One of the most common and earliest questions people have asked is whether a baby will be a girl or a boy. Today, science gives us a simple answer: A special gene called the SRY gene, found on the Y chromosome, decides this. If the SRY gene is present, the baby develops as a boy. If it is not there, the baby develops as a girl.

While this general rule applies to most cases, rare exceptions exist where some females possess the SRY gene but still develop as healthy biological females. These rare cases have been reported only three times in medical history. In 2024 alone, two such cases were found—one by researchers at Renato Dulbecco University Hospital in Italy and another by Cincinnati Children’s Hospital Medical Centre, USA. These findings help scientists understand the role of the SRY gene in male development.

How does a baby's sex get decided?

Every human has 23 pairs of chromosomes—a total of 46. These include 22 pairs of regular (autosomal) chromosomes and one pair of sex chromosomes (XX or XY). The sex chromosomes determine whether a baby will be male or female.

  • All eggs contain an X chromosome.
  • Sperm cells carry either an X or a Y chromosome.
  • If a sperm with an X chromosome fertilises the egg, the baby will be female (XX).
  • If a sperm with a Y chromosome fertilises the egg, the baby will be male (XY).

The Y chromosome carries the SRY gene, which plays a key role in directing the embryo to develop as a male. If this gene is absent or does not function, the embryo develops as a female by default.

What happens when the SRY gene is present in females?

Sometimes, a rare mutation occurs where the SRY gene moves from the Y chromosome to the X chromosome. This mutation is called a translocation. When a sperm carrying an X chromosome with the translocated SRY gene fertilises an egg, the resulting baby will have an XX chromosome pattern but also carry the SRY gene.

Typical effects of the SRY gene in XX individuals

In most cases, XX individuals with the SRY gene develop as males. They form testes, which produce the male hormone testosterone, leading to male characteristics. However, these individuals are usually infertile because they lack other Y chromosome genes necessary for sperm production. Since they cannot have children, the SRY gene is not passed down.

Exception: XX females with the SRY gene

In the exceptional cases found in Italy and the USA, researchers discovered XX females who had the SRY gene but still developed as normal females. The Italian study found four healthy females across three generations of a single family who carried the SRY gene. This suggests that the gene can be passed through generations via fertile females. The US study found a similar case in a female fetus, who was born healthy but was not studied further after birth.

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Why did these SRY-positive XX individuals develop as females?

The key factor that allowed these individuals to develop as females was the inactivation of the SRY gene. Here’s how:

  • Every female has two X chromosomes, but in each cell, one X chromosome is randomly inactivated to balance gene expression between males (XY) and females (XX).
  • In these rare cases, the X chromosome carrying the SRY gene was inactivated, preventing male development.
  • Additionally, these translocations also involved the loss of a small portion of the X chromosome, which affected how the gene worked.

Because the SRY gene was silenced, these individuals followed the usual female developmental pathway, even though they technically had a gene that should have made them male.

These findings reinforce the importance of the SRY gene in male development while also showing that specific genetic mutations can override its effects. Scientists believe that similar cases may be hidden in the population, undetected due to the lack of genetic testing.

Additionally, researchers are concerned that low levels of SRY gene expression later in life might lead to disorders in sexual development. However, long-term studies would be required to confirm this possibility. Unfortunately, in the US case, the family did not allow further follow-ups after the baby’s birth.

The future of genetic research

With advanced genome sequencing, scientists now have access to the genetic data of thousands of individuals. By identifying more cases of SRY-bearing translocations, genetic counsellors can help parents understand potential implications for their children. This knowledge could improve medical guidance for families dealing with rare genetic conditions.

The presence of the SRY gene on the Y chromosome is usually what makes a baby male. However, in rare cases, the SRY gene can move to an X chromosome, and if it becomes inactive, the baby develops as a female instead. Recent discoveries of SRY-positive females show that genetic mutations can create exceptions to established biological rules.

These cases highlight how complex genetics can be and how much more there is to learn about human development. With continued research, scientists hope to better understand these rare conditions and provide improved genetic counselling in the future.

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