Which of the following organisms does not exhibit X chromosome inactivation?
Humans
Fruit flies (Drosophila melanogaster)
Cats (Felis catus)
Birds (Aves)
In mammals, X chromosome inactivation occurs when one of the two X chromosomes in female cells is randomly inactivated to accomplish dosage adjustment between males and females. Birds (Aves) do not have X chromosomal inactivation. Both the Z and W chromosomes are sex chromosomes in birds, with females having ZW and males having ZZ. Unlike in mammals, inactivation of one of the Z chromosomes does not equalise gene expression on the sex chromosomes in birds. Instead, dosage compensation is accomplished in females by increasing transcription of the single active Z chromosome.
Option 4 is the correct answer.