Class 10 Science Lesson 4

Exercise

1. Choose the correct options for the given questions.

a) How many pairs of sex chromosomes are found in the human cell? 

Answer: iii. 23 pairs

b) What is a chromosome made up of? 

Answer: iii. DNA and protein

c) What is the main function of sex chromosomes?

 Answer: ii. To determine sex

d) What is the smallest unit of the chromosome that helps in the transmission of hereditary characteristics? 

Answer:  iv. Gene

e) Which of the following statements is correct for mitotic cell division? 

Answer: iv. This cell division helps to repair tissue.

f) A technician working in a radiotherapy laboratory was tested after a long time of marriage when there was no childbirth. After the test, it was found that his child production capacity was reduced because he worked in high-intensity radiation for a long time. Which part of the cell is affected in this case? 

Answer:  i. DNA

g) f there is no DNA transcription in a cell, which process is affected?

 Answer: ii. Protein synthesis

h) If a nucleotide is destructed during DNA replication, what happens to the organism? 

Answer:  i. Genetic disorder occurs

i) Which of the following indicates the set of chromosomes in a cell of a woman? 

Answer:  ii. 44+XX

j) Which of the following statements is true? 

Answer: iii. Ovum contains only an X chromosome and sperm contains either an X or Y chromosome.

2. Differences Between:

a. Autosome vs Sex Chromosome

Topic	Autosome	Sex Chromosome
Function	Determine body traits	Determine sex
Number in Humans	22 pairs	1 pair (XX or XY)
Found in	Both sexes equally	Varies: XX (female), XY (male)
Genes	Carry most genetic traits	Carry sex-determining genes

b) Mitosis vs Meiosis

Topic		Mitosis		Meiosis
Type of Cell		Somatic		Reproductive
Number of Divisions		One		Two
Number of Cells Formed		Two		Four
Function		Growth and repair		Gamete formation

c) DNA vs RNA

Topic		DNA	RNA
Structure		Double-stranded	Single-stranded
Sugar		Deoxyribose	Ribose
Bases		A, T, G, C	A, U, G, C
Location		Nucleus	Nucleus and cytoplasm
Role		Stores genetic information	Helps in protein synthesis

d) Haploid vs Diploid

Topic	Haploid	Diploid
Definition	Single set of chromosomes	Two sets of chromosomes
Symbol	n	2n
Example	Gametes (sperm/egg)	Somatic (body) cells
Produced by	Meiosis	Mitosis

3. Give reasons:

a) Offspring have the same characteristics as their parents.

Ans: Offspring resemble their parents because they inherit genetic material through DNA, which is passed from both the mother and the father during reproduction. The genes, which are segments of DNA, carry the instructions for traits such as eye color, hair texture, height, and even certain behaviors. Each parent contributes half of the genetic material, and these genes determine the observable traits of the offspring. Although offspring look like their parents, they are not identical due to genetic recombination, which introduces slight variations, making each individual unique.

b) The male has a main role in the determination of sex.

Ans: In humans, the sex of a child is determined by the combination of sex chromosomes inherited from the parents. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). The mother can only pass an X chromosome to her child, but the father can pass either an X or a Y chromosome. If the sperm carrying an X chromosome fertilizes the egg, the child will be a girl (XX). If the sperm carrying a Y chromosome fertilizes the egg, the child will be a boy (XY). Hence, it is the male’s sperm that ultimately determines the sex of the baby.

c) Though males have both X and Y sex chromosomes, some of them have only male or only female kids.

Ans: Males have both X and Y chromosomes, and during reproduction, they produce two types of sperm – one carrying the X chromosome and the other carrying the Y chromosome. The child’s sex depends on which type of sperm fertilizes the egg. Although the chances are 50-50, due to random events or genetic patterns, some families might end up having multiple children of the same sex. However, this is purely by chance and does not mean that a man can only produce one type of sperm.

d) Meiotic cell division is also called reductional cell division.

