Heredity and Evolution – Class 10, Chapter - 9 , Science (NCERT)
Dear learners,
Have you ever wondered why you look similar to your parents or why some animals can survive better in certain environments? The answers lie in heredity and evolution, two fascinating concepts that explain how traits are passed from one generation to another and how species change over time.
In this chapter, you will discover how genes determine our characteristics, how variations help species survive, and how life on Earth has evolved over millions of years. Through Mendel’s experiments, Darwin’s theory of natural selection, and the study of fossils, we will explore the amazing story of life’s journey.
Get ready to uncover the secrets of heredity and evolution—because understanding where we come from helps us shape the future!
You translate this topic to your own language for better understanding.
1. Heredity
Heredity is the process by which parents pass their traits (like eye color, height, or hair type) to their children through genes. These genes are present in the DNA, which is found in every cell of our body.
Mendel’s Experiments on Pea Plants
Gregor Mendel, the "Father of Genetics," performed experiments on pea plants to understand how traits are inherited. He found that:
Traits are controlled by factors (now called genes) that exist in pairs.
Some traits (dominant) are seen in the next generation, while others (recessive) may remain hidden.
Each parent contributes one gene for a trait.
Inheritance of Traits
Dominant Traits: The traits that appear in the next generation. (e.g., If one parent has brown eyes and the other has blue, the child may have brown eyes if the brown-eye gene is dominant.)
Recessive Traits: These traits appear only if both parents pass the same recessive gene.
Sex Determination in Humans
The father’s sperm determines the baby’s gender.
Males have XY chromosomes, and females have XX chromosomes.
If the sperm carries an X chromosome, the baby will be a girl (XX), and if it carries a Y chromosome, the baby will be a boy (XY).
2. Evolution
Evolution explains how different species have changed over millions of years and how new species are formed.
Variations and Their Importance
No two individuals are exactly the same because of variations in their genes.
Variations help species survive in changing environments. (e.g., Some bacteria become resistant to antibiotics due to variations.)
Natural Selection (Theory by Charles Darwin)
Nature selects organisms that are better adapted to their environment.
Example: Giraffes with longer necks could reach food on tall trees and survived better than those with short necks. Over generations, only long-necked giraffes remained.
Speciation
When a group of organisms becomes very different from others due to genetic changes, a new species is formed.
This can happen due to geographical barriers (like rivers or mountains) or reproductive isolation (when groups stop breeding with each other).
3. Tracing Evolution
Scientists study evolution using:
Fossils: Remains of ancient organisms preserved in rocks. (e.g., Dinosaurs’ fossils tell us about extinct species.)
Homologous Organs: Similar body structures in different species (e.g., human hands and bat wings).
Analogous Organs: Different structures with the same function (e.g., wings of birds and butterflies).
4. Human Evolution
Modern humans evolved from early ape-like ancestors millions of years ago.
Fossil evidence suggests humans first appeared in Africa and then spread across the world.
Dear learners, This is a summary of the lesson. Please go through with your textbook . Heredity and evolution explain how traits are passed down and how life has changed over time. Genetics helps us understand family traits, while evolution shows how species adapt and survive. By studying this chapter, we learn about the unity and diversity of life on Earth.
Here are some key questions and answers from the Class 10 NCERT Science Chapter 9-- "Heredity and Evolution":
Q. What is heredity?
Answer: Heredity refers to the transmission of traits and characteristics from parents to their offspring.
Q. Define variation.
Answer: Variation refers to the differences in traits among individuals of the same species, resulting from genetic differences or environmental factors.
Q. What is a gene?
Answer: A gene is a segment of DNA that codes for a specific protein, determining particular traits in an organism.
Q. How is sex determined in human beings?
Answer: Sex determination in humans depends on the combination of sex chromosomes inherited:
Females have two X chromosomes (XX).
Males have one X and one Y chromosome (XY).
The sperm determines the sex of the offspring:
An X-bearing sperm fertilizing an egg results in a female (XX).
A Y-bearing sperm fertilizing an egg results in a male (XY).
Q. Explain the terms 'homologous' and 'analogous' organs with examples.
Answer: Homologous organs: Structures with similar anatomy and origin but different functions. Example: The forelimbs of humans and the wings of bats.
Analogous organs: Structures with different anatomy and origin but similar functions. Example: The wings of birds and insects.
Q. What is evolution?
Answer: Evolution is the gradual change in the inherited traits of a population over successive generations, leading to the development of new species.
Q. How do fossils provide evidence for evolution?
Answer: Fossils are the preserved remains or traces of organisms from the past. They provide evidence for evolution by:
Showing the progression of life forms over time.
Revealing transitional forms between different groups.
Indicating common ancestry through structural similarities.
Q. What is natural selection?
Answer: Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.
Q. Describe Mendel's contribution to genetics.
Answer: Gregor Mendel, known as the father of genetics, conducted experiments on pea plants and formulated the foundational laws of inheritance:
Law of Segregation: Each organism carries two alleles for a trait, which segregate during gamete formation.
Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.
Q. How do embryological studies provide evidence for evolution?
Answer: Embryological studies show that embryos of different vertebrates exhibit similar stages of development, suggesting a common ancestry and providing evidence for evolution.
Q. What are traits?
Answer: Traits are characteristics or features inherited from parents, such as eye color, height, and hair type.
Q. What is a dominant trait?
Answer: A dominant trait is expressed in the offspring even if only one copy of the gene is inherited (e.g., brown eyes over blue eyes).
