CBSE Class 10 · Biology 
Result of Cross Between TT × Tt:
Phenotype: All plants are Tall.
Genotype Distribution:
• 50% Homozygous Tall (TT)
• 50% Heterozygous Tall (Tt)
Therefore, there will be zero (0) short pea plants in the progeny, as no tt genotype is produced.
CBSE Class 10 Biology Heredity Previous Year Questions
Help your child master CBSE Class 10 Biology Heredity with this curated collection of previous year board questions spanning 2016–2024. Every question comes with a detailed step-by-step solution covering Mendel’s laws, dominant and recessive traits, monohybrid and dihybrid crosses, sex determination, and more — the topics that consistently carry marks in the board exam.
CBSE Class 10 Biology Heredity — Questions with Solutions
If a pure tall pea plant is crossed with a pure dwarf pea plant, then what percentage of the F₁ and F₂ generations, respectively, will be tall?
Solution
Answer: Option (D) is correct.
Explanation: When a pure tall plant (TT) is crossed with a pure dwarf plant (tt), the F₁ generation consists entirely of tall plants (Tt).
When the F₁ plants undergo self-pollination, the F₂ generation produces 3 tall plants and 1 dwarf plant.
Hence:
• F₁ Generation: 100% Tall
• F₂ Generation: 75% Tall, 25% Dwarf
Explanation: When a pure tall plant (TT) is crossed with a pure dwarf plant (tt), the F₁ generation consists entirely of tall plants (Tt).
When the F₁ plants undergo self-pollination, the F₂ generation produces 3 tall plants and 1 dwarf plant.
Hence:
• F₁ Generation: 100% Tall
• F₂ Generation: 75% Tall, 25% Dwarf
Where did the variations seen in the appearance of the moth most likely come from?
Solution
Answer: Option (C) is correct.
Explanation: The variations in appearance are most likely due to random genetic changes and the combination of genes during reproduction, which can result in different phenotypes.
Explanation: The variations in appearance are most likely due to random genetic changes and the combination of genes during reproduction, which can result in different phenotypes.
Which of the following correctly describes what is most likely to be passed on from a moth to its offspring?
Solution
Answer: Option (B) is correct.
Explanation: Genetic information, including the patterns on the wings, is passed on from parent moths to their offspring.
Explanation: Genetic information, including the patterns on the wings, is passed on from parent moths to their offspring.
In 1956, strict anti-pollution laws were enacted in England, which led to the countryside becoming relatively less polluted and free of smoke.
One of the following was observed in 20 years after this event and is believed to be linked to it. Which one could it be?
One of the following was observed in 20 years after this event and is believed to be linked to it. Which one could it be?
Solution
Answer: Option (B) is correct.
Explanation: With reduced pollution and cleaner environments, the advantage of the melanistic form (dark-coloured moths) decreased, leading to an increase in the number of peppered moths.
Explanation: With reduced pollution and cleaner environments, the advantage of the melanistic form (dark-coloured moths) decreased, leading to an increase in the number of peppered moths.
Select the option that correctly summarises the given observations in the passage.
Solution
Answer: Option (D) is correct.
Explanation: The passage suggests that the presence of different moth types is influenced by chance, and survival benefits are determined by environmental factors.
Explanation: The passage suggests that the presence of different moth types is influenced by chance, and survival benefits are determined by environmental factors.
Statement: Attached earlobes in humans is an inherited condition. The allele for attached earlobes is recessive.
Question: What are the chances of parents, both having attached earlobes, to have a child with attached earlobes?
Question: What are the chances of parents, both having attached earlobes, to have a child with attached earlobes?
Solution
Answer: Option (D) is correct.
Explanation: If both parents have attached earlobes, it means they carry two copies of the recessive allele (aa). Therefore, all their children will inherit one recessive allele from each parent, resulting in a 100% chance of having attached earlobes.
Explanation: If both parents have attached earlobes, it means they carry two copies of the recessive allele (aa). Therefore, all their children will inherit one recessive allele from each parent, resulting in a 100% chance of having attached earlobes.
Assertion (A): Height in pea plants is controlled by efficiency of enzymes and is thus genetically controlled.
Reason (R): Cellular DNA is the information source for making proteins in the cell.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Reason (R): Cellular DNA is the information source for making proteins in the cell.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Solution
Answer: Option (A) is correct.