Ans: Meiotic cell division reduces the chromosome number of a cell by half. This is essential in the formation of gametes (sperm and egg) in sexually reproducing organisms. A normal human body cell has 46 chromosomes (diploid), but through meiosis, gametes are formed with only 23 chromosomes (haploid). During fertilization, these two haploid cells fuse to restore the diploid number. This reduction in chromosome number is why meiosis is called reductional division.

e) Mitotic cell division is also called equational cell division.

Ans: In mitosis, a single cell divides to produce two genetically identical daughter cells with the same number of chromosomes as the parent cell. For example, if the parent cell is diploid with 46 chromosomes, each daughter cell will also have 46 chromosomes. Since there is no change in the number of chromosomes and the division results in equal genetic distribution, it is termed equational division.

f) Sexual reproduction is impossible without meiotic cell division. 

Ans: Sexual reproduction involves the fusion of male and female gametes. For this to occur properly, each gamete must have half the number of chromosomes. This is achieved through meiotic cell division. Without meiosis, gametes would have the full number of chromosomes, and their fusion would result in offspring with double the required chromosomes, leading to genetic disorders. Hence, meiosis is vital for maintaining chromosome number and enabling sexual reproduction.

g) Meiotic cell division brings variation.

Ans: Meiosis contributes to genetic diversity through two main processes: crossing over and independent assortment. During crossing over, homologous chromosomes exchange genetic segments, leading to new combinations of genes. In independent assortment, chromosomes are randomly distributed into gametes. These mechanisms ensure that each gamete, and therefore each offspring, is genetically unique. This variation is important for evolution and the adaptability of species.

4. Answer the following questions.

a) What is a gene?

Ans: A gene is a specific sequence of nucleotides in DNA that carries the instructions to produce a particular protein or RNA molecule. Genes are considered the basic units of heredity because they are responsible for the inheritance of traits from parents to offspring. 

b) What is a chromosome? Clarify the role of chromosomes in the body of living beings.

Ans: A chromosome is a long, thread-like structure made up of DNA and proteins (mainly histones), found in the nucleus of eukaryotic cells. Humans have 46 chromosomes, organized into 23 pairs. Chromosomes play a key role in storing genetic information and transmitting it during cell division. They help in the proper segregation of genetic material during mitosis and meiosis. Chromosomes ensure that DNA is accurately replicated and distributed in the formation of new cells, which is essential for growth, reproduction, and survival.

c) Explain the importance of mitotic cell division in the growth and development of the body.

Ans: Mitotic cell division is fundamental for the growth, repair, and maintenance of an organism’s body. Through mitosis, a single cell divides to form two identical daughter cells. This process allows multicellular organisms to grow by increasing their cell number. It also plays a critical role in tissue repair after injury. For example, skin cells continuously divide to replace those lost due to abrasion. Mitosis ensures that each new cell receives an exact copy of the parent cell’s DNA, maintaining genetic stability.

d) Explain the role of mitosis and meiosis in the reproduction of organisms.

Ans: Mitosis and meiosis are both types of cell division that serve different functions in reproduction. Mitosis is responsible for the asexual reproduction seen in some plants and unicellular organisms. It produces genetically identical offspring by duplicating the parent cell. In contrast, meiosis occurs in sexually reproducing organisms and is essential for the formation of gametes. Meiosis ensures that offspring have the correct number of chromosomes and introduces genetic diversity through recombination. Both types of division are vital for life, growth, and evolution.

e) What will happen if meiotic cell division does not occur in the reproductive cell of an organism? Explain.

Ans: If meiosis does not occur in reproductive cells, gametes (sperm and egg) will not be formed correctly. The resulting cells might retain the diploid number of chromosomes instead of being haploid. This means that when two such gametes fuse during fertilization, the offspring will have double the number of chromosomes, leading to genetic abnormalities or developmental failure. Additionally, the lack of genetic variation would limit evolutionary adaptability. Thus, without meiosis, sexual reproduction and species survival would be at risk.

f) Clearly explain the role of genes in the transmission of hereditary characteristics in organisms. 

