NCERT CLASS 10 SCIENCE CHAPTER 8 HEREDITY
NCERT CLASS 10 SCIENCE
CHAPTER 8 – HEREDITY
IN-TEXT QUESTIONS AND ANSWERS
1. If a trait is inherited from one parent, what happens to the DNA contribution from the other parent?
In sexual reproduction, an offspring receives DNA from both parents. Each parent contributes one set of chromosomes. Therefore, traits inherited from one parent do not mean that the other parent has made no contribution.
2. How does the inheritance of two different traits in pea plants help us understand inheritance?
Studying two different traits together helps us understand how traits are passed from parents to offspring. Mendel observed that different pairs of traits can be inherited independently of one another.
3. How do variations arise during reproduction?
Variations arise due to small differences during DNA copying. In sexual reproduction, genetic material from two parents combines, producing new combinations of genes. These processes create differences among offspring.
4. Why are variations important for a species?
Variations increase the chances that some individuals will survive when environmental conditions change. They provide the raw material for evolution and help populations adapt over generations.
5. What is heredity?
Heredity is the process by which characteristics are transmitted from parents to their offspring through genetic information.
6. What is the difference between inherited and acquired traits?
Inherited traits are passed from parents to offspring through genes. Acquired traits develop during an individual's lifetime due to environmental influences or activities and are generally not passed to offspring through DNA.
7. Why are acquired traits generally not inherited?
Acquired traits usually affect body cells but do not change the DNA of reproductive cells. Therefore, these changes are generally not transferred to the next generation.
8. What is a gene?
A gene is a segment of DNA that carries information for a particular functional product and can influence a specific characteristic of an organism.
9. What is the role of chromosomes in heredity?
Chromosomes are structures made of DNA and proteins. They carry genes and help transmit hereditary information from parents to offspring.
10. What is the difference between dominant and recessive traits?
A dominant trait is expressed even when only one copy of its allele is present. A recessive trait is expressed only when two copies of the recessive allele are present in a simple Mendelian inheritance pattern.
EXERCISE QUESTIONS AND ANSWERS
1. What is the importance of heredity?
Heredity is important because it:
Transfers characteristics from parents to offspring.
Maintains similarities among members of a species.
Helps preserve basic features from one generation to the next.
Provides variations that contribute to evolution.
2. How do Mendel's experiments show that traits may be dominant or recessive?
Mendel crossed pure tall pea plants with pure dwarf pea plants.
In the first generation, all plants were tall.
When the first-generation plants were self-pollinated, both tall and dwarf plants appeared in the second generation.
The dwarf trait had not disappeared; it was hidden in the first generation.
This showed that the tall trait was dominant and the dwarf trait was recessive.
3. How do Mendel's experiments show that traits are inherited independently?
Mendel studied two pairs of contrasting traits in pea plants, such as seed shape and seed colour.
He observed new combinations of traits in the second generation. This indicated that the inheritance of one pair of traits could occur independently of the other pair. This is known as the law of independent assortment.
4. A man with blood group A marries a woman with blood group O. Their child has blood group O. Is this possible? Explain.
Yes, it is possible.
A person with blood group A may have the genotype IAIA or IAi. A person with blood group O has the genotype ii.
If the father has genotype IAi, he can pass either IA or i to the child. The mother can pass only i.
Therefore, the child may have genotype ii and blood group O.
5. How is the sex of a child determined in human beings?
In humans:
Females have two X chromosomes (XX).
Males have one X chromosome and one Y chromosome (XY).
The mother produces eggs carrying only the X chromosome. The father produces sperms carrying either X or Y chromosome.
X-bearing sperm + X-bearing egg = XX (female).
Y-bearing sperm + X-bearing egg = XY (male).
Thus, the sperm from the father determines the chromosomal sex of the child. The probability of either outcome is approximately equal.
6. Why is the inheritance of traits important for evolution?
Inheritance passes genetic information from one generation to the next. Variations arise during reproduction. Some variations may help organisms survive and reproduce more successfully in particular environments. Over many generations, these inherited differences can lead to evolutionary changes.
7. What is the difference between inherited and acquired traits? Give examples.
Inherited traits:
Passed from parents to offspring through genes.
Present due to genetic information.
Examples: Natural eye colour and blood group.
Acquired traits:
Develop during an individual's lifetime.
Usually result from environmental influences or personal activities.
Generally not passed to offspring through genes.
Examples: Learning a language and developing a skill through practice.
8. Explain Mendel's monohybrid cross with an example.
A monohybrid cross studies the inheritance of one pair of contrasting traits.
Mendel crossed a pure tall pea plant (TT) with a pure dwarf pea plant (tt).
Parental generation:
TT × tt
First filial generation (F1):
All offspring are Tt and tall because T is dominant over t.
