Twins can be different genders. Whether twins are the same or different sexes depends on the biological process of their formation. Understanding how twins develop clarifies why some pairs can be a boy and a girl, while others are always the same sex.
Understanding Different-Sex Twins
Different-sex twins, meaning one boy and one girl, are a common occurrence. They arise from dizygotic, or fraternal, twinning. This process begins when two separate eggs are released from the ovary during the same menstrual cycle, and each egg is then fertilized by a different sperm cell.
Two separate zygotes are formed from these distinct eggs and sperm. These zygotes develop independently within the womb, each with its own genetic blueprint. Fraternal twins are genetically as similar as any other siblings, sharing approximately 50 percent of their genes.
The sex of each fraternal twin is determined independently at fertilization. One twin may receive the chromosomal combination for a male, while the other receives the combination for a female, leading to different sexes. This explains why boy-girl twin pairs are always fraternal.
Why Identical Twins Are Always the Same Sex
In contrast to fraternal twins, identical twins, known as monozygotic twins, almost always share the same sex. They originate from a single fertilized egg. After fertilization, this single zygote splits into two separate embryos early in development.
Since both individuals develop from the same fertilized egg, they possess nearly identical genetic material, including the same sex chromosomes. If the original zygote carried the information for a female (XX chromosomes), both twins will be female. If it carried the information for a male (XY chromosomes), both twins will be male.
While extremely rare genetic events or chromosomal abnormalities can lead to identical twins with different sexes, these instances are exceptional and involve complex genetic conditions. For the vast majority of identical twin pairs, their shared genetic origin ensures they are of the same biological sex.
The Genetic Basis of Sex
Human biological sex is determined at conception by sex chromosomes. Females typically inherit two X chromosomes (XX), while males typically inherit one X and one Y chromosome (XY). Every egg cell produced by a female carries an X chromosome.
The sperm cell, however, can carry either an X or a Y chromosome. If a sperm carrying an X chromosome fertilizes the egg, the offspring will be female (XX). If a sperm carrying a Y chromosome fertilizes the egg, the offspring will be male (XY). Therefore, the father’s sperm determines the biological sex of the child.