The first step in the decoding process is to determine the gender of the baby. Is the baby a boy (XY) or a girl (XX)? Both male and female DNA profiles will have numbered chromosomes and both will have X chromosomes; however, only males will have the Y chromosome.
The DNA profile of a fetal sample has two variations of the same gene (alleles), one donated by the biological mother and the other from the biological father. The DNA profile of the biological mother will be compared to the DNA profile of the fetus. Any genetic markers that do not come from the biological mother would then come from the biological father. In the case of two XX chromosomes, the fetus is female. In the case of one X chromosome and one Y chromosome, the fetus is a male. The Y-chromosomes of the father’s DNA profile and the son’s DNA profile are compared. If the DNA profile provided is that of the real biological father, then the two Y-chromosomes should be identical because the Y chromosome is only passed from father to son. The X chromosome is passed from the biological mother, who as a female has two X chromosomes.
In DNA testing, profiles with exclusions indicate that the X and Y letters were not obtained from the mother or father’s DNA profiles as provided for testing and therefore the DNA profile belongs to another individual. Remember: every individual’s DNA profile is unique unless there is an identical twin, and therefore it is highly improbable that there would be a matching DNA profile from a random man in the human population.