Heredity | Dominant and Recessive Traits
Class 10 Science | Heredity | CBSE + Foundation + Competitive Level
🧬 Dominant and Recessive Traits
1. Introduction – Dominant and Recessive Traits
In organisms, a particular characteristic may be controlled by different forms of a gene.
These alternative forms of a gene are called alleles.
When two different alleles are present together, one may express itself while the effect of the other is masked.
The allele that expresses its effect in the heterozygous condition is called the: Dominant allele
The allele whose expression is masked in the presence of a dominant allele is called the: Recessive allele
When two different alleles are present together, one may express itself while the effect of the other is masked.
The allele that expresses its effect in the heterozygous condition is called the: Dominant allele
The allele whose expression is masked in the presence of a dominant allele is called the: Recessive allele
Example:
Let T represent the dominant allele for tallness and t represent the recessive allele for dwarfness.
TT → Tall
Tt → Tall
tt → Dwarf
Let T represent the dominant allele for tallness and t represent the recessive allele for dwarfness.
TT → Tall
Tt → Tall
tt → Dwarf
2. What is a Trait?
A trait is a specific characteristic of an organism.
Examples:
Examples:
- Height of a plant
- Seed colour
- Seed shape
- Flower colour
- Pod shape
3. What is an Allele?
An allele is an alternative form of a gene.
For a simple Mendelian example:
For a simple Mendelian example:
T = Tall allele
t = Dwarf allele
Thus T and t are two alleles associated with the same characteristic – plant height.
t = Dwarf allele
4. Dominant Trait
A dominant trait is the trait that is expressed when the organism has at least one dominant allele.
For example: Tt → Tall
Here:
For example: Tt → Tall
Here:
- T = dominant allele
- t = recessive allele
- Tt = heterozygous genotype
- Tall = phenotype
5. Recessive Trait
A recessive trait is generally expressed only when both alleles are recessive.
Example: tt → Dwarf
The recessive allele is not destroyed in a heterozygous organism. It is simply not expressed in the phenotype when the dominant allele is present.
Example: tt → Dwarf
The recessive allele is not destroyed in a heterozygous organism. It is simply not expressed in the phenotype when the dominant allele is present.
Very Important:
Dominant does NOT mean “better”, “stronger”, “more common” or “more powerful”.
Recessive does NOT mean “weak” or “inferior”.
Dominance describes the pattern of expression of alleles in a heterozygote.
Dominant does NOT mean “better”, “stronger”, “more common” or “more powerful”.
Recessive does NOT mean “weak” or “inferior”.
Dominance describes the pattern of expression of alleles in a heterozygote.
6. Genotype and Phenotype
| Term | Meaning | Examples |
|---|---|---|
| Genotype | Genetic constitution of an organism | TT, Tt, tt |
| Phenotype | Observable characteristic | Tall, Dwarf |
7. Homozygous Condition
When both alleles of a gene are identical, the organism is said to be homozygous for that gene.
Two possibilities:
Two possibilities:
TT
Homozygous dominant
tt
Homozygous recessive
8. Heterozygous Condition
When the two alleles of a gene are different, the organism is said to be heterozygous.
Example:
If T is dominant, the phenotype is Tall.
Example:
Tt
If T is dominant, the phenotype is Tall.
9. Comparison – Dominant vs Recessive
| Feature | Dominant | Recessive |
|---|---|---|
| Symbol | Usually represented by capital letter | Usually represented by small letter |
| Expression in heterozygote | Expressed | Masked |
| Expression in homozygote | Expressed | Expressed |
| Example | T | t |
| Example genotype | TT or Tt | tt |
10. Visual Representation
TT
→
Tall
Tt
→
Tall
tt
→
Dwarf
11. Why Does Dominant Trait Appear in Tt?
In the genotype Tt, two different alleles are present.
The allele T is dominant.
Therefore, the phenotype expresses the dominant characteristic.
Thus: Tt → Tall The recessive allele t remains present and can be passed to offspring.
