Brief Answer:
genetic determinant

Detailed Answer:
Thanks for posting on HealthCareMagic. Let me try to clarify your doubts. From our conception to our death, things that happen have either a genetic determinant or an external influence or both. Certain things are NOT influenced by external influence. For example, no matter what you do, you would not be able to change the number of bones in the human body (without breaking them of course). In other words, we can say that the fact that humans have 206 bones is genetically determined. But it does not mean that we have identified that gene A on Chromosome B segment C locus D is responsible for this. In a similar way, the size of penis is genetically determined and cannot be externally influenced. There are a number of unidentified genes behind and the final effect is the combination of all these. The components are chosen by a matter of probability and so the genes that have come to you are not the same as the genes that have gone to your brother and hence the difference in size. You would perhaps also note that his height is not exactly same as yours although height is also a genetically determined trait. And that does not mean that science cannot explain these or is not valid in real life scenarios like this.

Moreover, like height that does not grow after a certain age, the penis size also does not increase after the hormonal effects of puberty are completely done with.

Xq11-12 is the locus of Androgen receptor CAG polymorphism and though it might have some indirect correlation, it is related to other functions of the reproductive system, for example human spermatogenesis. X chromosome of a male comes from the mother and Y from the father. But that does not that X is the only determinant although Hox genes that reside on the X chromosome, have an identified role to play. Some genes on Y can also have a major role to play. Male sex hormones have a role to play but it is more on the size of testis than on the size of penis.

It is the head of the penis or glans that has the main friction with the initial one third of the vagina which is the sensitive part. The remaining sensation comes from the friction while the shaft of penis slides. Since it is basically a to and fro movement, the length does not matter and so long as you are going in and out, she keeps getting the sensation irrespective of the size of penis.

In phalloplasty, certain grafts from certain parts of the body are used to artificially increase the length of penis. But that does not mean that it will be fully functional. It would need something called an erectile prosthesis to achieve an erection. I think that you are better off without it. It has more relevance for persons with conditions like hypospadias.

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