You should probably read the relevant sections of the catechism of the Catholic Church, such as the material below. Click on the url to go to the section the has this material. In brief, the doctrine says that all human beings have been deprived of the original holiness and that can only be restored by baptism. The condition called original sin does arrive by propagation of the species, but has nothing to do with our genetic makeup. In the catholic understanding of human beings we are body and soul, not just genetics. The nature of a human being is the most fundamental attribute and is the "essence" of what it is to be human. The flesh we have ("matter") is not relevant. The soul that contains the "essence" of what it is to be human shapes the matter. It gives it "form" and substance. In our case, the soul posses a wounded human nature. These philosophical ideas predate science. We do not gain original sin by bad example or by handing on a defective genetic package. We are born in a wounded state, deficient in a fundamental way.
I have tried to understand this myself; it is usually stated that it is a mystery, something that cannot be fully uderstood. I have come to think that all sin, including original sin is a separation from God, the source of holiness and goodness. We are born separated from him, but when baptized the separation is overcome with the indwelling of the Holy Spirit. Looking at it this way, the question becomes one of relationship rather than physical, intellectual, or emotional defect. God created us to be in a close continual relationship with him. Adam, as father, made a decision for himself that has an effect on all of his offspring. He rejected God, or you could say that by his disobedience he said I don't care about you and what you want. I want this more than I want you. In driving God away, he drove away all the benefits that each human would possess by being involved closely and continuously with God from the earliest moment of the person's existence.
The difference this causes can be seen by looking at the lives of Jesus and Mary. Mary was given the grace of baptism from the eariest moment. Jesus is fully human and yet is not subject to original sin, and thus He is called the new Adam. In both cases, they were required to accept the human condition with its suffering and death. They were subject to temptation, but they did not sin. The close continuous relationship with God seems to have had profoundly beneficial consequence of enabling people to resist temptation. This means they were loving, self-sacrificing people who did not inflict suffering and harm on others. How much better would the world be if we were all like that?
Sec. 404. "How did the SIN of Adam become the SIN of all his descendants? The whole human race is in Adam 'as one body of one man'.[St. Thomas Aquinas, De malo 4, I.] By this 'unity of the human race' all men are implicated in Adam's SIN, as all are implicated in Christ's justice. Still, the transmission of ORIGINAL SIN is a mystery that we cannot fully understand. But we do know by Revelation that Adam had received ORIGINAL holiness and justice not for himself alone, but for all human nature. By yielding to the tempter, Adam and Eve committed a personal SIN, but this SIN affected the human nature that they would then transmit in a fallen state.[Cf. Council of Trent: DS 1511-1512.] It is a SIN which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of ORIGINAL holiness and justice. And that is why ORIGINAL SIN is called 'SIN' only in an analogical sense: it is a SIN 'contracted' and not 'committed' - a state and not an act."
Sec. 405. "Although it is proper to each individual,[Cf. Council of Trent: DS 1513.] ORIGINAL SIN does not have the character of a personal fault in any of Adam's descendants. It is a deprivation of ORIGINAL holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to SIN - an inclination to evil that is called concupiscence'. Baptism, by imparting the life of Christ's grace, erases ORIGINAL SIN and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle."
To view the context to these sections of the Catechism of the Catholic Church.