XX is the normal chromosomal makeup of a female mammal. XX chromosomes are also present in some female reptiles, insects, and plants. This chromosomal makeup is formed when a male sperm with an X chromosome fertilizes a female ovum. Usually we refer to XX chromosomes, rather than using the letters XX on their own.
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.
