CBSE Class 10 Science • Heredity Revision
Heredity Class 10 Worksheet with Answers
Why do offspring resemble their parents without being identical? How can two tall pea plants produce a short plant? Practise these ideas with the Heredity Class 10 worksheet with answers. Revise genes, inherited traits, dominant and recessive alleles, Mendel’s experiments and chromosome-based sex determination.
Heredity explains how genetic information passes from one generation to the next. Variation describes differences between individuals. Learning these concepts together helps you interpret genetic crosses, explain trait expression and avoid common mistakes about inheritance.
Begin with our Heredity Class 10 notes and mind map , then attempt the worksheet independently. Use the answers to check your reasoning, genetic symbols and ratios. The worked examples below provide additional practice before a chapter test.
Heredity Class 10 Summary and Key Terms
| Term | Meaning | Example |
|---|---|---|
| Heredity | Transmission of genetic information from parents to offspring. | Inheritance of alleles influencing pea seed shape. |
| Gene | A segment of DNA involved in producing a functional product. | A gene whose variants influence a pea plant characteristic. |
| Allele | An alternative version of a gene. | T and t in a simplified pea-height cross. |
| Genotype | The genetic constitution being considered. | TT, Tt or tt. |
| Phenotype | The expressed characteristic. | Tall or short in the pea-height example. |
| Homozygous | Having two identical alleles at a gene locus. | TT or tt. |
| Heterozygous | Having two different alleles at a gene locus. | Tt. |
DNA, Chromosomes and Trait Expression
Chromosomes contain DNA, and genes are regions of DNA. Gene products, including proteins, contribute to how characteristics develop. Offspring receive genetic information from their parents, but the environment can also influence the expression of many traits.
Use Simple Genetic Models Carefully
Mendel’s pea examples are useful for learning inheritance patterns. Human height, intelligence and many other human characteristics are complex and cannot be explained by a single dominant or recessive allele.
Mendel’s Experiments Class 10 Short Notes
Gregor Mendel studied inheritance using pea plants with contrasting characteristics. Pea plants were suitable because they could self-pollinate, could be cross-pollinated deliberately, produced many offspring and had identifiable contrasting traits.
Mendel followed characteristics across generations. In a monohybrid cross, one characteristic is studied. In a dihybrid cross, two characteristics are studied together.
Dominant and Recessive Alleles
Under complete dominance, the dominant allele determines the phenotype of a heterozygote. The recessive phenotype appears when both alleles are recessive. A recessive allele can therefore be present without its phenotype being visible.
Dominant Does Not Mean Stronger or More Common
Dominance describes expression in a heterozygote. It does not mean that an allele is better, more useful or necessarily more frequent in a population.
Monohybrid Cross: Tall and Short Pea Plants
Parental Cross and First Generation
Let T represent the dominant tall allele and t the recessive short allele in this pea example.
Parents: TT × tt
Gametes: T from one parent; t from the other.
F1 offspring: All Tt.
Phenotype: All tall, because T is dominant over t in this example.
Second Generation: Tt × Tt
Each heterozygous parent produces two types of gametes, T and t. Combine them in a Punnett square to find the expected offspring genotypes.
| Gametes | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotypic ratio: 1 TT : 2 Tt : 1 tt.
Phenotypic ratio: 3 tall : 1 short.
Probability of a short offspring: 1/4 or 25%.
These are expected probabilities. A small group of offspring need not show the exact ratio.
Dihybrid Cross and Independent Assortment
Consider pea seed shape and colour. Let R represent round, r wrinkled, Y yellow and y green. Crossing RRYY with rryy produces F1 offspring with genotype RrYy.
When the genes assort independently, an RrYy plant produces four gamete types: RY, Ry, rY and ry. Crossing two such plants gives the familiar F2 phenotypic ratio under complete dominance.
Expected Dihybrid Phenotypic Ratio
9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
The 9 : 3 : 3 : 1 ratio depends on the assumptions of the cross, including independent assortment and complete dominance. It is not a universal ratio for every pair of genes.
Sex Determination in Humans Class 10
In the standard XX/XY model studied at this level, an egg carries an X chromosome, while a sperm can carry X or Y. Their combination produces XX or XY offspring with approximately equal probability.
| Egg chromosome | Sperm: X | Sperm: Y |
|---|---|---|
| X | XX | XY |
The fertilising sperm’s chromosome determines the XX or XY combination in this model. This is a chance biological event that neither parent consciously controls. Each conception is a new event; earlier births do not determine the next outcome.