Thus: Tt → Tall The recessive allele t remains present and can be passed to offspring.
12. Why Does Recessive Trait Reappear?
Suppose two heterozygous plants are crossed:
Tt × Tt
During gamete formation:
Tt → T + t
Both parents can therefore produce T and t gametes.
When two t gametes combine:
t + t → tt
The recessive phenotype appears.
13. Punnett Square – Tt × Tt
| ♀ / ♂ | T | t |
|---|---|---|
| T | TT Tall |
Tt Tall |
| t | Tt Tall |
tt Dwarf |
Genotypic Ratio
1 TT : 2 Tt : 1 tt
Phenotypic Ratio
3 Tall : 1 Dwarf
1 TT : 2 Tt : 1 tt
Phenotypic Ratio
3 Tall : 1 Dwarf
14. Dominant Trait Does Not Mean Common Trait
A common misconception is:
Dominant = common
This is incorrect.
Dominance describes how an allele behaves in a heterozygous condition. It does not automatically tell us how frequent the allele is in a population.
Dominance describes how an allele behaves in a heterozygous condition. It does not automatically tell us how frequent the allele is in a population.
15. Dominant Trait Does Not Mean Stronger Trait
Dominant and recessive are genetic terms.
They do not mean:
- strong vs weak
- good vs bad
- healthy vs unhealthy
- common vs rare
- important vs unimportant
16. Relationship Between Gene, Allele and Trait
DNA
→
Gene
→
Alleles
→
Trait
DNA contains hereditary information.
A gene is a segment of DNA associated with a particular characteristic.
Different forms of a gene are called alleles.
The expression of genetic information contributes to the organism's observable trait, along with environmental influences where applicable.
A gene is a segment of DNA associated with a particular characteristic.
Different forms of a gene are called alleles.
The expression of genetic information contributes to the organism's observable trait, along with environmental influences where applicable.
17. Mendel’s Example of Plant Height
Mendel studied plant height using contrasting forms:
- Tall plant
- Dwarf plant
TT × tt → All Tt
All F₁ plants were tall.
When F₁ plants were self-pollinated:
Tt × Tt
the dwarf phenotype appeared again in F₂.
18. Important Crosses
| Cross | Genotypes | Phenotype |
|---|---|---|
| TT × TT | All TT | All Tall |
| TT × tt | All Tt | All Tall |
| Tt × Tt | TT, Tt, Tt, tt | 3 Tall : 1 Dwarf |
| TT × Tt | TT, TT, Tt, Tt | All Tall |
| tt × Tt | tt, Tt | 1 Tall : 1 Dwarf |
| tt × tt | All tt | All Dwarf |
19. Test Cross – Basic Concept
A cross between an individual showing a dominant phenotype and a homozygous recessive individual can help determine whether the dominant individual is homozygous or heterozygous.
For example: TT × tt → All offspring Tt → Tall.
But: Tt × tt → Tt and tt.
The appearance of recessive offspring indicates that the dominant-looking parent carried the recessive allele.
For example: TT × tt → All offspring Tt → Tall.
But: Tt × tt → Tt and tt.
The appearance of recessive offspring indicates that the dominant-looking parent carried the recessive allele.
20. Key Differences – Genotype and Phenotype
| Genotype | Phenotype |
|---|---|
| Genetic constitution | Observable characteristic |
| Written using symbols | Usually described using words |
| TT, Tt, tt | Tall, Dwarf |
| May be same for individuals with same genetic constitution | Can also be influenced by environment |
21. 30 MCQs – Dominant and Recessive Traits
Q1. Which allele is expressed in a heterozygous condition in complete dominance?
A) Dominant allele
B) Recessive allele
C) Both equally
D) Neither
Answer: A
Q2. Which genotype represents homozygous dominant condition?
A) Tt
B) tt
C) TT
D) T
Answer: C
Q3. Which genotype represents heterozygous condition?
