Is it correct to say: Confusing genotype with phenotype ratio in Mendel's cross?

Genotype 1:2:1; Phenotype 3:1 (for complete dominance) in Aa × Aa.

Is it correct to say: Assuming Hardy-Weinberg is always valid?

Requires no mutation, no selection, no gene flow, random mating, large population. State conditions.

Is it correct to say: Writing X-linked traits as autosomal in pedigree?

X-linked recessive: skips generations, affects males more; expressed in females only if both X carry allele.

Is it correct to say: Confusing codominance with incomplete dominance?

Codominance: both alleles fully expressed (AB blood). Incomplete dominance: intermediate phenotype (pink flower).

Is it correct to say: Assuming linkage always gives 1:1:1:1 ratio?

Linked genes give parental combinations in higher frequency. Only recombinants show independent-assortment ratio.

Is it correct to say: Writing 'test cross ratio 3:1'?

Test cross (heterozygote × recessive) gives 1:1 phenotypic ratio, NOT 3:1.

Genetics and Evolution

Mendelism, chromosomal theory, linkage, sex determination, evolution — Maharashtra HSC Biology Ch 3 & 4

📐 11 formulas✏️ 3 examples🎯 5 practice⚖️ 6 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

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Board Exam Tips

→Mendel's laws (dominance, segregation, independent assortment) with cross diagrams (dihybrid 9:3:3:1) is a Maharashtra HSC 4-marker every year.
→Pedigree analysis with a family tree diagram is a HOTS 3-marker in Balbharati.
→Sex determination — XY (mammal), XO (grasshopper), ZW (bird), ZO (moth) — 2-marker classification question.
→Linkage and crossing over — draw diagrams of both parental and recombinant types; state chromosomal theory of inheritance.
→Hardy–Weinberg equilibrium: p² + 2pq + q² = 1 and conditions for equilibrium — 3-marker.

📐 Formulas(11)

Monohybrid Cross Ratio★ Board fav

A a \times A a \to 1 AA : 2 A a : 1 aa (genotypic) = 3 : 1 (phenotypic)

Dihybrid Cross Ratio★ Board fav

9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb

Test Cross★ Board fav

A a \times aa \to 1 A a : 1 aa

Incomplete Dominance

R R \times r r \to R r (intermediate phenotype)

Codominance

I^{A} I^{B} \to AB blood group

Multiple Alleles — ABO Blood Group★ Board fav

I^{A}, I^{B}, i; I^{A} = I^{B} > i

Chromosomal Sex Determination★ Board fav

XY: mammals; XO: grasshopper; ZW: birds; ZO: moth

Sex-Linked Inheritance

X-linked recessive: haemophilia, colour blindness

Hardy–Weinberg Equation★ Board fav

p^{2} + 2 pq + q^{2} = 1, p + q = 1

Linkage Frequency

Recombination frequency (RF) = \frac{recombinants}{total offspring} \times 100

Pleiotropy

One gene \to multiple phenotypic effects

✏️ Solved Examples

1Solved Exampleeasy3 steps

In pea plants, tall (T) is dominant over dwarf (t). Predict the genotypic and phenotypic ratios in the F₂ generation of a cross Tt × Tt.

Punnett square of Tt × Tt.

Gametes: T, t from each parent

2Solved Exampleboard4 steps

In a population, 16% of individuals have blood type O (i i genotype). Assuming Hardy–Weinberg equilibrium, calculate frequencies of alleles I^A, I^B, i, if I^A frequency = 0.5, I^B = 0.1.

Given: i i homozygotes = 16% = 0.16.

q^{2} = 0.16 \Rightarrow q = 0.4

3Solved ExampleHOTS4 steps

In humans, colour blindness is X-linked recessive. A normal woman whose father was colour-blind marries a normal man. What is the probability that (i) her sons will be colour-blind, (ii) her daughters will be colour-blind?

Her father was colour-blind (X^c Y), so mother must be a carrier: X^C X^c.

Mother genotype: X^{C} X^{c}

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing genotype with phenotype ratio in Mendel's cross
✓Genotype 1:2:1; Phenotype 3:1 (for complete dominance) in Aa × Aa.
2
Assuming Hardy-Weinberg is always valid
✓Requires no mutation, no selection, no gene flow, random mating, large population. State conditions.
3
Writing X-linked traits as autosomal in pedigree
✓X-linked recessive: skips generations, affects males more; expressed in females only if both X carry allele.
4
Confusing codominance with incomplete dominance
✓Codominance: both alleles fully expressed (AB blood). Incomplete dominance: intermediate phenotype (pink flower).
5
Assuming linkage always gives 1:1:1:1 ratio
✓Linked genes give parental combinations in higher frequency. Only recombinants show independent-assortment ratio.
6
Writing 'test cross ratio 3:1'
✓Test cross (heterozygote × recessive) gives 1:1 phenotypic ratio, NOT 3:1.

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
State Mendel's law of independent assortment.
Q2
In a dihybrid cross AaBb × AaBb, what fraction of offspring is aabb?
Q3
Define pleiotropy with example.
Q4
A woman has blood group A (I^A i) and her husband has blood group B (I^B i). What blood groups can their children have?
Q5
State the chromosomal theory of inheritance.

📝 Notes

Genetics and Evolution — Maharashtra HSC Overview

Chapters 3 and 4 of the Balbharati Class 12 Biology textbook cover Mendelian and non-Mendelian inheritance, chromosomal theory, sex determination, pedigree analysis, and (in Ch 4) evolution.

Maharashtra syllabus specifics

Mendel's laws with cross diagrams (monohybrid, dihybrid, test cross) — always asked with detailed Punnett squares.
Non-Mendelian inheritance — incomplete dominance, codominance, multiple alleles (ABO), polygenic (skin colour) — Balbharati covers all four.
Sex determination in different organisms (XX-XY, XX-XO, ZW-ZZ, ZO-ZZ) — 2-marker classification.
Pedigree analysis for autosomal and X-linked traits — Maharashtra-specific.
Hardy–Weinberg equilibrium with its conditions is asked in evolution chapter.
Chromosomal disorders — Down (trisomy 21), Turner (XO), Klinefelter (XXY) — 2-marker theory.

Strategy

Draw a clear Punnett square for every cross problem.
For pedigree, use standard symbols (square = male, circle = female, filled = affected).
State Mendel's laws explicitly in theory answers; do not paraphrase.
Distinguish between codominance (AB) and incomplete dominance (pink) with named examples.
Learn Hardy-Weinberg conditions as a list — HSC asks them as a 2-marker.