Heredity Class 10 MCQs with Answers
1. Which genotype is heterozygous?
A. TT
B. tt
C. Tt
D. Both TT and tt
Answer: C. Tt. It contains two different alleles.
2. What is the expected genotypic ratio in Tt × Tt?
A. 3 : 1
B. 1 : 2 : 1
C. 9 : 3 : 3 : 1
D. 1 : 1
Answer: B. 1 : 2 : 1. The expected genotypes are one TT, two Tt and one tt.
3. What does a recessive phenotype indicate in this complete-dominance model?
A. Two recessive alleles
B. Two dominant alleles
C. One dominant and one recessive allele
D. No alleles
Answer: A. The recessive phenotype appears in the homozygous recessive genotype.
Assertion–Reason Practice
Assertion: A recessive phenotype can reappear in the F2 generation after being absent in F1.
Reason: F1 heterozygotes retain the recessive allele and can pass it to their offspring.
Answer: Both statements are true, and the reason correctly explains the assertion.
Heredity Class 10 Case-Based Questions
Case Study: Two Tall Pea Plants
Two heterozygous tall pea plants are crossed. Tallness shows complete dominance over shortness in this example.
Question 1: Write the parental genotypes.
Tt × Tt.
Question 2: Which gametes can each parent produce?
T and t.
Question 3: What is the probability of a short offspring?
1/4 or 25%.
Question 4: Must exactly one of every four offspring be short?
No. The ratio represents expected probabilities, not a fixed
sequence of births.
Heredity Class 10 Important Questions
- Define heredity and variation.
- Explain the relationship between DNA, genes and chromosomes.
- Distinguish genotype from phenotype with examples.
- Explain dominant and recessive alleles.
- Show the F1 and F2 generations of a monohybrid cross.
- Explain why a recessive phenotype can reappear in F2.
- List the gametes produced by RrYy under independent assortment.
- Explain the expected 9 : 3 : 3 : 1 dihybrid ratio.
- Describe chromosome-based sex determination using a cross.
How to Solve Genetic Crosses Correctly
- Define the symbols and state which allele is dominant.
- Write the parental genotypes.
- List the possible gametes.
- Combine the gametes in a Punnett square.
- Count genotypes and phenotypes separately.
- State the ratio or probability requested.
Use this method while attempting the Heredity worksheet. It makes your working clear and helps prevent confusion between genotypic and phenotypic ratios.
Frequently Asked Questions
What is the difference between heredity and variation?
Heredity concerns the transmission of genetic information. Variation concerns differences among individuals. Both help explain why offspring resemble their parents but are not identical.
Can two tall pea plants produce a short offspring?
Yes, if both are heterozygous in the simple complete-dominance model. A Tt × Tt cross has a 25% probability of producing tt.
Why do TT and Tt have the same phenotype?
Under complete dominance, one dominant allele is sufficient for the dominant phenotype. Their genotypes differ even though the characteristic being observed is the same.
Does a dominant allele remove the recessive allele?
No. A heterozygote retains both alleles. The recessive allele can enter a gamete and be inherited by the next generation.
What is the difference between the 3 : 1 and 1 : 2 : 1 ratios?
In the standard heterozygous monohybrid cross, 3 : 1 describes phenotypes, while 1 : 2 : 1 describes genotypes.
Are Mendelian ratios guaranteed in a small sample?
No. They are expected probabilities. Random sampling can produce different observed numbers, especially when there are few offspring.
Does every human characteristic follow simple dominant–recessive inheritance?
No. Many characteristics involve multiple genes and environmental influences. Simple pea crosses illustrate particular inheritance patterns.
Does the mother determine the XX or XY combination?
In the standard model, the egg contributes X and the fertilising sperm contributes X or Y. The combination occurs by chance and is not a conscious choice by either parent.
How should I prepare Heredity Class 10 extra questions?
Practise definitions, comparisons, monohybrid crosses and chromosome combinations. Explain your symbols and show the gametes before writing a final ratio.
Where can I find Heredity Class 10 notes and a mind map?
Visit our Heredity Class 10 notes and mind map for revision, then return to the worksheet for practice.
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