Some crossing over → parental > recombinant gametes.
• Recombination frequency (RF): RF = (Number of recombinant offspring / Total offspring) × 100
• RF < 50% → Linked genes
• RF = 50% → Unlinked (independent assortment)

5. Sex Determination

• Humans: XX (female), XY (male)
• Birds/Butterflies: ZW (female), ZZ (male)
• Honeybees: Haplodiploidy (males haploid, females diploid)

MEMORISE THIS: - Monohybrid & dihybrid ratios - Pedigree symbols & inheritance patterns - Recombination frequency formula

STEP-BY-STEP METHOD

A. Monohybrid Cross (Single Gene)

Steps:
1. Identify parental genotypes (e.g., TT × tt).
2. Write gametes (T and t for each parent).
3. Draw Punnett square (2×2 grid).
4. Fill in offspring genotypes (e.g., Tt, Tt, Tt, Tt).
5. Determine phenotypic ratio (e.g., 100% tall).
6. For F₂ generation, cross F₁ (Tt × Tt) and repeat steps 2-5.

B. Dihybrid Cross (Two Genes)

Steps:
1. Identify parental genotypes (e.g., RRYY × rryy).
2. Write gametes (RY and ry).
3. Draw Punnett square (4×4 grid).
4. Fill in offspring genotypes (e.g., RrYy for all).
5. For F₂ generation, cross F₁ (RrYy × RrYy).
6. Write all possible gametes (RY, Ry, rY, ry).
7. Fill 4×4 Punnett square.
8. Count phenotypes (9:3:3:1 ratio).

C. Pedigree Analysis

Steps:
1. Label generations (I, II, III…).
2. Identify affected individuals (shaded).
3. Check for skipping generations (recessive if yes, dominant if no).
4. Check sex bias (X-linked if mostly males affected).
5. Determine genotypes (use "A_" for dominant, "aa" for recessive).
6. Verify with test crosses (e.g., affected child from unaffected parents → recessive).

D. Linkage & Recombination

Steps:
1. Identify parental phenotypes (most frequent).
2. Identify recombinant phenotypes (least frequent).
3. Calculate recombination frequency (RF): - RF = (Recombinants / Total offspring) × 100
4. Determine linkage: - RF < 50% → Linked - RF = 50% → Unlinked

E. Sex Determination

Steps:
1. Identify species (humans, birds, bees).
2. Write sex chromosome combinations (e.g., XX, XY).
3. Determine gametes (e.g., female = X, male = X or Y).
4. Predict offspring sex ratio (e.g., 1:1 in humans).

WORKED EXAMPLES

Example 1 – Monohybrid Cross (Basic)

Question: In pea plants, tall (T) is dominant over dwarf (t). Cross a heterozygous tall plant with a dwarf plant. What is the phenotypic ratio of the offspring?

Solution:
1. Parental genotypes: Tt × tt
2. Gametes: T, t (from Tt) and t, t (from tt)
3. Punnett square: | | t | t | |---|---|---| | T | Tt | Tt | | t | tt | tt |
4. Offspring genotypes: 2 Tt, 2 tt
5. Phenotypic ratio: 2 tall : 2 dwarf → 1:1

What we did and why: - Used a test cross (heterozygous × homozygous recessive) to reveal hidden alleles. - Punnett square shows all possible gamete combinations.

Example 2 – Dihybrid Cross (Medium)

Question: In peas, yellow (Y) is dominant over green (y), and round (R) is dominant over wrinkled (r). Cross two plants heterozygous for both traits (YyRr × YyRr). What is the phenotypic ratio of the offspring?