A) TT
B) tt
C) Tt
D) TTT
Answer: C
Q4. If T is dominant over t, the phenotype of Tt is:
A) Dwarf
B) Tall
C) Both
D) Cannot be determined
Answer: B
Q5. Which genotype expresses the recessive phenotype?
A) TT
B) Tt
C) tt
D) T
Answer: C
Q6. Alternative forms of a gene are called:
A) Proteins
B) Alleles
C) Enzymes
D) Tissues
Answer: B
Q7. Which represents phenotype?
A) TT
B) Tt
C) Tall
D) tt
Answer: C
Q8. Which represents genotype?
A) Tall
B) Dwarf
C) Tt
D) Height
Answer: C
Q9. In Tt × Tt, the probability of tt is:
A) 25%
B) 50%
C) 75%
D) 100%
Answer: A
Q10. In Tt × Tt, the probability of tall phenotype is:
A) 25%
B) 50%
C) 75%
D) 100%
Answer: C
Q11. The genotypic ratio of Tt × Tt is:
A) 3:1
B) 1:2:1
C) 1:1
D) 9:3:3:1
Answer: B
Q12. The phenotypic ratio of a simple monohybrid cross is:
A) 1:2:1
B) 1:1
C) 3:1
D) 2:1
Answer: C
Q13. Which is homozygous recessive?
A) TT
B) Tt
C) tt
D) T
Answer: C
Q14. Dominance refers to:
A) Strength of an organism
B) Expression of one allele over another in a heterozygote
C) Frequency of an allele
D) Size of a gene
Answer: B
Q15. Which statement is correct?
A) Recessive allele is destroyed
B) Recessive allele is always absent in heterozygotes
C) Recessive allele can remain present but masked
D) Dominant allele is always more common
Answer: C
Q16. A tall plant having genotype Tt is:
A) Homozygous
B) Heterozygous
C) Haploid
D) Recessive
Answer: B
Q17. A dwarf plant with genotype tt produces which type of allele in its gametes?
A) Only T
B) Only t
C) T and t
D) Neither
Answer: B
Q18. A Tt individual produces:
A) Only T gametes
B) Only t gametes
C) T and t gametes
D) TT gametes
Answer: C
Q19. Which genotype can produce only dominant T gametes?
A) TT
B) Tt
C) tt
D) Tt and tt
Answer: A
Q20. In TT × tt, all offspring are:
A) TT
B) tt
C) Tt
D) TTT
Answer: C
Q21. If T is dominant, which genotype can have the same phenotype as TT?
A) tt
B) Tt
C) Only tt
D) None
Answer: B
Q22. Which cross gives a 1:1 tall:dwarf ratio?
A) TT × TT
B) TT × tt
C) Tt × tt
D) Tt × Tt
Answer: C
Q23. If a dominant-looking plant produces dwarf offspring when crossed with tt, the plant is likely:
A) TT
B) Tt
C) tt
D) TTT
Answer: B
Q24. Which statement about a dominant trait is correct?
A) It is always more common
B) It is always stronger
C) It can be expressed in a heterozygote
D) It cannot occur in homozygous condition
Answer: C
Q25. Which is NOT a correct meaning of recessive?
A) Its expression may be masked in a heterozygote
B) It may be expressed in homozygous condition
C) It is genetically weaker
D) It can be inherited
Answer: C
Q26. Which genotype has two identical alleles?
A) Tt
B) TT
C) Both TT and tt
D) Neither
Answer: C
Q27. The observable expression of a genotype is called:
A) Allele
B) Phenotype
C) Genotype
D) Chromosome
Answer: B
Q28. Which combination is correct?
A) TT – heterozygous
B) Tt – homozygous
C) tt – homozygous recessive
D) Tt – homozygous recessive
Answer: C
Q29. In a heterozygous individual, the recessive allele:
A) Is destroyed
B) Is absent
C) Remains present but may be masked
D) Changes into a dominant allele
Answer: C
Q30. Which ratio represents the expected phenotype in Tt × Tt?
