Which of the following organs,when found to be present in two animals,indicate that these two animals have been evolved from a common ancestor?

"It is a matter of chance whether a couple will give birth to a male child or a female child." Justify this statement with the help of a flow chart showing the fusion of sex chromosomes.

"The chromosome number of the sexually reproducing parents and their offspring is the same." Justify this statement.

$A$ woman having blood group $B$ marries a man having blood group $A$ and they have four offspring. Two children have blood group $O$,one has blood group $B$,and one has blood group $AB$. Now,answer the following:
$(i)$ Which blood group is dominant?
$(ii)$ Which blood group is recessive?
$(iii)$ What are the genotypes of the parents?
$(iv)$ Show the possible genotypes and phenotypes of the offspring.

Which organism can change its sex?

What are fossils? How are they formed? Describe in brief two methods of determining the age of fossils. State any one role of fossils in the study of the process of evolution.