Related Experiment Video
Updated: May 6, 2026

Dissection of Larval Zebrafish Gonadal Tissue
Published on: April 26, 2017
Two monozygotic (identical) twin pairs discordant for gender identity
1UCLA School of Medicine, Los Angeles, California, USA.
Abstract:
Two pairs of monozygotic twins discordant for gender identity are described. The first set are 8-year-old males, one of whom enjoys doll play, cross-dressing, and taking the role of a female in fantasy games. His brother shows no feminine behavior, enjoys sports, and is more masculine in gestures and speech. The second set are 24-year-old females, one of whom wants to undergo sex-change surgery to male status. Her cotwin is a feminine woman desirous of marrying and bearing children. Differential childhood experiences are described for the cotwins. These highlight environmental influences, which may contribute to variances in masculinity and femininity, when genetic influences are held relatively constant.
Related Concept Videos
The Ratio of X Chromosome to Autosomes
Normal male Drosophila has a ratio of one X chromosome to two sets of autosomes. In contrast, normal female...
X and Y Chromosomes
The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or...
The Y Chromosome Determines Maleness
Evolution
Around 300 million years ago, the two sex chromosomes diverged from two identical autosomal chromosomes. Over time, the Y chromosome has lost most of its genes, shrinking in size....
Sex-linked Disorders
Sex Linked Disorders
Nondisjunction