When F1 plants are self-pollinated:
Tt × Tt
Second filial generation (F2):
TT, Tt, Tt and tt
Phenotypic ratio:
3 Tall : 1 Dwarf
Genotypic ratio:
1 TT : 2 Tt : 1 tt
9. Explain Mendel's dihybrid cross.
A dihybrid cross studies the inheritance of two pairs of contrasting traits together.
Mendel studied seed shape and seed colour in pea plants.
Round seed shape (R) is dominant over wrinkled seed shape (r).
Yellow seed colour (Y) is dominant over green seed colour (y).
When plants with genotype RRYY and rryy are crossed, all F1 plants have genotype RrYy and show round yellow seeds.
On self-pollination of F1 plants, the F2 generation shows the phenotypic ratio:
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
This supports the law of independent assortment.
10. Why do all offspring in the F1 generation of a cross between pure tall and pure dwarf pea plants have tall stems?
The allele for tallness (T) is dominant over the allele for dwarfness (t). All F1 offspring receive one T allele and one t allele, giving genotype Tt. Since T is dominant, all plants are tall.
FEW IMPORTANT ADDITIONAL QUESTIONS AND ANSWERS
1. Define heredity and variation.
Heredity: The transmission of characteristics from parents to offspring.
Variation: Differences in characteristics among individuals of the same species.
2. What is the importance of variation?
Variation:
Helps populations adapt to changing environments.
Increases the chances of survival of some individuals.
Provides the basis for natural selection.
Plays an important role in evolution.
3. What are genes?
Genes are specific segments of DNA that carry hereditary information. They influence the development and functioning of organisms and are passed from parents to offspring.
4. What are alleles?
Alleles are alternative forms of the same gene that occur at the same position on corresponding chromosomes.
For example, T and t may represent alleles for tallness and dwarfness in pea plants.
5. What is a dominant trait?
A dominant trait is a characteristic that is expressed even when an organism has only one copy of the dominant allele.
Example: Tallness in pea plants is dominant over dwarfness in Mendel's experiment.
6. What is a recessive trait?
A recessive trait is expressed when both copies of the allele are recessive in a simple Mendelian inheritance pattern.
Example: Dwarfness in pea plants.
7. What is genotype?
Genotype refers to the genetic makeup or combination of alleles possessed by an organism.
Examples: TT, Tt and tt.
8. What is phenotype?
Phenotype refers to the observable characteristics of an organism, which result from its genetic makeup and environmental influences.
Examples: Tall plant, dwarf plant and yellow seeds.
9. What is homozygous condition?
An organism is homozygous for a particular gene when it has two identical alleles.
Examples:
TT – Homozygous dominant.
tt – Homozygous recessive.
10. What is heterozygous condition?
An organism is heterozygous for a particular gene when it has two different alleles.
Example: Tt.
11. What is a monohybrid cross?
A monohybrid cross is a genetic cross that studies the inheritance of one pair of contrasting characteristics.
Example: A cross between tall and dwarf pea plants.
12. What is a dihybrid cross?
A dihybrid cross is a genetic cross that studies the inheritance of two pairs of contrasting characteristics at the same time.
Example: A cross involving seed shape and seed colour in pea plants.
13. State Mendel's law of dominance.
The law of dominance states that when two contrasting alleles are present together in a heterozygous organism, only the dominant allele is expressed in the phenotype, while the recessive allele remains hidden.
14. State Mendel's law of segregation.
The law of segregation states that the two alleles of a gene separate during the formation of gametes. Each gamete receives only one allele of the pair.
15. State Mendel's law of independent assortment.
The law of independent assortment states that alleles of different genes are inherited independently of one another when the genes are unlinked or sufficiently far apart on the same chromosome.
16. Why did Mendel select pea plants for his experiments?
Mendel selected pea plants because:
They have easily distinguishable contrasting traits.
They have a relatively short life cycle.
They produce many seeds.
They can self-pollinate.
Cross-pollination can also be carried out easily.
17. What is the difference between dominant and recessive alleles?
Dominant allele:
Expresses its effect in both homozygous and heterozygous conditions.
Represented by a capital letter.
Example: T.
Recessive allele:
Expresses its effect in the homozygous condition in simple Mendelian inheritance.
Represented by a small letter.
Example: t.
18. What is DNA?
DNA stands for Deoxyribonucleic Acid. It is the hereditary material that carries genetic information in most living organisms. It controls the formation of proteins and influences the characteristics of organisms.
19. What are chromosomes?
Chromosomes are thread-like structures made of DNA and proteins. They are present in the nucleus of most cells and carry genes responsible for hereditary characteristics.
