XX

The XX chromosomes come from the sex cells of two parents. The female ovum always contains an X chromosome. Sperm can have an X chromosome or a Y chromosome. When a sperm with an X chromosome fertilizes an ovum, the new cell has both X chromosomes. This new cell, called a zygote grows into an embryo and later a baby.

Babies with XX chromosomes are usually assigned the female gender at birth. These chromosomes usually give babies female characteristics including vaginas and working ovaries.

XX chromosomes usually make someone a cisgender female. However, this doesn’t always hold true. People with XX Male Syndrome appear as male, despite having XX chromosomes. It can occur when the tip of the Y chromosome is translocated to the X chromosome in utero. There are also people born with XX chromosomes and ambiguous genitalia. Transgender people born with XX chromosomes usually identify as male.

Genes on the X chromosome are X-linked genes. Red-green colorblindness is an example of an x-linked gene. As people with XY chromosomes have just one X chromosome, any X-linked genes they have are always expressed. This isn’t the case for people with XX chromosomes. If they have one X chromosome with a mutation and one without, the normal X chromosome could mask the abnormal trait. This is why more men are colorblind than women.