CBSE Class 10 Heredity and Evolution — Complete Chapter Guide with Punnett Squares

In short: Heredity and Evolution carries 5-7 marks in CBSE Class 10. This chapter covers Mendel's experiments with pea plants, the monohybrid cross (3:1 phenotypic ratio), the dihybrid cross (9:3:3:1), sex determination through XX-XY chromosomes, and the basics of evolution. Master Punnett squares, know the difference between homologous and analogous organs, and understand how fossils and DNA evidence support evolution.

Heredity and Evolution combines genetics with evolutionary biology. The genetics half is logical and diagram-based — once you understand Punnett squares, you can solve any cross. The evolution half requires clear conceptual understanding of how species change over time. Board exams focus on Mendel's experiments, Punnett square diagrams, sex determination, and comparing inherited vs acquired traits.

This guide covers everything you need with practice problems. For marks distribution, check the CBSE Class 10 Science chapter-wise marks weightage.

What Is Heredity?

Heredity is the transmission of characteristics (traits) from parents to offspring. The branch of biology that studies heredity is called genetics. The traits that are passed from one generation to the next are called inherited traits.

Variation refers to differences in traits among individuals of the same species. For example, all humans belong to the same species but differ in height, skin color, eye color, and so on. Variations arise during reproduction and are the basis for evolution.

Mendel's Experiments with Pea Plants

Gregor Johann Mendel (1822-1884), an Austrian monk, is called the "Father of Genetics." He conducted experiments on garden pea plants (Pisum sativum) for seven years and discovered the fundamental laws of inheritance.

Why pea plants?

• Short generation time (results visible quickly).
• Many contrasting traits that are easy to observe.
• Self-pollination is natural, but cross-pollination can also be done easily.
• Large number of offspring produced.

Mendel studied seven contrasting traits:

Key terms:

• Gene: A unit of inheritance. A section of DNA that codes for a specific protein/trait.
• Allele: Different forms of the same gene. For example, T (tall) and t (dwarf) are alleles.
• Dominant trait: The trait that expresses itself in the presence of the other allele (e.g., tallness).
• Recessive trait: The trait that is hidden in the presence of the dominant allele (e.g., dwarfness). It only appears when both alleles are recessive (tt).
• Homozygous: Both alleles are the same (TT or tt).
• Heterozygous: Alleles are different (Tt).
• Genotype: The genetic makeup of an organism (e.g., Tt).
• Phenotype: The physical appearance of an organism (e.g., Tall).

Monohybrid Cross (One Trait)

A monohybrid cross studies the inheritance of one trait at a time.

Mendel's experiment: He crossed a pure tall plant (TT) with a pure dwarf plant (tt).

F1 generation (first filial): All plants were tall. This showed that tallness is dominant over dwarfness.

Punnett Square for F1:

All F1 offspring are Tt (heterozygous tall). 100% tall.

F2 generation: Mendel then self-pollinated the F1 plants (Tt × Tt).

Punnett Square for F2:

Results:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Dwarf (the famous 3:1 ratio)

The recessive trait (dwarf) reappeared in the F2 generation even though it was hidden in F1. This proved that traits do not blend — they are inherited as discrete units (genes).

Board Exam Tip: Drawing the Punnett square for F2 of a monohybrid cross is a very common 3-mark question. Always label the gametes, fill the square correctly, and clearly state both the genotypic and phenotypic ratios.

Dihybrid Cross (Two Traits)

A dihybrid cross studies the inheritance of two traits simultaneously.

Mendel's experiment: He crossed a plant with round yellow seeds (RRYY) with a plant with wrinkled green seeds (rryy).

F1 generation: All plants had round yellow seeds (RrYy). Both round and yellow are dominant.

F2 generation: F1 plants (RrYy) were self-pollinated.

Punnett Square for F2 (4 × 4 grid):

Each parent produces four types of gametes: RY, Ry, rY, ry.

The 16-cell Punnett square gives the following phenotypic results:

Phenotypic ratio: 9:3:3:1

This experiment proved the Law of Independent Assortment — the inheritance of one trait is independent of the inheritance of another trait. The genes for seed shape and seed color assort independently during gamete formation.

Sex Determination in Humans

In humans, sex is determined by the sex chromosomes.

• Females have two X chromosomes: XX
• Males have one X and one Y chromosome: XY

All eggs produced by a female carry the X chromosome. Sperm produced by a male can carry either X or Y.

Punnett Square for Sex Determination:

Result: 50% chance of a boy, 50% chance of a girl.

Key point: The sex of the child is determined by the father's sperm (whether it carries X or Y), not by the mother. This is an important point both scientifically and socially, as it counters the misconception that mothers are "responsible" for the sex of the child.

Board Exam Tip: "How is the sex of a child determined in humans? Explain with a diagram" is a 3-mark question that appears very frequently. Draw the Punnett square clearly.

Inherited Traits vs Acquired Traits

Why acquired traits are not inherited: Changes that occur in non-reproductive tissues (somatic cells) cannot be passed to the next generation. Only changes in the germ cells (eggs and sperm) can be inherited. For example, if a mouse's tail is cut off, its offspring will still be born with tails because the tail-cutting did not change the DNA in the germ cells.

Evolution — How Species Change Over Time

Evolution is the process by which living organisms have changed over millions of years from simple forms to more complex forms.

