It is my belief that Mary’s DNA became Jesus’ DNA. Something like a clone produced by genetic engineers. However, if this alone were the case, the Messiah would have been female.
A male human or any mammal has a Y chromosome. The mother cannot supply this chromosome since she only has X chromosomes; only the father can pass on the Y chromosome.
Thus, a second miracle must have taken place.
The Y chromosome is much shorter than the X chromosome, but it carries one crucial gene for maleness. This gene is called TDF standing for testis-determining factor. The same gene has also been named SYR standing for sex-determining Y region.
This TDF or SYR gene is switched on very early in fetal development. It produces a protein transcription factor that causes the gonads to develop into testes. The testes produce testosterone that inhibits the development of female structures and causes male structures to develop instead in the fetus, the child and later in the adult.
Both males and females have testosterone, but males produce five times more testosterone than females; it is the higher level of this hormone in males that has masculinising effects.
The sex chromosomes in most males are XY. However, in rare cases a male can have an XXY genotype. It has also been known that a male had a genotype that was XX, but the TDF gene had been transferred to an X chromosome through mutation. Despite having female chromosomes, this one gene made the individual male. The condition is called XX male syndrome. These individuals are infertile. There are also people with an XY genotype, but who lack the TDF gene, and they are female.
Therefore, for Mary to give birth to a male Messiah, one of two possibilities must have occurred:
Either God created a whole Y chromosome and added it to the genome of the egg cell provided by Mary giving an XXY genotype, or God created only the TDF gene and inserted it into one of the X chromosomes.
Thus, the minimum requirement for the incarnation of a male Messiah is firstly that Mary supplied a diploid egg cell rather than the normal haploid egg cell. Secondly, that God created one gene – the TDF gene directly and inserted it into the genome.
The other option is that God created a full set of autosomal chromosomes carrying every human gene and a whole Y chromosome. I have opted to believe in the minimum requirement.
The Principles of Creation 