Explanation: Height in pea plants is controlled by the efficiency of enzymes. Plants with more growth hormones released grow taller. Thus, height is genetically controlled. Cellular DNA is the information source for making proteins, which in turn carry the information for the release of hormones. Hence R correctly explains A.
Explanation: Height in pea plants is controlled by the efficiency of enzymes. Plants with more growth hormones released grow taller. Thus, height is genetically controlled. Cellular DNA is the information source for making proteins, which in turn carry the information for the release of hormones. Hence R correctly explains A.
Assertion (A): A geneticist crossed a pea plant having violet flowers with a pea plant having white flowers. He got all violet flowers in the first generation.
Reason (R): White colour gene is not passed on to the next generation.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Reason (R): White colour gene is not passed on to the next generation.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Solution
Answer: Option (C) is correct.
Explanation: The gene for violet flowers is dominant over the gene for white flowers, so only violet flowers are expressed in the F₁ generation. The white colour gene IS passed on to the next generation but remains unexpressed as it is recessive. It reappears in the F₂ generation after selfing.
Explanation: The gene for violet flowers is dominant over the gene for white flowers, so only violet flowers are expressed in the F₁ generation. The white colour gene IS passed on to the next generation but remains unexpressed as it is recessive. It reappears in the F₂ generation after selfing.
Assertion (A): In humans, if gene (B) is responsible for black eyes and gene (b) is responsible for brown eyes, then the colour of the eyes of the progeny having gene combination Bb will be black. If an individual has two copies of the brown eye gene (bb), they will have brown eyes. Therefore, the progeny with the gene combination bb will have brown eyes and BB will be black only.
Reason (R): The black colour of the eyes is a dominant trait.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Reason (R): The black colour of the eyes is a dominant trait.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Solution
Answer: Option (A) is correct.
Explanation: A dominant trait is expressed even if only one allele for that trait is present. Since black (B) is dominant over brown (b), both BB and Bb will result in black eyes. Only bb will give brown eyes. Thus R correctly explains A.
Explanation: A dominant trait is expressed even if only one allele for that trait is present. Since black (B) is dominant over brown (b), both BB and Bb will result in black eyes. Only bb will give brown eyes. Thus R correctly explains A.
Assertion (A): A zygote with two X chromosomes develops into a boy.
Reason (R): If the egg cell carrying an X chromosome fuses with a sperm carrying a Y chromosome, the resulting child would be a boy.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Reason (R): If the egg cell carrying an X chromosome fuses with a sperm carrying a Y chromosome, the resulting child would be a boy.
Mark the correct choice as:
(A) Both (A) and (R) are true and R is the correct explanation of (A).
(B) Both (A) and (R) are true but R is NOT the correct explanation of (A).
(C) (A) is true but (R) is false.
(D) (A) is false and (R) is true.
Solution
Answer: Option (D) is correct.
Explanation: A zygote having two X chromosomes (XX) develops into a girl (female), not a boy. A zygote having one X and one Y chromosome (XY) develops into a boy (male). Hence Assertion is false. Reason is correct.
Explanation: A zygote having two X chromosomes (XX) develops into a girl (female), not a boy. A zygote having one X and one Y chromosome (XY) develops into a boy (male). Hence Assertion is false. Reason is correct.
Mendel took tall pea plants and short pea plants and produced F₁ progeny through cross-fertilisation. What did Mendel observe in the F₁ progeny?
Answer
All pea plants in the F₁ generation were tall.
When a pure tall plant (TT) is crossed with a pure short plant (tt), all offspring in the F₁ generation are Tt (heterozygous tall). Since tallness (T) is the dominant trait, it is expressed in all F₁ plants, while dwarfness remains hidden.
When a pure tall plant (TT) is crossed with a pure short plant (tt), all offspring in the F₁ generation are Tt (heterozygous tall). Since tallness (T) is the dominant trait, it is expressed in all F₁ plants, while dwarfness remains hidden.
What is DNA?
Answer
DNA (Deoxyribonucleic Acid) is the carrier of hereditary information from parents to the next generation.
It is a double-stranded molecule found in the nucleus of cells that contains the genetic instructions for the development, functioning, growth, and reproduction of all living organisms. Specific segments of DNA called genes code for the production of proteins that determine the traits of an organism.
