CBSE Class 10 Science Chapter 9: Heredity and Evolution NCERT Solutions

In species that reproduce asexually, new individuals are formed from a single parent, and their genetic material is a copy of the parent's DNA. However, this copying process is not always perfectly accurate, leading to small changes or variations in the DNA. These variations are the source of new traits.

The frequency of a trait within a population can give us clues about when it originated. If a trait is present in a very small percentage of the population (like 10%), it suggests that this variation is relatively new and has not had much time to spread. Conversely, if a trait is present in a much larger percentage of the population (like 60%), it indicates that the variation has existed for a longer time and has had more opportunities to be passed down through generations.

Therefore, trait B, which exists in 60% of the population, is likely to have arisen earlier than trait A, which is present in only 10% of the population.

Species survival is often challenged by changes in their environment. These environmental changes can include shifts in temperature, availability of food, or the presence of predators. When environmental conditions change drastically, individuals within a species may find it difficult to survive.

Variations within a species are crucial because they provide a range of traits among individuals. If an environmental change occurs, such as a sudden increase in water temperature, most individuals with less heat-resistant traits might die. However, if some individuals possess variations that make them more resistant to the heat, they are more likely to survive.

These surviving individuals, carrying the advantageous variations, can then reproduce and pass on these traits to their offspring. Over time, this leads to a population that is better adapted to the new environmental conditions. Thus, variations increase the probability that at least some members of a species will survive significant environmental challenges, promoting the long-term survival of the species as a whole. It is important to note that not all variations are beneficial; some may be neutral or even harmful to the individual organism.

Gregor Mendel conducted experiments using pea plants to understand how traits are inherited. He started with 'true-breeding' plants, meaning plants that consistently produced offspring with the same trait when self-pollinated (e.g., pure tall plants always produced tall offspring, and pure dwarf plants always produced dwarf offspring).

Mendel crossed a true-breeding tall pea plant (represented by genotype TT) with a true-breeding dwarf pea plant (represented by genotype tt). The seeds produced from this cross were grown, forming the first filial or $F\_1$ generation. Interestingly, all the $F\_1$ generation plants were tall.

This observation suggested that the trait for tallness masked the trait for dwarfness. Mendel then allowed these $F\_1$ tall plants to self-pollinate. When he examined the second filial or $F\_2$ generation, he observed that the traits segregated again. Out of the $F\_2$ plants, approximately three-fourths were tall, and one-fourth were short (dwarf).

The reappearance of the short trait in the $F\_2$ generation, even though it was not visible in the $F\_1$ generation, indicated that the dwarf trait had been present but masked. Mendel concluded that the trait for tallness is 'dominant' over the trait for dwarfness, which is 'recessive'. The $F\_1$ plants, although appearing tall, carried the genetic information for both tallness and dwarfness.