How a Baby's Gender is Determined

Females have an XX pair of sex chromosomes, and males, an XY pair. A baby’s gender is determined by the sperm cell that fertilizes a woman’s egg. Sperm carry one sex chromosome, either an Y (male) or X (female).

To better understand, let’s look at how sperm develop. A human sperm results from the division of a cell containing 46 chromosome pairs, including one pair of sex chromosomes: one X and one Y. So exactly half of a man’s sperm have Y (male) chromosomes and half X (female) chromosomes. Other than the two sex chromosomes, the 22 other chromosome pairs that the sperm carries are genetically identical.

The sperm also carries half of an embryo’s DNA; the egg holds the other half. When the two combine, following sexual intercourse or fertilization in a lab (IVF), the resulting embryo will have a full 46 chromosome pairs. One pair determines the gender, and the other 22 pairs determine everything else about the baby, including eye color, height and body type.   

The basics…

  • If an egg is fertilized by an X-bearing sperm, the resulting embryo will have two X chromosomes (XX), and will grow into a baby girl. If a Y-bearing sperm fertilizes the egg, the embryo will have XY chromosomes, a boy. (Except in the rare case of testicular feminization.) 
  • Since men’s sperm carry both male and female chromosomes (Y & X) in 50:50 proportion, and women’s eggs carry only one female chromosome (only X, because her sex chromosomes are XX), the man’s sperm holds the key to a baby’s gender.

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