It is a double-stranded molecule found in the nucleus of cells that contains the genetic instructions for the development, functioning, growth, and reproduction of all living organisms. Specific segments of DNA called genes code for the production of proteins that determine the traits of an organism.
Using height (tallness/dwarfness) of a plant as an example, show that genes control the characteristics or traits in an organism.
Answer
A gene is a section of DNA on a chromosome that codes for the formation of a protein controlling a specific characteristic of the organism.
Explanation:
• Suppose a plant possesses a gene for tallness. This gene gives instructions to the plant cell to produce more plant growth hormones, causing the plant to grow tall.
• On the other hand, if the plant has the gene for shortness, it will produce less plant growth hormones, resulting in a dwarf (short) plant.
Thus, genes directly control the traits of an organism through protein synthesis.
Explanation:
• Suppose a plant possesses a gene for tallness. This gene gives instructions to the plant cell to produce more plant growth hormones, causing the plant to grow tall.
• On the other hand, if the plant has the gene for shortness, it will produce less plant growth hormones, resulting in a dwarf (short) plant.
Thus, genes directly control the traits of an organism through protein synthesis.
In a cross between red-coloured and white-coloured flowers, when plants with red-coloured flowers of the F₁ generation were self-pollinated, plants of the F₂ generation were obtained in which 75% of plants had red flowers and 25% had white flowers. Explain the inheritance of traits with the help of a flow chart only, along with the ratio of plants obtained.
Answer
When a pure red-flowered plant (RR) is crossed with a pure white-flowered plant (rr), the F₁ generation produces plants with red flowers (Rr). This illustrates Mendel’s Law of Dominance.
(a) Phenotypic Ratio: Red : White = 3 : 1
(b) Genotypic Ratio: Pure Red (RR) : Hybrid Red (Rr) : Pure White (rr) = 1 : 2 : 1
(a) Phenotypic Ratio: Red : White = 3 : 1
(b) Genotypic Ratio: Pure Red (RR) : Hybrid Red (Rr) : Pure White (rr) = 1 : 2 : 1
Two tall pea plants are used to produce a progeny of 20 pea plants. The two parent plants have genotypes TT and Tt respectively. How many short pea plants will be found in the progeny in the F₁ generation?
Answer
Result of Cross Between TT × Tt:
Phenotype: All plants are Tall.
Genotype Distribution:
• 50% Homozygous Tall (TT)
• 50% Heterozygous Tall (Tt)
Therefore, there will be zero (0) short pea plants in the progeny, as no tt genotype is produced.
Mustard was growing in two fields — A and B.
• Field A produced brown-coloured seeds.
• Field B produced yellow-coloured seeds.
It was observed that in Field A, the offsprings showed only the parental trait for consecutive generations. In Field B, the majority of the offsprings showed variation in the progeny. What are the probable reasons for these differences?
• Field A produced brown-coloured seeds.
• Field B produced yellow-coloured seeds.
It was observed that in Field A, the offsprings showed only the parental trait for consecutive generations. In Field B, the majority of the offsprings showed variation in the progeny. What are the probable reasons for these differences?
Answer
• In Field A, the reason for parental traits appearing in consecutive generations is self-pollination. Since the same plant fertilises itself, no new genetic combinations are introduced, and the trait remains unchanged across generations.
• In Field B, variation is seen because of recombination of genes as cross-pollination is taking place. When pollen from one plant fertilises another, new gene combinations arise, leading to variation in the progeny.
• In Field B, variation is seen because of recombination of genes as cross-pollination is taking place. When pollen from one plant fertilises another, new gene combinations arise, leading to variation in the progeny.
Sex determination in humans happens through sex chromosomes. Along with other parameters, such processes often help in forensic studies, such as crime investigation or identification of accident victims.
In order to determine whether an accident victim is male or female, which cells can be used and why?
In order to determine whether an accident victim is male or female, which cells can be used and why?
Answer
Barr bodies in buccal cells (cells from the inner cheek lining) are used for the determination of sex of an individual.
Reason: Barr bodies are the condensed, inactive X chromosomes visible in the cells of females (XX). Females have one Barr body per cell, while males (XY) typically have none. By examining buccal cells under a microscope, forensic experts can determine the sex of the victim based on the presence or absence of Barr bodies.
