Is it correct to say: Confusing genotype with phenotype?

Genotype = genetic makeup (TT, Tt, tt). Phenotype = visible trait (tall, short).

Is it correct to say: Saying mother determines sex of child?

Mother's eggs are always X. Father contributes X or Y — so father decides the sex.

Is it correct to say: Treating vestigial organs as useless in ancestors?

They were functional in ancestors (appendix helped digest cellulose in earlier primates). Now reduced.

Is it correct to say: Confusing acquired traits with inherited traits?

Acquired traits (muscles from exercise) are not passed on. Only genetic (inherited) traits are transmitted to offspring.

Is it correct to say: Mixing up Darwin and Lamarck?

Lamarck: 'use and disuse' + inheritance of acquired traits (now discredited). Darwin: variation + natural selection.

Is it correct to say: Assuming all mutations are harmful?

Mutations can be neutral, harmful or beneficial. Beneficial ones drive evolution.

Heredity and Evolution

Mendel's laws, DNA, evolution, natural selection, human evolution — Maharashtra SSC Science Part 2

📐 6 formulas✏️ 3 examples🎯 6 practice⚖️ 5 marks🏫 Maharashtra State Board📚 Class 10✓ 2025–26 syllabus

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Board Exam Tips

→Mendel's monohybrid cross — with Punnett square — repeat 3-mark question.
→Difference between chromosome, DNA and gene — 2 marks.
→Sex determination in humans (XX vs XY) — repeat 2-mark diagram.
→Homologous, analogous, vestigial organs — with one example each. 3 marks.
→Darwin's theory of natural selection in 4 points — 4 marks.

📐 Formulas(6)

Monohybrid Ratio (F₂ Generation)★ Board fav

3 : 1 (phenotype); 1 : 2 : 1 (genotype)

Dihybrid Ratio (F₂)

9 : 3 : 3 : 1

Sex Determination in Humans★ Board fav

Father XY, Mother XX \to Offspring XX or XY

DNA Base Pairing

A \equiv T, G \equiv C

Gene, Chromosome, DNA★ Board fav

Gene \subset DNA \subset Chromosome

Natural Selection (Darwin's Formula)

Variation + Struggle for existence + Survival of fittest \to Evolution

✏️ Solved Examples

1Solved Exampleeasy4 steps

A tall pea plant (TT) is crossed with a short pea plant (tt). Predict the phenotypic and genotypic ratio of the F₂ generation.

F₁ generation: all Tt (tall — heterozygous)

T T \times tt \to T t

2Solved Exampleboard5 steps

Explain sex determination in humans with a chart. Why is the father responsible for the sex of the child?

Chromosomal makeup

Male: 22 + XY; Female: 22 + XX

3Solved ExampleHOTS6 steps

Distinguish homologous, analogous and vestigial organs. Give one example of each and explain what each tells us about evolution.

Homologous — same basic structure, different function; common ancestor

Human arm, whale flipper, bat wing

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing genotype with phenotype
✓Genotype = genetic makeup (TT, Tt, tt). Phenotype = visible trait (tall, short).
2
Saying mother determines sex of child
✓Mother's eggs are always X. Father contributes X or Y — so father decides the sex.
3
Treating vestigial organs as useless in ancestors
✓They were functional in ancestors (appendix helped digest cellulose in earlier primates). Now reduced.
4
Confusing acquired traits with inherited traits
✓Acquired traits (muscles from exercise) are not passed on. Only genetic (inherited) traits are transmitted to offspring.
5
Mixing up Darwin and Lamarck
✓Lamarck: 'use and disuse' + inheritance of acquired traits (now discredited). Darwin: variation + natural selection.
6
Assuming all mutations are harmful
✓Mutations can be neutral, harmful or beneficial. Beneficial ones drive evolution.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
State Mendel's laws in one line each.
Q2
Define the term 'gene'.
Q3
Name three types of evidence supporting evolution.
Q4
What is meant by speciation?
Q5
Which scientist proposed the theory of natural selection?
Q6
Why are fossils considered important in the study of evolution?

📝 Notes

Heredity and Evolution — Maharashtra SSC

Chapter 1 of Science Part 2 (Biology). The chapter connects two big ideas: heredity (how traits pass from parents to offspring) and evolution (how species change over time).

Mendel — the father of genetics

Working with pea plants, Gregor Mendel established:

Law of Dominance: in a heterozygote (Tt), the dominant allele (T) masks the recessive one (t).
Law of Segregation: during gamete formation, alleles of a gene separate — each gamete gets one.
Law of Independent Assortment: alleles of different genes assort independently.

Monohybrid vs dihybrid cross

Monohybrid (one trait): F₂ ratio 3:1 phenotype, 1:2:1 genotype.
Dihybrid (two traits): F₂ ratio 9:3:3:1.

DNA — chromosomes — genes — traits

Traits come from genes; genes are stretches of DNA; DNA is packed into chromosomes; chromosomes reside in the nucleus.

Humans have 23 pairs of chromosomes (46 total).
Of these, 22 pairs are autosomes; 1 pair is sex chromosomes.
Female: 22 pairs autosomes + XX. Male: 22 pairs autosomes + XY.

Sex determination — flow

Egg always carries X.
Sperm carries X (→ girl) or Y (→ boy) with 50/50 probability.
Father's chromosome contribution determines the child's sex.

Evidence for evolution

| Evidence | Example | |---|---| | Homologous organs | Human arm, whale flipper, bat wing | | Analogous organs | Insect wings vs bird wings | | Vestigial organs | Appendix, wisdom teeth, tail bone | | Fossils | Archaeopteryx (dinosaur–bird link) | | Embryological | Similar early embryos in vertebrates |

Human evolution — a rough timeline

Australopithecus (~4 mya) — bipedal.
Homo habilis (~2.4 mya) — tool user.
Homo erectus (~1.8 mya) — fire, migration.
Homo sapiens (~200 000 ya to present) — modern humans.

Board vs CBSE

Both boards teach identical content. Maharashtra SSC leans on Punnett square problems and terminology (homologous/analogous/vestigial). Draw diagrams and Punnett squares neatly — half of the marks depend on presentation.