Human beings have 22 sets of paired chromosomes plus two sex chromosomes which in the female are XX and in the male are XY.
Mary would certainly have supplied the maternal set of chromosomes, but what of the paternal set in the absence of a biological father?
It is not possible for a human being to have a haploid genome. Only bacteria have haploid genomes with only one copy of each gene. All multicellular organisms including humans have diploid genomes with two sets of chromosomes so that each gene is represented by two copies called gene alleles. One gene allele is of maternal origin and the other corresponding gene allele is of paternal origin for each gene.
It is our belief as Christians that Jesus was fully man and fully God.
There are two possibilities: Either God created a complete set of 22 paternal chromosomes and a Y chromosome carrying one copy of every human gene at the moment of the overshadowing. Or the Virgin Mary miraculously supplied an egg cell that was diploid and contained the full set of 23 paired chromosomes that included the two X chromosomes.
Normal egg cells are haploid because they have undergone the reduction divisions of meiosis when they were formed as gametes. The eggs in a woman’s ovaries were formed while the woman was still a fetus herself in her mother’s womb. When the woman becomes an adult, the eggs are released one per month from the ovary until they run out and menopause is reached.
The more minimal belief is that the Virgin Mary supplied a diploid egg cell that had not undergone reduction divisions like other egg cells. This diploid egg cell contained the full complement of chromosomes containing the two copies of each human gene.
If this were the case, the special egg cell would have been prepared in the ovary of the Virgin Mary while she herself was a fetus in the womb of St Anne. Traditionally, Catholics believe that Mary was chosen to be the mother of the Messiah at her conception in the womb of St Anne, her mother. (This is named the Immaculate Conception).
Thus, Mary would have been chosen to be the mother of the Messiah before she was born. A special egg cell that had not undergone reduction divisions would have to have been designated in Mary’s ovaries before her birth, awaiting her ‘yes’ to God’s plan at the appointed time.