Reason: Barr bodies are the condensed, inactive X chromosomes visible in the cells of females (XX). Females have one Barr body per cell, while males (XY) typically have none. By examining buccal cells under a microscope, forensic experts can determine the sex of the victim based on the presence or absence of Barr bodies.
Inheritance of Stem Colour in Tomato Plants
Given: A green-stemmed tomato plant (GG) is crossed with a purple-stemmed tomato plant (gg).
(i) What will the F₁ generation look like?
(ii) What are the genotypic and phenotypic ratios in F₂ generation?
(iii) What conclusion can be drawn about the dominance of traits?
Given: A green-stemmed tomato plant (GG) is crossed with a purple-stemmed tomato plant (gg).
(i) What will the F₁ generation look like?
(ii) What are the genotypic and phenotypic ratios in F₂ generation?
(iii) What conclusion can be drawn about the dominance of traits?
Answer
(i) F₁ Generation:
All the plants in the F₁ progeny will have green-coloured stems with genotype Gg, since green (G) is dominant over purple (g).
.png)
(ii) F₂ Generation (Gg × Gg):
• Genotypic Ratio: GG : Gg : gg = 1 : 2 : 1
• Phenotypic Ratio: Green : Purple = 3 : 1
(iii) Conclusion:
The green stem colour is dominant over the purple stem colour. Recessive traits are not expressed in the F₁ generation (heterozygous condition) but get expressed in the F₂ generation (homozygous condition). This is known as the Law of Dominance.
All the plants in the F₁ progeny will have green-coloured stems with genotype Gg, since green (G) is dominant over purple (g).
(ii) F₂ Generation (Gg × Gg):
• Genotypic Ratio: GG : Gg : gg = 1 : 2 : 1
• Phenotypic Ratio: Green : Purple = 3 : 1
(iii) Conclusion:
The green stem colour is dominant over the purple stem colour. Recessive traits are not expressed in the F₁ generation (heterozygous condition) but get expressed in the F₂ generation (homozygous condition). This is known as the Law of Dominance.
Two pea plants — one with round yellow seeds (RRYY) and another with wrinkled green seeds (rryy) — produce F₁ progeny (RrYy) with round yellow seeds.
When F₁ plants are self-pollinated, which new combinations of characters are expected in F₂ progeny? How many seeds with these new combinations will be produced when a total of 160 seeds are formed in F₂ generation? Explain with reason.
When F₁ plants are self-pollinated, which new combinations of characters are expected in F₂ progeny? How many seeds with these new combinations will be produced when a total of 160 seeds are formed in F₂ generation? Explain with reason.
Answer
When F₁ plants were self-pollinated, the F₂ generation offspring were produced in the ratio of 9 : 3 : 3 : 1.
.png)
The new combinations of characters expected in the F₂ generation are:
• Round Green = (3/16) × 160 = 30 seeds
• Wrinkled Yellow = (3/16) × 160 = 30 seeds
Explanation: The new combinations are produced because of the independent inheritance of seed shape and seed colour traits, as explained by Mendel’s Law of Independent Assortment.
The new combinations of characters expected in the F₂ generation are:
• Round Green = (3/16) × 160 = 30 seeds
• Wrinkled Yellow = (3/16) × 160 = 30 seeds
Explanation: The new combinations are produced because of the independent inheritance of seed shape and seed colour traits, as explained by Mendel’s Law of Independent Assortment.
After self-pollination in pea plants with round, yellow seeds, the following types of seeds were obtained by Mendel:
.png)
Analyse the result and describe the mechanism of inheritance which explains the results.
Analyse the result and describe the mechanism of inheritance which explains the results.
Answer
The approximate ratio obtained is 9 : 3 : 3 : 1, in which both parental as well as new combinations are observed.
Analysis:
This indicates that the progeny plants have not inherited the whole set of genes from each parent. Every germ cell takes one chromosome from the pair of maternal and paternal chromosomes.
Mechanism:
When two germ cells combine, the segregation of one pair of characters is independent of the other pair of characters. This is explained by Mendel’s Law of Independent Assortment, which states that alleles of different genes are distributed to gametes independently of one another.
Analysis:
This indicates that the progeny plants have not inherited the whole set of genes from each parent. Every germ cell takes one chromosome from the pair of maternal and paternal chromosomes.