Ans: Genes are responsible for passing characteristics from parents to their offspring. They do this by encoding instructions for the production of proteins, which carry out essential functions in the body. Each person receives two copies of every gene — one from each parent. These genes determine traits such as physical appearance, blood type, and even behavior. Genes also allow for genetic diversity through recombination and mutations, which is crucial for evolution and adaptability. Without genes, heredity and the continuation of species would not be possible.

g) How is sex determined in humans? Explain with a chart.

Ans: In humans, sex determination is based on the combination of sex chromosomes received from the parents. Females have XX chromosomes, while males have XY. During fertilization:

• Mother (egg): always contributes an X chromosome.
• Father (sperm): can contribute either X or Y.

h) A woman is pregnant. What is her probability of giving birth to a daughter? Write in percent.

Ans: The probability of a woman giving birth to a daughter is 50%. This is because the sex of the child depends on the chromosome contributed by the father’s sperm. Since the sperm can carry either an X (for a girl) or a Y (for a boy) with equal probability, the chance of having a daughter is 1 in 2, or 50%.

i) A couple gave birth to only a son. Does it mean that the testes of those male-produced sperm have Y-chromosomes only?

Ans: No, it does not mean that the male can only produce sperm with Y chromosomes. Each male produces both X and Y chromosome-carrying sperm in roughly equal numbers. The fact that a couple gave birth to only a son is due to chance. Each fertilization event is independent, and whether an X- or Y-carrying sperm fertilizes the egg is a random event. Thus, over multiple pregnancies, the couple can still have a daughter in the future.

4.8 Genetics and genetic technology

Exercise

1. Choose the correct option for the following questions:

a) What plant did Mendel use for his experiment?
i. Pea
ii. Gram
iii. Maize
iv. Bean

Answer: i. Pea

b) Which of the following is the genotypic ratio for Monohybrid cross?
i. 1:2
ii. 3:1
iii. 1:2:1
iv. 9:2:3:1

Answer: iii. 1:2:1

c) What is the term for a characteristic that is passed down from generation to generation?
i. Dominant character
ii. Recessive character
iii. Hereditary character
iv. Imported character

Answer: iii. Hereditary character

d) A white-skinned child was born to a dark-skinned parent. What is the reason for this?
i. The parent was hybrid
ii. Both the father and mother have pure black characters
iii. White colour is dominant
iv. Black colour is recessive

Answer: i. The parent was hybrid

e) In order to produce good meat, farmers look for Boer goats and cross them with local goats. What kind of breeding method is this?
i. Artificial insemination
ii. Selective breeding
iii. IVF
iv. Natural selection

Answer: ii. Selective breeding

f) Ramit has produced a new plant by crossing an orange plant and a lemon plant. What type of plant is this?
i. Advanced variety plant
ii. Pure plant
iii. Hybrid plant
iv. Artificial plant

Answer: iii. Hybrid plant

g) Roshani is a student from Himalayan region. A mule is reared in her home for transportation of the goods. But the mule is getting older, and her family members are considering getting a new mule. In this situation, she asked her father how a mule gives birth to a child. Which of the following is the correct answer given by her father?
i. Mules produce offspring naturally
ii. There is inbreeding in mule.
iii. Mule cannot produce offspring naturally
iv. Mules produce offspring by AI.

Answer: iii. Mule cannot produce offspring naturally

2) Differences between:

i) Dominant and Recessive Characters

Aspect		Dominant Character	Recessive Character
Definition		A trait that is expressed when at least one dominant allele is present.	A trait that is only expressed when both alleles are recessive.
Symbol (usually)		Represented by a capital letter (e.g., A)	Represented by a lowercase letter (e.g., a)
Expression		Seen in both homozygous (AA) and heterozygous (Aa) forms.	Only seen in homozygous (aa) form.

