In genetics, the sex chromosomes determine whether an individual is male or female. An individual will get an X-chromosome from their mother and a second X-chromosome from their father if they are a female (XX). Males get an X-chromosome from their mother and a Y-chromosome from their father (XY). Therefore, sexual development is based on genes.
Variations on chromosomes can lead to variations in the sexual development and sexual expression of an individual. For example, if a male has an extra X-chromosome (XXY) they will have biological features resembling a female and if a male has an extra Y-chromosome (XYY) they will be taller and often have a learning disability. There are other genetic conditions that do not affect an individual’s sexual development but are the cause of a learning disabilities that make it difficult for them to have the cognitive and emotional structure to manage their sexual expression and sexual health in socially acceptable ways. Fragile X syndrome is an example of this.