Q. What is a recessive trait?
Answer : A recessive trait is only expressed when both copies of the gene carry the same trait (e.g., blue eyes require two blue eye alleles).
Q. What is a chromosome?
Answer: A chromosome is a thread-like structure in the nucleus that carries genetic information in the form of DNA.
Q. What is an allele?
Answer: An allele is a variant form of a gene that determines specific traits.
Q. What is the difference between genotype and phenotype?
Answer: Genotype: The genetic makeup of an organism (e.g., BB, Bb, or bb for eye color).
Phenotype: The physical appearance of an organism (e.g., brown or blue eyes).
Q. What is monohybrid cross?
Answer : A monohybrid cross studies the inheritance of a single trait (e.g., tall vs. short pea plants).
Q. What is dihybrid cross?
Answer: A dihybrid cross studies the inheritance of two different traits at the same time (e.g., seed color and seed shape in pea plants).
Q. What is meant by acquired traits?
Answer: Acquired traits are characteristics developed during an organism’s lifetime due to environmental factors (e.g., muscle growth due to exercise).
Q. Why are acquired traits not inherited?
Answer: Acquired traits do not change the DNA sequence and cannot be passed to offspring.
Q. What is the theory of evolution?
Answer : Evolution is the process by which species change over time due to genetic variations and natural selection.
Q. Who proposed the theory of natural selection?
Answer: Charles Darwin proposed the theory of natural selection.
Q. What is speciation?
Answer: Speciation is the formation of new species due to genetic variations, natural selection, and reproductive isolation.
Q. What is genetic drift?
Answer : Genetic drift is a random change in gene frequency in a population, especially in small populations.
Q. What is artificial selection?
Answer : Artificial selection is the process by which humans selectively breed plants or animals to develop desired traits (e.g., breeding dogs for specific features).
Q. What is the importance of variations in evolution?
Answer : Variations help organisms adapt to their environment, increasing survival chances and leading to evolution over time.
Q. What is an evolutionary tree?
Answer: An evolutionary tree is a diagram showing how different species have evolved from common ancestors.
Q. What is the importance of homologous structures in evolution?
Answer: Homologous structures indicate common ancestry, even if their functions differ (e.g., human hands and bat wings).
Q. What is the importance of analogous structures in evolution?
Answer: Analogous structures show how different organisms adapt similarly to their environment despite not having a common ancestor (e.g., wings of birds and insects).
Q. Why do species become extinct?
Answer : Species become extinct due to environmental changes, habitat destruction, predation, competition, and inability to adapt.
Q. If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B is likely to have arisen earlier because it is present in a larger percentage (60%) of the population compared to trait A (10%). In asexually reproducing species, traits are passed on directly to offspring, so a higher prevalence indicates an older origin.
Q. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as:
a) TTWW
b) TTww
c) TtWW
d) TtWw
Answer: c) TtWW. The tall parent must be heterozygous for height (Tt) to produce both tall and short offspring and homozygous dominant for violet flowers (WW) since all progeny had violet flowers.
Q. An example of homologous organs is:
a) Our arm and a dog's fore-leg.
b) Our teeth and an elephant's tusks.
c) Potato and runners of grass.
d) All of the above.
Answer: b) Our teeth and an elephant's tusks. Both structures have a similar origin but may serve different functions.
Q. In evolutionary terms, we have more in common with:
a) A Chinese schoolboy.
b) A chimpanzee.
c) A spider.
d) A bacterium.
Answer: a) A Chinese schoolboy. Both are humans and belong to the same species, sharing the most recent common ancestry.
Q. A study found that children with light-colored eyes are likely to have parents with light-colored eyes. On this basis, can we say anything about whether the light eye color trait is dominant or recessive? Why or why not?
Answer: No, we cannot determine whether the light eye color trait is dominant or recessive based solely on this information. Both dominant and recessive traits can appear in offspring if the parents carry the respective alleles.
Q. Aditya was observing some organisms in a lab and tried to compare them. The presence of which organs will confirm to him that they share evolutionary history?
a) Analogous organs
b) Paralogous organs
c) Homologous organs
d) None of these
Answer: c) Homologous organs. These organs have similar structures due to shared ancestry, even if their functions differ.
Q. New species may be formed if:
I. DNA undergoes significant changes in germ cells.
II. Chromosome number changes in the gamete.
III. There is no change in the genetic material.
IV. Mating does not take place.
a) I and II
b) I and III
c) II, III, and IV
d) I, II, and III
Answer: a) I and II. Significant genetic changes or changes in chromosome number can lead to the formation of new species.
Q. Which of the following statements is not true with respect to variation?
a) All variations in a species have an equal chance of survival.
b) Change in genetic composition results in variation.
c) Selection of variants by environmental factors forms the basis of evolutionary processes.
d) Variation is minimum in asexual reproduction.
Answer: a) All variations in a species do not have an equal chance of survival. Some variations may offer advantages or disadvantages in specific environments.
Q. Select the statement that describes characteristics of genes:
a) Genes are specific sequences of bases in a DNA molecule.
b) A gene does not code for proteins.
c) In individuals of a given species, a specific gene is located on a particular chromosome.
d) Each chromosome has only one gene.
Answer: a) Genes are specific sequences of bases in a DNA molecule.
Q. If a round, green-seeded pea plant (RRyy) is crossed with a wrinkled, yellow-seeded pea plant (rrYY), the seeds produced in the F1 generation will be:
a) Round and yellow
b) Round and green
c) Wrinkled and green
d) Wrinkled and yellow
Answer: a) Round and yellow. Round (R) and yellow (Y) are dominant traits.
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