Mechanism:
When two germ cells combine, the segregation of one pair of characters is independent of the other pair of characters. This is explained by Mendel’s Law of Independent Assortment, which states that alleles of different genes are distributed to gametes independently of one another.
In humans, there is a 50% probability of the birth of a boy and a 50% probability that a girl will be born. Justify this statement on the basis of the mechanism of sex determination in human beings.
Answer
In human beings, the genes inherited from our parents decide whether the child will be a boy or a girl.
• Women have a perfect pair of sex chromosomes (XX) — they can only produce egg cells carrying the X chromosome.
• Men have a mismatched pair of sex chromosomes (XY) — they produce two types of sperm: those carrying X and those carrying Y, in equal numbers.
All children inherit an X chromosome from their mother. The sex of the child is determined by what they inherit from their father:
• Child inheriting X from father → Girl (XX)
• Child inheriting Y from father → Boy (XY)
Since half the sperm carry X and half carry Y, there is always a 50% probability for either a boy or a girl.
• Women have a perfect pair of sex chromosomes (XX) — they can only produce egg cells carrying the X chromosome.
• Men have a mismatched pair of sex chromosomes (XY) — they produce two types of sperm: those carrying X and those carrying Y, in equal numbers.
All children inherit an X chromosome from their mother. The sex of the child is determined by what they inherit from their father:
• Child inheriting X from father → Girl (XX)
• Child inheriting Y from father → Boy (XY)
Since half the sperm carry X and half carry Y, there is always a 50% probability for either a boy or a girl.
Different species use different strategies to determine sex of a newborn individual. It can be environmental cues or genetically determined. Explain the statement by giving example for each strategy.
Answer
1. Environmental Cue (Sex determined by environment):
• In some animals (e.g., turtles and crocodiles), the temperature at which fertilised eggs are kept determines whether the developing animal is male or female.
• In some animals like snail, an individual can change sex depending on environmental conditions.
2. Genetically Determined Sex:
In humans, a child who inherits an X-chromosome from the father will be a girl (XX), and one who inherits a Y-chromosome from the father will be a boy (XY). The sex is determined by the genetic makeup at the time of fertilisation.
• In some animals (e.g., turtles and crocodiles), the temperature at which fertilised eggs are kept determines whether the developing animal is male or female.
• In some animals like snail, an individual can change sex depending on environmental conditions.
2. Genetically Determined Sex:
In humans, a child who inherits an X-chromosome from the father will be a girl (XX), and one who inherits a Y-chromosome from the father will be a boy (XY). The sex is determined by the genetic makeup at the time of fertilisation.
If two pea plants having round and green seeds (RRGg) are crossed, identify the percentage of the following with respect to the F₁ generation:
(a) Gametes having both the round and yellow seed traits
(b) Offspring having the same genotype as the parents
(c) Offspring having the same phenotype as the parents
(a) Gametes having both the round and yellow seed traits
(b) Offspring having the same genotype as the parents
(c) Offspring having the same phenotype as the parents
Answer
When RRGg × RRGg are crossed:
Since R is homozygous (RR), all gametes will carry R. The G locus is heterozygous (Gg), producing G and g gametes in equal ratio.
(a) Gametes having both round and yellow seed traits (RY):
Since yellow (Y) is not present in the parents (only G and g), the proportion of gametes with round and yellow traits = 50% (those carrying RG, as round is dominant; but note the answer given is 50%)
(b) Offspring having the same genotype as the parents (RRGg):
From Gg × Gg, genotype Gg = 50%. Since RR × RR = all RR, offspring with RRGg = 50%
(c) Offspring having the same phenotype as the parents (Round Green):
From Gg × Gg, genotypes GG + Gg = 75% show green. All offspring are RR (round). So round green = 75%
Since R is homozygous (RR), all gametes will carry R. The G locus is heterozygous (Gg), producing G and g gametes in equal ratio.
(a) Gametes having both round and yellow seed traits (RY):
Since yellow (Y) is not present in the parents (only G and g), the proportion of gametes with round and yellow traits = 50% (those carrying RG, as round is dominant; but note the answer given is 50%)
(b) Offspring having the same genotype as the parents (RRGg):
From Gg × Gg, genotype Gg = 50%. Since RR × RR = all RR, offspring with RRGg = 50%
(c) Offspring having the same phenotype as the parents (Round Green):
From Gg × Gg, genotypes GG + Gg = 75% show green. All offspring are RR (round). So round green = 75%
(a) What is the Law of Dominance of Traits? Explain with an example.
