Turner syndrome is a genetic condition that happens when a female is missing all or part of one of her two X chromosomes. Normally, females have two X chromosomes (XX), but in Turner syndrome, one X chromosome is either completely absent or partially missing. This means instead of having the usual 46 chromosomes, a person with Turner syndrome has 45 chromosomes, with only one full X chromosome present. This missing or incomplete X chromosome disrupts normal development, especially affecting sexual development and growth.
The cause of Turner syndrome lies in errors during the formation of reproductive cells (eggs or sperm) or very early after fertilization. When an egg or sperm cell divides to form a new individual, chromosomes are supposed to separate evenly so that each new cell gets the correct number. Sometimes, however, a chromosome fails to separate properly, a process called nondisjunction. If this happens with the sex chromosomes, the resulting embryo may end up with only one X chromosome instead of two. This error can occur in the mother’s egg or the father’s sperm, or shortly after fertilization when the embryo’s cells begin to divide.
In some cases, Turner syndrome results from mosaicism. This means that some cells in the body have the normal two X chromosomes, while others have only one. Mosaicism happens when the chromosome loss occurs after fertilization, during one of the early cell divisions. Because of this mixture of cells, the symptoms of Turner syndrome can vary widely depending on how many cells are affected and where they are in the body.
Another cause can be structural abnormalities of the X chromosome. Instead of being completely missing, part of the X chromosome may be deleted or altered. This partial loss can also disrupt the genes necessary for normal development. The X chromosome carries many genes important for growth, sexual development, and other bodily functions, so losing part of it can have significant effects.
Since the Y chromosome is absent in Turner syndrome, the embryo develops as female, but the lack of a second X chromosome means the ovaries do not develop properly. Instead of fully formed ovaries, individuals with Turner syndrome have streak ovaries, which are mostly fibrous tissue and cannot produce eggs or the hormones needed for puberty and fertility. This leads to many of the physical features and medical issues associated with the syndrome.
The exact reason why nondisjunction or chromosome loss happens is not fully understood. It is generally considered a random event during the formation of reproductive cells or early embryonic development. It is not usually inherited from parents, meaning most cases of Turner syndrome occur sporadically without a family history.
Because the X chromosome contains many genes that regulate growth and development, losing one copy affects multiple systems in the body. This explains why Turner syndrome is associated with short stature, distinctive facial features like a webbed neck and low-set ears, heart abnormalities, and underdeveloped sexual characteristics. The missing genetic material disrupts the normal signaling pathways that guide how cells grow and organs form.
In summary, Turner syndrome is caused by the absence or abnormality of one X chromosome in females, due to errors in chromosome separation during the formation of eggs or sperm or early embryonic cell division. This leads to a wide range of developmental challenges because the genes on the missing or altered X chromosome are crucial for normal growth and sexual development.
