Adam's posterity - yes. Who was Jesus's father? Who was Him mother?

It is the privilege of every sinner to ask his teacher what sin really is. Give me a definition of sin. We have one in 1 John 3. "Sin is the transgression of the law." Now this is the only definition of sin in the whole Bible. {1SAT 228.2}

Our only definition of sin is that given in the word of God; it is "the transgression of the law;" it is the outworking of a principle at war with the great law of love which is the foundation of the divine government. {GC 492.2}

The only definition of sin in the Word of God is given us in 1 John 3:4. "Sin is the transgression of the law." {OHC 141.3}

"Sin is the transgression of the law." This is the only definition of sin. {7BC 951.3}

"Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law." This is the only definition of sin given in the Holy Scriptures, and we should seek to understand what sin is, lest any of us be found in opposition to the God of heaven. {RH, July 15, 1890 par. 2}

Sin is selfishness. It is more a characteristic than an act.

"Sin is the transgression of the law." 1 John 3:4.

Sinning requires choice and behavior

It is true that all suffering results from the transgression of God's law {DA 471.3}

"I present the Word of the Lord God of Israel, because of transgression the curse of God has come upon the earth itself, upon the cattle, and upon all flesh. Human beings are suffering the results of their own course of action in departing from the commandments of God. The beasts also suffer under the curse. Disease in cattle is making meat-eating a dangerous matter. The Lord's curse is upon the earth, upon man, upon beasts, upon the fish, and as transgression becomes almost universal, the curse will be permitted to become as broad and as deep as the transgression. Disease is contracted by the use of meat. The diseased flesh of these dead carcasses is sold in the market-places, and disease among men is the sure result. The Lord would bring His people into a position where they will not touch or taste the flesh of dead animals. There is no safety in eating of the flesh of dead animals, and in a short time the milk of the cows will also be excluded from the diet of God's commandment-keeping people. In a short time it will not be safe to use anything that comes from the animal creation."--Unpublished Testimony, July 26, 1898. {PUR, November 7, 1901 par. 1}

You are right - selfishness is a characteristic.

Leaves are green, that is a characteristic. Leave produce sugars by photosynthesis, that is an act. Both being green and producing sugar do not describe nature of the leaf.

What is the nature of sin?

You have now gotten too abstract for my understanding. And repeating the same question ad infinitum will not help me.

Sin is incongruence with God's character. If you want something more exotic than that, I'll leave it to others to fill your need.

No, because when we are born again, we have the same power to be transformed to be dead to sin in the path to sanctification, at what point the Holy Spirit is involved is moot as the end result is the same.

Let's consider that. John the Baptist was born filled with the Spirit. Was he in the same condition as Jesus was at birth? Were they equally holy?

And that this is likeness to man as he is in his fallen, sinful nature and not as he was in his original, sinless nature is made certain by the word: "We see Jesus, who was made a little lower than the angels for the suffering of death." Therefore, as man is since he became subject to death, this is what we see Jesus to be, in His place as man. {1905 ATJ, CWCP 21.2}