 

ii) Phenotype and Genotype

Aspect		Phenotype			Genotype
Definition		The observable physical or biochemical  characteristics of an organism.			The genetic makeup of an organism.
Examples		Tall plant, blue eyes, blood type			TT, Tt, or tt (for a trait like plant height)
Influenced by		Genotype and environment			Inherited from parents

iii) Inbreeding and Crossbreeding

Aspect	Inbreeding		Crossbreeding
Definition	Mating of closely related individuals within the same breed.		Mating of individuals from different breeds or species.
Purpose	To maintain or fix desired traits within a breed.		To combine desirable traits from two different breeds.
Risk	Higher risk of genetic disorders due to reduced diversity.		Often leads to hybrid vigor (heterosis).

iv) Artificial Insemination and In Vitro Fertilization

Aspect	Artificial Insemination (AI)	In Vitro Fertilization (IVF)
Definition	Introduction of sperm into the female reproductive tract artificially.	Fertilization of egg and sperm outside the body in a lab.
Where fertilization occurs	Inside the female body	Outside the female body (in lab conditions)
Common use in	Animal husbandry, human fertility treatments	Assisted reproductive technologies in humans and animals

v) Tigon and Liger

Aspect		Tigon		Liger
Parentage		Male tiger × Female lion		Male lion × Female tiger
Size		Generally smaller than ligers		Largest of all big cat hybrids
Physical Traits		May show features of both species but are usually smaller and tiger-like		Larger size, often lion-like in appearance

3. Give Reasons 

a) Children look like their parents, but not exactly the same.

Answer: Children inherit half of their genes from each parent, leading to a unique combination of traits. In addition, during sexual reproduction, the process of recombination and independent assortment causes variation, making each child different from their parents and siblings.

b) Mendel selected pea plants for his experiment.

Answer: Mendel chose pea plants because they had clear and contrasting traits like tall/dwarf and green/yellow seeds, which made it easier to observe inheritance patterns. Also, pea plants had a short life cycle and could self-pollinate and cross-pollinate, making them ideal for genetic studies.

c) When tall pea plants and dwarf pea plants are cross-pollinated, tall plants are produced in the first filial generation.

Answer: This is because the tall trait is dominant over the dwarf trait. Even when one gene for dwarfness is present, the dominant tall gene masks its expression, resulting in tall offspring in the F₁ generation.

d) When self-breeding is done between hybrids, different types of offspring are produced.

Answer: When two hybrids (Tt) are crossed, the resulting genotypes can be TT, Tt, or tt. This leads to a mix of phenotypes—some plants will be tall and some will be dwarf. This happens due to the random combination of alleles during fertilization.

e) DNA testing is a reliable technique for criminal investigation.

Answer: DNA is unique to each individual, making it a precise tool for identifying people involved in a crime. Even a small biological sample like hair, blood, or saliva can link a suspect to a crime scene with high accuracy, which is why it’s widely used in forensic science.

f) Genetic engineering involves the detailed study of DNA.

Answer: Genetic engineering works by modifying the DNA of an organism to produce desired traits or cures. To do this, scientists must understand the structure, function, and sequence of DNA. Only with this deep knowledge can they identify specific genes and alter them effectively.

g. Offspring produced by cross-breeding may be sterile.

Answer:

Explanation:
Yes, in some cases, especially when crossbreeding involves two different species (e.g., horse × donkey → mule), the offspring may be sterile. This is because the chromosomes from each parent may not pair properly during gamete formation, preventing reproduction. Example: Liger and Tigon are often sterile.

h. Special attention should be given while collecting samples for DNA testing.

Answer:

Explanation:
DNA samples must be free from contamination and correctly labeled. Poor handling may lead to incorrect results in paternity tests, criminal investigations, or disease diagnosis. Sterile tools and proper storage (e.g., refrigeration) are essential.

4. Answer the following:

a. What is genetics?