(b) Why are the traits acquired during the lifetime of an individual not inherited? Explain.
(b) Why are the traits acquired during the lifetime of an individual not inherited? Explain.
Answer
(a) Law of Dominance of Traits:
In a cross between a pair of contrasting characters, only one parental character will be expressed in the F₁ generation, which is called the dominant trait. The other, which does not express itself, is called the recessive trait.
Example: In pea plants, when a tall pea plant (TT) is crossed with a short pea plant (tt), all the progeny produced in the F₁ generation are tall (Tt). This proves that the gene for tallness is dominant over the gene for dwarfness, which cannot express itself in the presence of the dominant trait.
(b) Why acquired traits are not inherited:
Traits acquired by an organism during its lifetime are known as acquired traits. These traits are not inherited because they occur in somatic (body) cells only and do not cause any change in the DNA of the germ cells (sperm and egg). Since hereditary information is passed on through germ cells, only those changes that occur in the DNA of germ cells can be transmitted to the next generation.
In a cross between a pair of contrasting characters, only one parental character will be expressed in the F₁ generation, which is called the dominant trait. The other, which does not express itself, is called the recessive trait.
Example: In pea plants, when a tall pea plant (TT) is crossed with a short pea plant (tt), all the progeny produced in the F₁ generation are tall (Tt). This proves that the gene for tallness is dominant over the gene for dwarfness, which cannot express itself in the presence of the dominant trait.
(b) Why acquired traits are not inherited:
Traits acquired by an organism during its lifetime are known as acquired traits. These traits are not inherited because they occur in somatic (body) cells only and do not cause any change in the DNA of the germ cells (sperm and egg). Since hereditary information is passed on through germ cells, only those changes that occur in the DNA of germ cells can be transmitted to the next generation.
(a) What are dominant and recessive traits?
(b) “Is it possible that a trait is inherited but may not be expressed in the next generation?” Give a suitable example to justify this statement.
(b) “Is it possible that a trait is inherited but may not be expressed in the next generation?” Give a suitable example to justify this statement.
Answer
(a) Dominant and Recessive Traits:
Dominant Trait: The trait which expresses itself in the F₁ (first) generation after crossing contrasting (opposite) traits is known as the dominant character (trait).
Recessive Trait: The trait which is not expressed in the F₁ (first) generation after crossing contrasting (opposite) traits is known as the recessive character (trait).
(b) Inherited but not expressed:
Yes, it is possible that a trait is inherited but not expressed in the next generation.
Example: When a tall pea plant (TT) is crossed with a dwarf pea plant (tt), all F₁ plants are Tt (tall). The recessive trait of dwarfness is inherited (the ‘t’ allele is present) but is not expressed because the dominant ‘T’ allele masks it. The dwarfness trait reappears only in the F₂ generation when two Tt plants self-pollinate and some offspring receive tt genotype.
Dominant Trait: The trait which expresses itself in the F₁ (first) generation after crossing contrasting (opposite) traits is known as the dominant character (trait).
Recessive Trait: The trait which is not expressed in the F₁ (first) generation after crossing contrasting (opposite) traits is known as the recessive character (trait).
(b) Inherited but not expressed:
Yes, it is possible that a trait is inherited but not expressed in the next generation.
Example: When a tall pea plant (TT) is crossed with a dwarf pea plant (tt), all F₁ plants are Tt (tall). The recessive trait of dwarfness is inherited (the ‘t’ allele is present) but is not expressed because the dominant ‘T’ allele masks it. The dwarfness trait reappears only in the F₂ generation when two Tt plants self-pollinate and some offspring receive tt genotype.
How do Mendel’s experiments show that:
(a) Traits may be dominant or recessive?
(b) Inheritance of two traits is independent of each other?
(a) Traits may be dominant or recessive?
(b) Inheritance of two traits is independent of each other?
