NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

Ans : In asexual reproduction, the reproducing cells produce a copy of their DNA through some chemical reactions. However, this copying of DNA is not accurate and therefore, the newly formed DNA has some variations. 
 

  
It can be easily observed in the above figure that in asexual reproduction, very few variations are allowed. Therefore, if a trait is present in only 10% of the population, it is more likely that the trait has arisen recently. Hence, it can be concluded that trait B that exists in 60% of the same population has arisen earlier than trait A. 

Ans : Sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, most of the bacteria living in that water would die. Only few variants resistant to heat would be able to survive. If these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species.  
However, not all variations are useful. Therefore, these are not necessarily beneficial for the individual organisms. 

Ans : Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these two plants. The seeds formed after fertilization were grown and these plants that were formed represent the first filial or Fl generation. All the Fl plants obtained were tall. 
  
Cross-pollination of tall and short plant Then, Mendel self-pollinated the Fl plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short. 
   
Self-pollination of F1 plants From this experiment, Mendel concluded that the F1 tall plants were not true breeding. They were carrying traits of both short height and tall height. They appeared tall only because the tall trait is dominant over the dwarf trait. 

Ans : Mendel crossed pea plants having round green seeds (RRyy) with pea plants having wrinkled yellow seeds (rrYY).  
   
Since the Fl plants are formed after crossing pea plants having green round seeds and pea plants having yellow wrinkled seeds, Fl generation will have both these characters in them. However, as we know that yellow seed colour and round seeds are dominant characters, therefore, the Fl plants will have yellow round seeds.  
Then this Fl progeny was self-pollinated and the F2 progeny was found to have yellow round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds in the ratio of 9:3:3:1. 
  
Independent inheritance of two different traits  
In the above cross, more than two factors are involved, and these are independently inherited. 

Ans : No. This information is not sufficient to determine which of the traits - blood group A or O — is dominant. This is because we do not know about the blood group of all the progeny.  
Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.