Answer: Genetics is the branch of biology that studies heredity and variation—how traits are passed from parents to offspring through genes.

b. What is DNA testing? For what purposes is it used?

Answer: DNA testing is the analysis of an individual’s genetic material (DNA) to identify genetic relationships or mutations.

Uses include:

• Paternity/maternity tests
• Forensic investigations
• Identifying genetic disorders
• Tracing ancestry or genealogy

c. Give some examples of genetic technology.

Answer:

• Genetic engineering (e.g., insulin-producing bacteria)
• CRISPR gene editing
• GMOs (genetically modified organisms)
• Cloning
• DNA fingerprinting

d) Mention the importance of DNA in genetic technology.

Answer: DNA (Deoxyribonucleic Acid) is the fundamental molecule that carries genetic instructions used in the growth, development, functioning, and reproduction of all living organisms. In genetic technology, DNA is crucial because:

• It stores genetic information: Scientists study DNA to understand genes and how they control traits.
• It enables gene identification: By analyzing DNA, researchers can locate specific genes linked to diseases or desirable traits.
• It allows genetic modification: DNA can be altered or transferred between organisms to improve or change genetic traits.
• It supports forensic and medical applications: DNA analysis is used in criminal investigations, ancestry tracing, and diagnosing genetic disorders.

e) Explain the importance of genetic engineering.

Answer: Genetic engineering is the direct manipulation of an organism’s DNA to change its characteristics. Its importance includes:

• Improving agriculture: Crops can be engineered to resist pests, tolerate drought, and produce higher yields.
• Treating diseases: Genetic engineering is used to create medicines, like insulin, and develop gene therapies for genetic disorders.
• Producing genetically modified organisms (GMOs): These can have enhanced nutrition or other beneficial traits.
• Environmental benefits: Engineered organisms can help clean up pollutants or reduce the need for chemical inputs.

g) What is monohybrid cross? Show in the filial chart, the result obtained by cross pollinating first and then self-pollinating of a red flowering pea plant and white flowering pea plant.

Answer:

A monohybrid cross is a genetic cross between two organisms that differ in one trait. It shows how alleles for a single gene are inherited from each parent.

Let’s assume:

• Red flower (R) is dominant
• White flower (r) is recessive

First Generation (F₁): Cross-Pollination

Parent:

RR (Red) × rr (White)

Cross		R		R
r		Rr		Rr
r		Rr		Rr

Second Generation (F₂): Self-Pollination of F₁ plants (Rr × Rr)

Cross		R		r
R		RR		Rr
r		Rr		rr

Result (F₂):

• Genotypic ratio: 1 RR : 2 Rr : 1 rr
• Phenotypic ratio: 3 Red : 1 White

g) Explain with an example that Mendel’s experiment can be done not only in plants but also in animals.

Answer:

Yes, Mendel’s experiments can also apply to animals. For example, in mice:

• Let black fur (B) be dominant
• Let white fur (b) be recessive

A cross between a pure black (BB) and pure white (bb) mouse gives:

• F₁ generation: All Bb – black fur
• F₂ generation (Bb × Bb):

Cross		B		b
B		BB		Bb
b		Bb		bb

• Phenotypic ratio: 3 Black : 1 White

• Genotypic ratio: 1 BB : 2 Bb : 1 bb

This shows that Mendel’s laws apply to both plants and animals.

h) Explain the Mendel’s law of dominance and purity of gametes.

Answer:

1) Law of Dominance:

• When two different alleles are present in a pair, one allele (dominant) expresses itself and the other (recessive) is masked.
• Example: In Rr (Red flower), R is dominant over r.

2) Law of Purity of Gametes (Law of Segregation):

• During gamete formation, the two alleles for a trait separate so that each gamete receives only one allele.
• Example: A plant with genotype Rr forms gametes with either R or r, not both.

i) Round seeded pea plant and wrinkle seeded pea plant are cross-pollinated first and then the offspring obtained were self-pollinated again. The result of the second filial generation is shown in the table below. Now answer the following questions:

Cross		R		r
R		RR		Rr
r		Rr		rr

i) What is the ratio of plants showing dominant and recessive characters?