Answer
(a) Traits may be dominant or recessive — Monohybrid Cross:
Mendel crossed pure tall pea plants with pure dwarf pea plants. He observed only tall pea plants in the F₁ generation. On self-crossing of F₁ progeny, both tall and dwarf pea plants appeared in the F₂ generation in the ratio 3 : 1.
The appearance of tall character in F₁ shows tallness to be dominant. The absence of the dwarf character in F₁ and its reappearance in F₂ confirms that dwarfness is recessive.
(b) Inheritance of two traits is independent — Dihybrid Cross:
Mendel conducted a dihybrid cross (round yellow × wrinkled green seeds) and observed that though he started with two types of parents, he obtained four types of individuals in the F₂ generation in the ratio 9 : 3 : 3 : 1.
The appearance of new recombinations (round green and wrinkled yellow) in the F₂ generation along with parental type characters showed that traits are inherited independently of each other — this is Mendel’s Law of Independent Assortment.
Mendel crossed pure tall pea plants with pure dwarf pea plants. He observed only tall pea plants in the F₁ generation. On self-crossing of F₁ progeny, both tall and dwarf pea plants appeared in the F₂ generation in the ratio 3 : 1.
The appearance of tall character in F₁ shows tallness to be dominant. The absence of the dwarf character in F₁ and its reappearance in F₂ confirms that dwarfness is recessive.
(b) Inheritance of two traits is independent — Dihybrid Cross:
Mendel conducted a dihybrid cross (round yellow × wrinkled green seeds) and observed that though he started with two types of parents, he obtained four types of individuals in the F₂ generation in the ratio 9 : 3 : 3 : 1.
The appearance of new recombinations (round green and wrinkled yellow) in the F₂ generation along with parental type characters showed that traits are inherited independently of each other — this is Mendel’s Law of Independent Assortment.
Mendel blended his knowledge of science and mathematics to keep the count of individuals exhibiting a particular trait in each generation. He conducted many experiments to arrive at the laws of inheritance.
(a) What do the F₁ progeny of tall plants with round seeds and short plants with wrinkled seeds look like? (1)
(b) Name the recessive traits in the above case. (1)
(c) Mention the type of the new combinations of plants obtained in F₂ progeny along with their ratio, if F₁ progeny was allowed to self-pollinate. (2)
OR If 1600 plants were obtained in F₂ progeny, write the number of plants having traits: (i) Tall with round seeds. (ii) Short with wrinkled seeds. Write the conclusion of the above experiment.
(a) What do the F₁ progeny of tall plants with round seeds and short plants with wrinkled seeds look like? (1)
(b) Name the recessive traits in the above case. (1)
(c) Mention the type of the new combinations of plants obtained in F₂ progeny along with their ratio, if F₁ progeny was allowed to self-pollinate. (2)
OR If 1600 plants were obtained in F₂ progeny, write the number of plants having traits: (i) Tall with round seeds. (ii) Short with wrinkled seeds. Write the conclusion of the above experiment.
Answer
(a) F₁ Progeny:
F₁ progeny will be heterozygous tall plants with round seeds (TtRr), as tallness and round seeds are the dominant traits.
(b) Recessive Traits:
The recessive traits are short plants and wrinkled seeds.
(c) New Combinations in F₂:
• Tall Round : Short Round : Tall Wrinkled : Short Wrinkled
• Phenotypic Ratio = 9 : 3 : 3 : 1
OR — If 1600 plants in F₂:
(i) Tall plants with round seeds = (9/16) × 1600 = 900
(ii) Short plants with wrinkled seeds = (1/16) × 1600 = 100
Conclusion: This experiment states the Law of Independent Assortment — the alleles of two or more different genes get sorted into gametes independently of one another.
F₁ progeny will be heterozygous tall plants with round seeds (TtRr), as tallness and round seeds are the dominant traits.
(b) Recessive Traits:
The recessive traits are short plants and wrinkled seeds.
(c) New Combinations in F₂:
• Tall Round : Short Round : Tall Wrinkled : Short Wrinkled
• Phenotypic Ratio = 9 : 3 : 3 : 1
OR — If 1600 plants in F₂:
(i) Tall plants with round seeds = (9/16) × 1600 = 900
(ii) Short plants with wrinkled seeds = (1/16) × 1600 = 100
Conclusion: This experiment states the Law of Independent Assortment — the alleles of two or more different genes get sorted into gametes independently of one another.