A) 1 Tall : 1 Dwarf
B) 2 Tall : 2 Dwarf
C) 3 Tall : 1 Dwarf
D) 1 Tall : 3 Dwarf
Answer: C
22. 30 Subjective Questions with Answers
2 Marks
Q1. What is a dominant trait?
Answer: A dominant trait is expressed when at least one dominant allele is present in a heterozygous condition.
2 Marks
Q2. What is a recessive trait?
Answer: A recessive trait is generally expressed when both alleles are recessive.
2 Marks
Q3. Define homozygous condition.
Answer: A condition in which both alleles of a gene are identical is called homozygous condition, e.g. TT or tt.
2 Marks
Q4. Define heterozygous condition.
Answer: A condition in which the two alleles of a gene are different is called heterozygous condition, e.g. Tt.
2 Marks
Q5. What is an allele?
Answer: An allele is an alternative form of a gene.
3 Marks
Q6. Differentiate between dominant and recessive traits.
Answer:
Dominant trait can be expressed in heterozygous condition, while recessive trait is generally expressed in homozygous recessive condition. For example, Tt is tall and tt is dwarf when T is dominant.
3 Marks
Q7. Differentiate between genotype and phenotype.
Answer:
Genotype is the genetic constitution, such as TT, Tt or tt. Phenotype is the observable characteristic, such as tall or dwarf.
3 Marks
Q8. Explain TT, Tt and tt.
Answer:
TT is homozygous dominant, Tt is heterozygous and tt is homozygous recessive. If T represents tallness, TT and Tt are tall while tt is dwarf.
3 Marks
Q9. Why does a recessive trait reappear in F₂ generation?
Answer:
The recessive allele remains present in F₁ heterozygotes. During gamete formation the alleles segregate, and two recessive alleles may combine to form tt in F₂.
3 Marks
Q10. Does dominant mean stronger or better? Explain.
Answer:
No. Dominance describes the expression of one allele in a heterozygous condition. It does not mean stronger, better, healthier or more common.
4 Marks
Q11. Explain the expression of dominant and recessive alleles using Tt.
Answer:
Tt contains one dominant allele T and one recessive allele t. The dominant allele T is expressed, so the plant is tall. The recessive allele remains present but its effect is masked.
4 Marks
Q12. Explain homozygous dominant and homozygous recessive conditions.
Answer:
TT is homozygous dominant because both alleles are dominant. tt is homozygous recessive because both alleles are recessive.
4 Marks
Q13. What is the difference between Tt and tt?
Answer:
Tt is heterozygous and has one dominant and one recessive allele. tt is homozygous recessive and has two recessive alleles. Therefore Tt is tall while tt is dwarf if T is dominant.
4 Marks
Q14. Explain why TT and Tt can have the same phenotype.
Answer:
TT and Tt both contain at least one dominant T allele. Therefore, when T is completely dominant over t, both genotypes produce the same tall phenotype.
4 Marks
Q15. Explain the relationship between gene, allele and trait.
Answer:
A gene is a segment of DNA associated with hereditary information. Alternative forms of a gene are alleles. Different alleles can contribute to different forms of a characteristic or trait.
5 Marks
Q16. Explain Tt × Tt cross with a Punnett square.
Answer:
Each Tt parent produces T and t gametes.
Genotypic ratio = 1:2:1.
Phenotypic ratio = 3:1.
| × | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
5 Marks
Q17. Why is the recessive allele not destroyed in a heterozygous organism?
Answer:
In Tt, both T and t alleles are present. T masks the expression of t, but t remains part of the genetic constitution and can pass to gametes.
5 Marks
Q18. Explain the significance of dominant and recessive alleles in Mendelian inheritance.
Answer:
Dominant and recessive relationships help explain why certain phenotypes appear in heterozygous organisms and why recessive phenotypes can reappear in later generations.
5 Marks
Q19. Explain why phenotype alone may not reveal genotype.