Answer:

• Dominant (Round seeds): RR, Rr, Rr → 3 plants
• Recessive (Wrinkled seeds): rr → 1 plant
  Ratio: 3 : 1

ii) Write the genotypic and phenotypic ratio of this generation.

Answer:

• Genotypic ratio:
  1 RR : 2 Rr : 1 rr
• Phenotypic ratio:
  3 Round : 1 Wrinkled

iii. Among them, which plant is purely round-seeded? Why?

Answer:

• The plant with genotype RR is purely round-seeded.
• Because it has two dominant alleles (RR) with no recessive gene, so it will always produce round seeds.

j) When a cross is made between a black guinea pig and a white guinea pig, the offspring of first filial generation were all black. Explain why white guinea pigs did not appear in this generation?

Answer:

The black color in guinea pigs is dominant (B) and the white color is recessive (b).

If a pure black (BB) guinea pig is crossed with a white (bb) guinea pig, all F₁ offspring will be Bb (black).

Cross		B		B
b		Bb		Bb
b		Bb		Bb

White guinea pigs did not appear in the first generation because the dominant black allele (B) masked the effect of the recessive white allele (b).

k) A teenage girl who has lost her mental balance became the victim of rape and gave birth to a child. How can the father of the child be detected?

Answer:

The father of the child can be detected through DNA fingerprinting.

• This technique compares the DNA of the child, the mother, and the suspected father(s).
• The DNA inherited from the father can be matched, confirming biological paternity with high accuracy.

l) The district animal development centre conducted a camp to fertilize many cows at once. Which technique did that organization adopt at that time? Explain this technique in brief.

Answer:

The technique used is called Artificial Insemination (AI).

Explanation:

• In this process, sperm from a high-quality bull is collected and preserved.
• The sperm is then artificially inserted into the uterus of multiple cows during their fertile period.
• This helps in controlled breeding, improves genetic quality, and ensures mass fertilization in a short time.

m) Is genetic engineering a boon or a bane for the present era? Give your arguments.

Answer:

Genetic engineering is mostly a boon. Here’s why:

Arguments for Boon:

• Agricultural benefits: Increases crop yield and resistance to pests/drought.
• Medical advancements: Helps produce insulin, vaccines, and gene therapy for genetic disorders.
• Environmental protection: Reduces the need for harmful pesticides and chemicals.
• Industrial uses: Genetically modified bacteria can help clean oil spills and waste.

However, caution is needed:

• It may cause ethical concerns (e.g., designer babies).
• Possible environmental impact if not properly regulated.

So, while it has some risks, responsible use of genetic engineering is a major boon to society.

n) How has AI technology helped to bring happiness to the farmer? Explain.

Answer:

AI (Artificial Intelligence) has helped farmers in many ways:

• Crop monitoring: Drones and AI tools detect pests, diseases, and crop health early.
• Weather prediction: Accurate forecasts help in planning sowing and harvesting.
• Automated farming: AI-powered machines reduce manual labor and increase efficiency.
• Soil analysis: AI apps help farmers understand soil health and nutrient needs.
• Market support: AI platforms suggest best prices and buyers for crops.

These benefits lead to higher productivity, reduced costs, and better profits, bringing happiness to farmers.

o) IVF is proved to be a boon for childless couples. Justify this statement.

Answer:

IVF (In Vitro Fertilization) is a medical technique where fertilization happens outside the body, and the embryo is placed in the uterus.

Why it’s a boon:

• Helps couples who are unable to conceive naturally due to medical conditions like blocked fallopian tubes, low sperm count, or hormonal issues.
• Offers hope to older couples or those with genetic disorders.
• Donor sperm or eggs can be used if needed.
• Multiple embryos can be stored for future use.

It has given millions of childless couples the joy of parenthood, making it a true boon of modern science.