Please explain your question more specified
Population decreases by 15.
Birth rate means more population (positive), death rate means less population (negative), immigration means more (positive), and emigration means less (negative). So, just add and subtract the appropriate numbers:

Negative 15 people means that the population decreases by 15.
The lymphatic system destroy harmful substances 'By producing antibodies'.
The lymphocytes contained in lymphatic organs are produced with other defence cells by the bone marrow. They are then dispersed to eliminate pathogens.
Gene regulation or Gene expression can be defined as a synthesis or make up of protein/RNA(Non-coding). In turn b-globin is (developing red blood cells in and only because its already programmed to).
The answer is a mutation. Its a mutation because a mutation can cause an organism to be deformed an in some cases due to the deformation it will not be able to feed or defend for it self. For example, if a Giraffe were to be born with a short neck it would not be able to feed because trees are tall. The answer is mutilation.