Some of the most significant criteria of science are experiments and observations. This applies to medicine science as well. The most suitable treatment for an illness among all possibilities can be determined through experiments on first animals and then humans itself. The prevailing result surely comes out of the experiments conducted on humans. The condition of whether a drug can be used in the treatment of a disease is whether people who have the disease can be cured through that drug. And this can be determined by a series of trials. In other words, it is the experiments conducted on humans that will determine the answer to the question of whether the drug has a healing effect on humans. The use of a human as a test subject in an experiment has certain criteria in legal and moral frameworks. With these criteria, it is aimed to protect the rights of the person within the legal and moral frameworks and to prevent unregulated experiments as well. Ethical principles, which are based on the fact that human life is single and precious and has to be protected against sufferings, are also guaranteed by law. While some of the principles determined in medical ethics are based on the relationship between physician and patient, the other part is based on the quality of the research that the physician will do to solve the problems of the patient. Thus, medical ethics includes the physician and patient relationship as well as the principles of the experiments conducted by the physician. Today’s medical ethics generally refers to four principles: respect for autonomy, beneficence, non-maleficence, justice. These principles provide a framework as universal principles in the process of establishing medical ethics. The main reason for needing these principles is to determine the relationship between physician and patient and to set a standard for the human experiment conducted throughout the history. Especially in the 20th century, the experiments on humans are tried to be regulated by these principles. In this study, the morality of a problem - the use of human guinea pigs in the experiments- within the framework of medical ethics, which is a subfield of applied ethics, will be discussed in terms of basic moral theorists. In this context, we will try to find possible answers that Deontological, Moral Sense and Consequentialist/Utilitarian moral theorists can provide to a problem encountered in real life and determined by principles within the framework of medical ethics. The case study is an experiment applied to twins. The study is about whether genetic or environmental factors are more effective in human life. How will moral theorists look at this experiment applied to twins? First and foremost, it should be noted that no moral theory can approve the sacrifice of human life, which is a fundamental human right. Every life is one and only, and precious. Therefore, it is the same for Kant, who represents the deontological view. Deontological morals or obligations morality binds human actions to certain obligations. It sets rules and presents the keys on how to act. Looking at this case study, in no situation or condition, Kant morality sees lying and disrupting family integrity as legitimate. Kant argues that in cases of crisis, the maxims will continue to operate unchanged. Therefore, Kant will never affirm such a scientific experiment. While deontological perspective evaluates an action through principles and obligations, consequentialist moral theorists discuss that the value of an action is determined according to its result. In other words, from the consequentialist perspective, such an action that will result in the trauma of two people may not be considered morally negative. If the results of an experiment will be of the great benefit to humanity, there will be no objections to the experiment. Thus, experiments conducted for the general health of society can be considered moral. In terms of Moral Sense Theory, the first premise in such an experiment is the happiness and well-being of individuals. The unhappiness of the experiment subjects can be grasped with our inner feelings. When we put ourselves in the place of those experimental subjects, we may not want to go through the same processes with them and can understand them. Likewise, the society called “neutral observers” which the individual is also a part of, might approve some experiments while rejecting others. Every society has the ability to determine whether such experiments should be approved or not within itself. This study aims to make an ethical analysis by discussing a problem of applied ethics through the perspective of basic moral theories.