The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
John 1:18 - "only begotten Son"
"No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him."
There are really two problems here, although only one appears on the surface. Should the proper translation be "only begotten Son" or should it be as the New American Standard Version renders it, "only begotten God"? This particular problem is not translational but textual because there is a difference in the Greek texts underlining these two translations. However, there is another problem that has to do with the Greek word monogenes. Both the King James and the New American Standard correctly translate it as only begotten. There is a growing movement to understand this word as unique, one of a kind, or simply only. We will deal with this difference first.
Many of the current handbooks on Greek syntax state that monogenes should not be translated as only begotten.  Instead, they take the word to mean only or unique. If this were true, the translation of the KJV would not be alone in its "error" for this is the translation of the New American Standard Version, the New King James Version, and several other translations of the twentieth century.
The problem here is a misunderstanding of the Greek language (both Koine and Modern). The word monogenes does means one or unique in the sense that an only child is the only one of his parents. It does not mean unique, as in special, such as in the phrase, "his work is very unique." Here the Greek would be monadikos, not monogenes. As we examine the New Testament we find the word monogenes used eight times (not counting its usage here in John 1:18). In every case it is used to describe a relationship between a parent and child (Luke 7:12; 8:42; 9:38; John 1:14; 3:16, 18; Hebrews 11:17; 1 John 4:9). Since this is how the Holy Spirit uses the word in the New Testament, we must accept this definition when reading John 1:18. 
The evidence establishes that Jesus Christ, although God (John 1:1), is also the only begotten Son of God. None other can claim hold to this title. Those who accept Christ as their personal Savior are spiritually born of God and are called His sons (John 1:12). But no human can lay claim to the title of only begotten Son. This phrase has not only to do with Christ's virgin birth, but also His eternal place within the Trinity.
Having established this point, we are now faced with the question of the word following monogenes. Should it be heios (Son) or theos (God)? The oldest known Greek manuscripts, P66 and P75, read only begotten God. However, these manuscripts all come from the Alexandrian line and smack of ancient Gnosticism. The Gnostics taught that Christ was a begotten god, created by God the Father, whom they called the Unbegotten God.
When those who had been tainted with Gnosticism cite John 1:18, they cite it as only begotten God. Such is true of Tatian (second century), Valentinus (second century), Clement of Alexandria (215 AD), and Arius (336 AD). On the other hand, we find many of the orthodox fathers who opposed Gnosticism quoting John 1:18 as only begotten Son (Irenaeus, Tertullian, Basil, Gregory Nazianzus, and Chrysostom). 
Even some that served on the textual committee for the UBS-4 recognized that the proper reading of John 1:18 is only begotten Son. Dr. Allen Wilkgren, who served on the committee, writes, "It is doubtful that the author (i.e., John) would have written monogenes theos, which may be a primitive, transcriptional error in the Alexandrian tradition."  Additionally, Professor Bart Ehrman of the University of North Carolina at Chapel Hill has noted that he believes the original reading is monogenes heios and not monogenes theos.  Although Professor Ehrman did not serve on the UBS-4 committee, he is a recognized scholar in the field of Biblical textual criticism.  Thus, not all scholars agree as to the original reading in this regard.
The majority of orthodox church fathers support the reading monogenes heios, as do the majority of existing Greek cursive manuscripts. The reading contained in the majority of uncials (such as A, C3, K, W, Q, Y, D, P, X, and 063), Old Latin, Latin Vulgate, and the Old Syrian also support the reading monogenes heios.
Since we know the Greek word monogenes concerns the parent/child relationship, and that God is never called monogenes (accept for Christ in His relationship to the Father), it is clear that monogenes heios is the correct reading.
 See, Newman and Nida, A Translator's Handbook on the Gospel of John (New York: United Bible Societies, 1980), 24. Also, Moulton and Milligan, The Vocabulary of the Greek Testament (Grand Rapids: Eerdman's, 1930), 416-417. However, others recognize that monogenesmeans only begotten. See, Thayer, Greek-English Lexicon of the New Testament (Milford, MI: Mott Media, 1977 ed.), 417-418. Moulton, The Analytical Greek Lexicon Revised (Grand Rapids: Zondervan, 1978 ed.), 272. And, Prestige, God in Patristic Thought (London: SPCK, 1952) 37-51, 135-141, 151-156.
 It has further been established that the word monogeneshas as its root word genos. Again, some have suggested that this root word means kind or type. This is true, but again in the sense that those who are born of a given parentage are a certain type or kind. The Greek word genos appears twenty-one times in the New Testament. It is translated as kind, nation, stock (of Abraham), nation, offspring, kindred, generation, and country in the KJV, demonstrating the word has to do with descendents. The New International Version translates it as born in Mark 7:26, and the New American Standard Version translates it as birth in Acts 4:36.
 It is also interesting to note that the New World Translation of the Jehovah's Witnesses also uses the phrase only begotten god. This is, of course, in line with their teaching that Christ is a created god. Once we accept the reading only begotten god, we have opened the door to reinterpret all other verses concerning the deity of Jesus Christ.
 Bruce Metzger, A Textual Commentary On The Greek New Testament (New York: United Bible Societies, 2nd ed.), 170.
 Bart D. Ehrman, The Orthodox Corruption Of Scripture (New York: Oxford University Press, 1993), 78-82.
 In fairness to Professor Ehrman's position he does not support the Traditional Text, and his support for the traditional reading here should not be taken as an endorsement of that text. Ehrman believes that many of the textual variants are a result of scribes seeking to establish orthodox Christianity by altering the text in favor of orthodoxy. It is his hypothesis that John 1:18 in the Critical Text is an orthodox corruption, stating that Christ was the unique God. Thereby supporting the orthodox view regarding the Deity of Jesus Christ.
My hypothesis is that many of the textual variants were caused by scribal corruption. However, not by orthodox scribes seeking to establish orthodoxy, but by heretical scribes seeking to corrupt Scripture to support their false doctrines. Once we understand that monogenes theos does not mean that Christ is uniquely God, but instead would be understood as a begotten god, we have a reading that would support the Gnostic teaching which proclaimed this very heresy. When we consider those in the second, third, and fourth centuries who support this false reading and the doctrine they held in this regard, it is not far fetched to draw such conclusions. If one accepts Ehrman's position as feasible than they should also be willing to accept the possibility of the opposite being true. Namely, that the corruption of the text may be afforded to various heretical groups who sought to move the text of Scripture away from Biblical orthodoxy and toward their heretical position.