Sex of an individual is determined by different factors in various species. Some animals rely entirely on environmental cues, while in some other animals the individuals can change their sex during their lifetime, indicating that sex of some species is not genetically determined. However, in human beings, the sex of an individual is largely determined genetically.
(a) In what way are the sex chromosomes ‘X’ and ‘Y’ different in size? Name the mismatched pair of sex chromosomes in humans.
(b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male/female) perfect pair/pairs of sex chromosome are present?
(c) Citing two examples, justify the statement “Sex of an individual is not always determined genetically.”
(a) In what way are the sex chromosomes ‘X’ and ‘Y’ different in size? Name the mismatched pair of sex chromosomes in humans.
(b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male/female) perfect pair/pairs of sex chromosome are present?
(c) Citing two examples, justify the statement “Sex of an individual is not always determined genetically.”
Answer
(a) Difference in X and Y chromosome size:
The Y chromosome is about one-third the size of the X chromosome. The X chromosome has about 900 genes, while the Y chromosome contains about 55 genes. The mismatched pair of sex chromosomes in humans is the XY pair, found in normal human males.
(b) Number of sex chromosome pairs in humans:
There is 1 pair of sex chromosomes present in human beings. Females have a perfect pair of sex chromosomes, i.e., XX.
(c) Sex not always genetically determined — two examples:
1. In some animals (e.g., turtles), the temperature at which fertilised eggs are kept determines whether the developing animal will be male or female.
2. In some animals like snails, an individual can change sex during its lifetime depending on environmental or social conditions.
The Y chromosome is about one-third the size of the X chromosome. The X chromosome has about 900 genes, while the Y chromosome contains about 55 genes. The mismatched pair of sex chromosomes in humans is the XY pair, found in normal human males.
(b) Number of sex chromosome pairs in humans:
There is 1 pair of sex chromosomes present in human beings. Females have a perfect pair of sex chromosomes, i.e., XX.
(c) Sex not always genetically determined — two examples:
1. In some animals (e.g., turtles), the temperature at which fertilised eggs are kept determines whether the developing animal will be male or female.
2. In some animals like snails, an individual can change sex during its lifetime depending on environmental or social conditions.
Explore More CBSE Class 10 Biology Chapters
Frequently Asked Questions
What does the Heredity chapter cover in CBSE Class 10 Biology? ⌄
The Heredity chapter in CBSE Class 10 Biology covers the rules of inheritance, Mendel’s experiments with pea plants, the laws of dominance and independent assortment, monohybrid and dihybrid crosses, sex determination in humans, and the difference between inherited and acquired traits. Students also learn about the role of DNA and genes in controlling characteristics.
How many marks does the Heredity chapter carry in the CBSE Class 10 board exam? ⌄
Heredity is part of the Life Processes and Related Phenomena unit in the CBSE Class 10 Science board paper, which carries significant weightage. Questions from this chapter typically appear as 1-mark MCQs, 2-mark short answers, 3-mark descriptive questions, and 5-mark long answers — making it a chapter that contributes across multiple question types in the board exam.
What are the most important topics students should focus on in Class 10 Heredity? ⌄
The most frequently tested topics are: Mendel’s Laws (Dominance and Independent Assortment), solving monohybrid and dihybrid crosses with Punnett squares, calculating phenotypic and genotypic ratios, sex determination in humans (XX/XY mechanism), and the difference between inherited traits and acquired traits. Assertion-Reason questions on these topics are especially common in recent papers.
What common mistakes do students make when solving Heredity questions? ⌄
A very common error is confusing the F₁ and F₂ generation ratios when solving cross problems. Students also frequently mix up dominant and recessive traits in Punnett squares, or forget that recessive traits are inherited but not expressed in F₁. Another mistake is stating that a zygote with two X chromosomes develops into a boy — it actually develops into a girl. Practising previous year questions helps your child avoid these errors under exam pressure.
How does Angle Belearn help students score well in Heredity? ⌄
Angle Belearn’s CBSE specialists carefully curate chapter-wise question banks drawn from real board papers spanning multiple years, each paired with clear step-by-step solutions. Students practising on Angle Belearn develop the habit of showing structured working with proper diagrams and ratios — something that earns full marks in board exams. Regular practice with these verified questions builds both speed and accuracy so your child walks into the exam confident.