Answer:
A dominant phenotype may result from either TT or Tt. Therefore, observing a tall plant does not always tell us whether its genotype is TT or Tt.
5 Marks
Q20. Explain the statement: “Dominant does not mean common.”
Answer:
Dominance describes allele expression in a heterozygote. It does not indicate the frequency of the allele in a population. A dominant allele may be less common than a recessive allele.
6 Marks
Q21. Explain dominant and recessive traits with suitable examples.
Answer:
A dominant trait is expressed in a heterozygous condition. A recessive trait is generally expressed when both alleles are recessive.
For example, if T is dominant for tallness and t is recessive:
TT → Tall
Tt → Tall
tt → Dwarf
Thus T is dominant and t is recessive in this model.
6 Marks
Q22. Explain how a recessive trait can remain hidden for one generation and appear in the next.
Answer:
A recessive allele may be present in a heterozygous individual but its expression is masked by the dominant allele. When two heterozygous individuals reproduce, their recessive alleles can combine to form a homozygous recessive offspring. Thus the recessive phenotype can reappear.
6 Marks
Q23. Explain the genotypic and phenotypic ratios of Tt × Tt.
Answer:
The offspring are TT, Tt, Tt and tt.
Genotypic ratio:
1 TT : 2 Tt : 1 tt
Phenotypic ratio:
3 Tall : 1 Dwarf
This occurs because TT and Tt are tall while tt is dwarf.
6 Marks
Q24. Explain why two tall plants can produce a dwarf plant.
Answer:
Two tall plants may both be heterozygous Tt. When crossed:
Tt × Tt
one possible offspring is tt. Since tt is homozygous recessive, the dwarf phenotype appears.
6 Marks
Q25. Explain homozygous and heterozygous conditions with examples.
Answer:
Homozygous means identical alleles, such as TT or tt.
Heterozygous means different alleles, such as Tt.
TT is homozygous dominant, tt is homozygous recessive and Tt is heterozygous.
6 Marks
Q26. A tall plant is crossed with a dwarf plant and all offspring are tall. Explain.
Answer:
If the tall parent is TT and dwarf parent is tt:
TT × tt → all Tt.
Since T is dominant, all offspring are tall. The recessive allele t is present but masked.
6 Marks
Q27. A tall plant is crossed with tt and half the offspring are dwarf. What is the genotype of the tall parent?
Answer:
The tall parent must be Tt.
Tt × tt produces:
50% Tt → Tall
50% tt → Dwarf
Therefore the tall parent is heterozygous.
6 Marks
Q28. Explain why a dominant phenotype can have two different genotypes.
Answer:
When complete dominance occurs, both TT and Tt express the dominant phenotype. Therefore a tall phenotype can have genotype TT or Tt.
6 Marks
Q29. Explain the role of segregation in the inheritance of dominant and recessive traits.
Answer:
In a heterozygous organism such as Tt, the two alleles separate during gamete formation. Thus some gametes receive T and others receive t. During fertilisation, different combinations can form, including tt, allowing the recessive phenotype to reappear.
6 Marks
Q30. Explain how Mendel's observations helped distinguish dominant and recessive traits.
Answer:
Mendel crossed pure-breeding contrasting pea plants. In the F₁ generation only one phenotype appeared. When F₁ plants were self-pollinated, the masked phenotype reappeared in F₂. This showed that one hereditary factor could mask another in the heterozygous condition and established the basis for the concepts of dominance and recessiveness.
23. HOTS / Competency-Based Questions
HOTS 1.
A plant is tall. Can you always say that its genotype is TT?
Answer: No. Its genotype could be TT or Tt if T is dominant.
HOTS 2.
Two tall plants produce one dwarf offspring. What does this tell you?
Answer: Both parents must carry the recessive allele. In the simple model, both are Tt.
HOTS 3.
Why does the recessive phenotype not appear in the F₁ generation of TT × tt?
Answer: All offspring are Tt, and the dominant T masks the recessive t.
HOTS 4.