20. What is the relationship between DNA, genes and chromosomes?
DNA is the hereditary material. A gene is a specific segment of DNA that carries information. Chromosomes are organised structures made of DNA and proteins that contain many genes.
21. Why do offspring resemble their parents?
Offspring resemble their parents because they inherit genetic information from them. Genes influence characteristics such as appearance, body structure and other biological features.
22. Why are offspring not exactly identical to their parents?
Offspring are not exactly identical because small variations arise during DNA copying. In sexual reproduction, genetic material from two parents combines to create new combinations of genes.
23. What is the difference between inherited and acquired characteristics?
Inherited characteristics:
Transmitted through genes.
Passed from parents to offspring.
Examples: Blood group and natural eye colour.
Acquired characteristics:
Develop during an individual's lifetime.
Usually result from environmental conditions or activities.
Generally not transmitted through reproductive DNA.
Examples: Learning to swim and developing a skill.
24. Why are acquired characteristics not generally inherited?
Acquired characteristics usually affect body cells and do not change the genetic information in reproductive cells. Therefore, they are generally not passed to the next generation.
25. Explain sex determination in humans.
Human females have XX sex chromosomes, while human males have XY sex chromosomes.
The mother produces eggs carrying an X chromosome. The father produces two types of sperms: X-bearing and Y-bearing.
X sperm fertilising an X egg produces XX offspring.
Y sperm fertilising an X egg produces XY offspring.
Therefore, the father determines whether the offspring receives XX or XY sex chromosomes. Both possibilities have approximately equal probability.
26. What is the role of the Y chromosome in human sex determination?
The Y chromosome usually carries the SRY gene, which plays an important role in initiating the development of testes and the male developmental pathway.
27. What is the probability of having a boy or a girl in human reproduction?
The probability is approximately 50% for an XX child and 50% for an XY child, assuming equal chances of fertilisation by X-bearing and Y-bearing sperms.
28. Why should a mother not be blamed for the sex of a child?
The mother contributes an X chromosome through the egg. The father contributes either an X or a Y chromosome through the sperm. Therefore, the sperm determines whether the child receives XX or XY sex chromosomes. Blaming the mother is scientifically incorrect.
29. What is the difference between male and female sex chromosomes in humans?
Female:
Has XX sex chromosomes.
Produces eggs carrying an X chromosome.
Male:
Has XY sex chromosomes.
Produces X-bearing and Y-bearing sperms.
30. How does variation contribute to evolution?
Variation creates differences among individuals in a population. If certain inherited variations help individuals survive and reproduce in a particular environment, those variations may become more common over generations. This process contributes to evolution.
IMPORTANT DIFFERENCES
1. Difference between genotype and phenotype
Genotype:
Genetic makeup of an organism.
Refers to the combination of alleles.
Example: TT, Tt or tt.
Phenotype:
Observable characteristics of an organism.
Influenced by genes and environment.
Example: Tall or dwarf plant.
2. Difference between homozygous and heterozygous
Homozygous:
Contains two identical alleles.
Examples: TT and tt.
Heterozygous:
Contains two different alleles.
Example: Tt.
3. Difference between inherited and acquired traits
Inherited traits:
Passed through genetic information.
Come from parents.
Examples: Blood group and natural eye colour.
Acquired traits:
Develop during an individual's lifetime.
Usually do not change reproductive DNA.
Examples: Learned skills and language.
IMPORTANT GENETIC CROSSES
1. Monohybrid Cross
Parental generation:
TT × tt
Gametes:
T and t
F1 generation:
All Tt (Tall)
Self-cross:
Tt × Tt
F2 generation:
TT, Tt, Tt, tt
Phenotypic ratio:
3 Tall : 1 Dwarf
Genotypic ratio:
1 TT : 2 Tt : 1 tt
2. Dihybrid Cross
Parental generation:
RRYY × rryy
F1 generation:
All RrYy (Round Yellow)
Self-cross:
RrYy × RrYy
F2 phenotypic ratio:
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
IMPORTANT TERMS TO REMEMBER
Heredity – Transfer of characteristics from parents to offspring.
Variation – Differences among individuals of the same species.
Gene – A segment of DNA carrying hereditary information.
Allele – Alternative form of a gene.
Dominant trait – Trait expressed in the presence of a dominant allele.
Recessive trait – Trait expressed when both alleles are recessive.
Genotype – Genetic makeup of an organism.
Phenotype – Observable characteristics of an organism.
Homozygous – Having two identical alleles.
Heterozygous – Having two different alleles.
Chromosome – DNA and protein structure carrying genes.
DNA – Deoxyribonucleic Acid.
Monohybrid cross – Cross involving one pair of contrasting traits.
Dihybrid cross – Cross involving two pairs of contrasting traits.
Zygote – Cell formed by fusion of male and female gametes.
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