Solution:
1. Parental genotypes: YyRr × YyRr
2. Gametes: YR, Yr, yR, yr (for both parents)
3. Punnett square (4×4): | | YR | Yr | yR | yr | |---|----|----|----|----| | YR | YYRR | YYRr | YyRR | YyRr | | Yr | YYRr | YYrr | YyRr | Yyrr | | yR | YyRR | YyRr | yyRR | yyRr | | yr | YyRr | Yyrr | yyRr | yyrr |
4. Phenotypes: - 9 Yellow, Round (Y_R_) - 3 Yellow, Wrinkled (Y_rr) - 3 Green, Round (yyR_) - 1 Green, Wrinkled (yyrr)
5. Ratio: 9:3:3:1

What we did and why: - Used the 2ⁿ rule (2² = 4 gametes per parent). - Counted phenotypes systematically to avoid missing combinations.

Example 3 – Pedigree Analysis (Exam-Style)

Question: The pedigree below shows the inheritance of a genetic disorder. What is the most likely mode of inheritance?

![Pedigree: Generation I has one affected male (shaded square). Generation II has 4 children: 2 unaffected females, 1 affected male, 1 unaffected male. Generation III has 3 children from the affected male: 1 affected female, 2 unaffected males.]

Solution:
1. Label generations: I, II, III.
2. Check affected individuals: - Affected in every generation → likely dominant. - Affected father (II-3) passes to daughter (III-1) → not Y-linked. - No sex bias → autosomal.
3. Test autosomal dominant: - Affected individuals = Aa or AA. - Unaffected = aa. - Generation I: Affected male (Aa) × unaffected female (aa) → 50% affected (matches II-3). - Generation II-3 (Aa) × unaffected female (aa) → 50% affected (matches III-1).
4. Conclusion: Autosomal dominant.

What we did and why: - Eliminated X-linked (father passes to daughter) and recessive (no skipping). - Verified with genotype predictions.

COMMON MISTAKES

1. MISTAKE: Forgetting to write all gametes in dihybrid crosses. WHY IT HAPPENS: Overconfidence in the 9:3:3:1 ratio. CORRECT APPROACH: Always list all 4 gametes (RY, Ry, rY, ry) for YyRr.

2. MISTAKE: Misidentifying pedigree inheritance (e.g., calling autosomal recessive dominant). WHY IT HAPPENS: Not checking for skipping generations. CORRECT APPROACH: Recessive skips, dominant doesn’t.

3. MISTAKE: Calculating recombination frequency incorrectly. WHY IT HAPPENS: Counting parental as recombinant. CORRECT APPROACH: Recombinants = least frequent phenotypes.

4. MISTAKE: Assuming all traits are autosomal. WHY IT HAPPENS: Ignoring sex bias in pedigrees. CORRECT APPROACH: More males affected → X-linked recessive.

5. MISTAKE: Mixing up monohybrid and dihybrid ratios. WHY IT HAPPENS: Memorizing without understanding. CORRECT APPROACH: Monohybrid = 3:1, Dihybrid = 9:3:3:1.

EXAM TRAPS

1. TRAP: "A test cross is always with a homozygous recessive." HOW TO SPOT IT: Question says "test cross" but shows a heterozygous parent. HOW TO AVOID IT: Test cross = homozygous recessive (aa or tt).

2. TRAP: "Linked genes always show 1:1:1:1 ratio." HOW TO SPOT IT: Question mentions "linked" but shows 9:3:3:1. HOW TO AVOID IT: Linked genes → parental > recombinant (not 9:3:3:1).

3. TRAP: "Sex determination is the same in all species." HOW TO SPOT IT: Question asks about birds or bees. HOW TO AVOID IT: Humans = XY, Birds = ZW, Bees = Haplodiploidy.

1-MINUTE RECAP

"Listen up—this is your last-minute crash course for NEET inheritance questions. For monohybrid crosses, remember 3:1 phenotypic ratio in F₂. Dihybrid? 9:3:3:1. Pedigrees: dominant never skips, recessive does. X-linked? More males affected. Linkage? Recombinants are the least frequent. Sex determination? Humans are XY, birds are ZW. Write down the gametes, draw the Punnett square, count the phenotypes. Don’t guess—calculate. And if a question mentions ‘test cross,’ it’s always with a homozygous recessive. You’ve got this—go ace it!