Evidence for Evolution

1. Homologous Organs

Organs that have the same basic structure and origin but different functions in different species. They indicate common ancestry.

Examples:

• Forelimbs of a human (grasping), whale (swimming), bat (flying), and horse (running) all have the same bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges).
• They have adapted to different functions through evolution.

2. Analogous Organs

Organs that have different basic structures and origins but similar functions. They indicate convergent evolution (similar environments leading to similar adaptations) but not common ancestry.

Examples:

• Wings of a bird (modified forelimb with feathers) and wings of an insect (thin membranous extension of body wall).
• Eyes of an octopus and eyes of a human.

Homologous vs Analogous Organs

3. Fossils

Fossils are preserved remains or impressions of organisms from the past. They provide direct evidence of evolution by showing intermediate forms and the gradual change in species over time.

How fossils form: Dead organisms are buried under sediment. Over millions of years, layers of sediment compress and turn to rock, preserving the shape of the organism. Deeper layers contain older fossils.

Dating fossils:

• Relative dating: Deeper layers of rock are older, so fossils in deeper layers are older.
• Absolute dating (Carbon-14 dating): Measures the ratio of C-14 to C-12 in fossils to determine their age.

4. Molecular evidence (DNA comparison)

Species that are more closely related have more similar DNA sequences. Human DNA is about 98.7% similar to chimpanzee DNA, confirming a recent common ancestor.

Speciation

Speciation is the formation of new species from existing ones. It happens through:

1. Geographical isolation: Physical barriers (mountains, rivers) separate a population. The separated groups evolve independently due to different environmental pressures, eventually becoming so different they can no longer interbreed.

2. Genetic drift: Random changes in gene frequency, especially in small populations. This can cause significant changes over many generations.

3. Natural selection: Organisms with traits better suited to the environment survive and reproduce more, passing on those advantageous traits. Over time, this leads to adaptation and eventually speciation.

Human Evolution — A Brief Overview

Modern humans (Homo sapiens) evolved in Africa and migrated to other continents. Key points:

• All humans are a single species regardless of skin color or other physical differences.
• Variations in skin color, body type, and other features are adaptations to different climates and environments.
• Tool use, bipedalism (walking on two legs), and brain size increased over human evolutionary history.
• Common ancestors with apes existed millions of years ago — humans did not "come from" modern apes; both evolved from a common ancestor.

Practice: Punnett Square Problems

Problem 1: Cross a heterozygous tall plant (Tt) with a dwarf plant (tt). Find the genotypic and phenotypic ratios.

Genotypic ratio: 2 Tt : 2 tt = 1:1 Phenotypic ratio: 2 Tall : 2 Dwarf = 1:1

Problem 2: In humans, brown eyes (B) are dominant over blue eyes (b). Both parents are heterozygous (Bb). What is the probability of a blue-eyed child?

Probability of blue-eyed child (bb) = 1/4 = 25%.

Problem 3: A couple has had three daughters. What is the probability that their fourth child will be a son?

The probability is still 50% (1/2). Each pregnancy is an independent event. Previous outcomes do not affect the sex of future children.

Frequently Asked Questions

How many marks does Heredity and Evolution carry in CBSE Class 10?

This chapter carries 5 to 7 marks in the board exam. Typical questions include a 3-mark Punnett square question (monohybrid cross or sex determination), a 2-3 mark question on homologous vs analogous organs, and 1-mark MCQs on Mendel's ratios or acquired vs inherited traits. The Punnett square question is almost guaranteed every year.

What is the difference between genotype and phenotype?

Genotype is the genetic composition of an organism — the alleles it carries (like Tt or TT). Phenotype is the observable physical trait (like tall or dwarf). Two organisms can have the same phenotype but different genotypes — both TT and Tt plants are tall in appearance, but their genotypes are different. The genotype determines the phenotype.

How do I draw a Punnett square correctly?

Write the gametes of one parent along the top and the other parent along the side. Fill in each cell by combining the column gamete with the row gamete. For a monohybrid cross, the square is 2×2; for a dihybrid cross, it is 4×4. Always write the dominant allele first (Tt, not tT). After filling the square, count the genotypes and phenotypes and express as ratios.

Why are acquired traits not inherited?

Acquired traits affect somatic (body) cells, not germ (reproductive) cells. Since only changes in germ cells are passed to offspring through DNA, traits acquired during an organism's lifetime — such as a muscular body from exercise or scars from injuries — cannot be inherited. For inheritance to occur, the change must be present in the DNA of the eggs or sperm.

What is the significance of the 3:1 ratio in Mendel's experiments?

The 3:1 ratio in the F2 generation proved several fundamental concepts. It showed that traits are controlled by discrete factors (genes), not blending. It proved that the recessive trait is not lost but merely hidden in the F1 generation. And it demonstrated that each parent contributes one allele for each trait, which segregate during gamete formation (the Law of Segregation). This ratio is the foundation of modern genetics.

How are fossils evidence for evolution?

Fossils show us what organisms looked like millions of years ago and how they have changed over time. By studying fossils in sequential rock layers, scientists can trace the gradual changes in species. Transitional fossils (like Archaeopteryx, which had features of both reptiles and birds) provide direct evidence of evolutionary links between groups. Fossil evidence, combined with DNA analysis, gives a comprehensive picture of evolutionary history.

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