Girl or boy, a question of chromosomes

Girl or boy, a question of chromosomes


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The sex of your future baby is determined genetically from fertilization. The girl is born from the fusion between an egg and a sperm carrying an X chromosome. The boy is born from the fusion between an egg and a sperm carrying a Y chromosome. Explanations.

Girl or boy, how's it going?

  • At the time of fertilization, the egg and sperm are the only cells in the body that have only 23 chromosomes each (instead of 46 in all other cells): 22 identical chromosomes and 1 sex chromosome.
  • The egg contains: 22 chromosomes + 1 sex chromosome X. The spermatozoon contains: 22 chromosomes + 1 sex chromosome X or 1 sex chromosome Y.
  • When spermatozoa and ovules unite, two solutions are possible. If the egg is fertilized by a sperm with sex chromosomes XY ... it is a boy who will see the light of day. Fertilized by a spermatozoon with XX chromosomes, it will be a girl.

1 girl (XX) = union of a spermatozoon X and an egg X

1 boy (YX) = union of a Y sperm and an ovum X

How is your baby's sex formed?

  • It is only around the 8th and 9th weeks of pregnancy that the genitals of the fetus take shape. Before, they are identical in boys and girls. Gradually, a small bud appears between the legs of the embryo. It will grow to make up the boy's penis or the girl's clitoris. Tissues are formed on each side of a cleft: they merge and constitute the scrotum (skin envelope of the testicles) in boys, the vaginal wall in the girl whose slit does not close.
  • Around the 17th week, we can easily distinguish the penis. The testicles are still in the abdominal cavity. They will not descend to the scrotum until the end of the pregnancy.

Girl or boy, it's nature that decides

  • It is therefore understandable that it is the sex chromosome of the sperm (X or Y), therefore the father, which determines the sex of the child. The spermatozoa carrying Y are a little more numerous, faster but also more fragile than the X. It is thus born 105 boys for 100 girls. This is called parity!
  • And you do not want to cheat to have a girl or rather a boy: no method - increase or decrease the acidity of the vaginal environment, have sex as close as possible or remote from ovulation, eat sweeter or saltier - has no scientific justification. Worse, some unbalanced diets are to be avoided.

Baby sex: our file