According to the evolutionary theory,the formation of a new species is generally due to:

In peas,a pure tall plant $(TT)$ is crossed with a short plant $(tt).$ The ratio of pure tall plants to short plants in $F_{2}$ is

$(a)$ What are monohybrid and dihybrid crosses?
$(b)$ How did Mendel prove that tallness is the dominant trait and dwarfness is the recessive trait in a pea plant? Explain with the help of a monohybrid cross.

Where are the genes located? What is the chemical nature of a gene?

Select the statements that describe characteristics of genes:
$(i)$ Genes are specific sequences of bases in a $DNA$ molecule.
$(ii)$ $A$ gene does not code for proteins.
$(iii)$ In individuals of a given species,a specific gene is located on a particular chromosome.
$(iv)$ Each chromosome has only one gene.

Which of the following pairs of organs are $NOT$ homologous organs?