If 200 offspring result from Tt × Tt, approximately how many are expected to show the recessive phenotype?
Answer: Approximately 50, because the expected probability of tt is 1/4.
HOTS 5.
A student says, “A recessive allele disappears in F₁.” Is this correct?
Answer: No. The recessive allele remains present in heterozygous offspring but its expression is masked.
24. Assertion–Reason
Q1.
Assertion: Tt is a heterozygous genotype.
Reason: T and t are different alleles.
Reason: T and t are different alleles.
Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Q2.
Assertion: tt expresses the recessive phenotype.
Reason: Both alleles in tt are recessive.
Reason: Both alleles in tt are recessive.
Answer: Both statements are true, and the Reason correctly explains the Assertion.
Q3.
Assertion: A dominant trait is always more common in a population.
Reason: Dominance means an allele is expressed in a heterozygote.
Reason: Dominance means an allele is expressed in a heterozygote.
Answer: Assertion is false, but Reason is true.
Q4.
Assertion: TT and Tt can have the same phenotype.
Reason: Both contain at least one dominant allele.
Reason: Both contain at least one dominant allele.
Answer: Both statements are true, and the Reason correctly explains the Assertion.
Q5.
Assertion: The recessive allele is destroyed in Tt.
Reason: The recessive allele is not expressed in the heterozygous condition under complete dominance.
Reason: The recessive allele is not expressed in the heterozygous condition under complete dominance.
Answer: Assertion is false, but Reason is true.
25. Important Diagram – Dominance
26. CBSE Golden Points
- Dominant and recessive describe the expression of alleles.
- Dominant allele can express in a heterozygous condition.
- Recessive phenotype is generally expressed in homozygous recessive condition.
- TT = homozygous dominant.
- Tt = heterozygous.
- tt = homozygous recessive.
- TT and Tt can have the same phenotype under complete dominance.
- A recessive allele is not destroyed when masked.
- Dominant does not mean stronger.
- Dominant does not necessarily mean common.
- Genotype = genetic constitution.
- Phenotype = observable characteristic.
- Tt × Tt gives 1:2:1 genotypic ratio.
- Tt × Tt gives 3:1 phenotypic ratio.
- Alleles segregate during gamete formation.
27. Quick Revision Table
| Concept | Remember |
|---|---|
| Dominant | Expressed in heterozygote |
| Recessive | Usually expressed in homozygous condition |
| Homozygous | Same alleles |
| Heterozygous | Different alleles |
| Genotype | TT, Tt, tt |
| Phenotype | Tall, Dwarf |
| Monohybrid F₂ Genotype | 1:2:1 |
| Monohybrid F₂ Phenotype | 3:1 |
28. Memory Trick
“D-H-R” Trick
D → Dominant
Expressed in heterozygous condition.
H → Heterozygous
Different alleles → Tt.
R → Recessive
Usually expressed when two recessive alleles combine → tt.
D → Dominant
Expressed in heterozygous condition.
H → Heterozygous
Different alleles → Tt.
R → Recessive
Usually expressed when two recessive alleles combine → tt.
29. One-Line Revision
Dominant = Expressed in Heterozygote
Recessive = Masked in Heterozygote
TT / Tt → Dominant Phenotype
tt → Recessive Phenotype
Recessive = Masked in Heterozygote
TT / Tt → Dominant Phenotype
tt → Recessive Phenotype
🧬 Final Summary – Dominant and Recessive Traits
Dominant and recessive traits explain how different alleles of a gene are expressed.
TT → Tall
Tt → Tall
tt → Dwarf
Tt × Tt → 1 TT : 2 Tt : 1 tt
Phenotypic Ratio → 3 Tall : 1 Dwarf
Dominant and recessive traits explain how different alleles of a gene are expressed.
TT → Tall
Tt → Tall
tt → Dwarf
Tt × Tt → 1 TT : 2 Tt : 1 tt
Phenotypic Ratio → 3 Tall : 1 Dwarf
