Gender and Sex Chromosomes

Rewritten: March 24, 2025

A person's biological sex is determined by the combination of sex chromosomes (X and Y), and the SRY gene on the Y chromosome drives differentiation toward male development. With seeDNA's next-generation fetal sex testing, highly accurate sex determination is possible from as early as 7 weeks of pregnancy.

A person's sex is determined by the combination of sex chromosomes

In general, a person's biological sex is determined by the combination of sex chromosomes, known as the X chromosome and the Y chromosome. Males have the combination "XY," while females have "XX." Human somatic cells contain a total of 46 chromosomes (23 pairs), of which 22 pairs are autosomes and the remaining pair are sex chromosomes. This combination of sex chromosomes is the fundamental factor determining the sex of a fetus [ref:1].

Basic structure and role of the sex chromosomes

The X chromosome is relatively large and contains approximately 800 to 900 genes. The Y chromosome, on the other hand, is the smallest chromosome in humans, containing only about 50 to 60 genes [ref:2]. However, the Y chromosome carries the SRY gene (Sex-determining Region Y), which plays an extremely important role in human sex determination.

Every human, regardless of sex, has at least one X chromosome, which also encodes many genes essential for life, including those involved in immune function and blood clotting factors. Because females have two X chromosomes, one of them is largely inactivated through a phenomenon called "X-chromosome inactivation," which maintains a balanced level of gene expression.

The mechanism of sexual differentiation in the fertilized egg

To briefly explain sexual differentiation, a human fertilized egg is fundamentally destined to become female by default. This is known as the "default female hypothesis," one of the core concepts in the study of sexual differentiation.

When it receives a signal from the SRY gene on the Y chromosome, the undifferentiated gonad develops into testes, male genitalia form, and the individual differentiates as male. Without that signal, ovaries form instead, and the individual differentiates as female [ref:1].

More specifically, when the SRY gene is expressed, it activates a transcription factor called SOX9, which promotes testis formation. The testes then secrete testosterone (a male hormone) and anti-Müllerian hormone (AMH); testosterone promotes the development of male reproductive organs (such as the vas deferens, seminal vesicles, and prostate), while AMH causes regression of the Müllerian ducts (the precursor of female reproductive organs). Through the coordinated action of these hormones, a male body is formed [ref:3].

The complexity and diversity of sexual differentiation

While the sex chromosome combination is usually either "XX" or "XY," there are rare cases in which variations occur in the number or structure of the sex chromosomes. For example, in Klinefelter syndrome (47,XXY) a male has an extra X chromosome, and in Turner syndrome (45,X) a female has only a single X chromosome [ref:4].

Recent research has also revealed that sex determination is not a simple binary choice but, at the genetic level, has aspects that resemble a gradient. Besides the SRY gene, genes such as WNT4, RSPO1, and FOXL2 play an active role in maintaining differentiation toward female development, and it has become clear that sexual differentiation is established through a delicate balance between "masculinizing signals" and "feminizing signals" [ref:1].

How sex chromosomes are inherited

To understand how a baby's sex is determined, it is important to know which sex chromosome the sperm and egg each carry.

• Egg: Always carries one X chromosome from the mother (XX)
• Sperm: Carries either one X chromosome or one Y chromosome from the father (XY)
• If a sperm carrying an X chromosome fertilizes the egg, the result is "XX" → a girl
• If a sperm carrying a Y chromosome fertilizes the egg, the result is "XY" → a boy

In other words, a baby's sex is determined by whether the sperm from the father carries an X chromosome or a Y chromosome. In theory, the probability of having a boy or a girl is roughly 50:50, but it is statistically known that the actual birth ratio tends to be slightly higher for boys [ref:5].

Next-generation fetal sex testing can be performed from 7 weeks of pregnancy

Generally, when determining fetal sex via ultrasound examination at an obstetric clinic, it can be confirmed from around 16 weeks of pregnancy. Because ultrasound relies on visually checking the shape of the fetus's external genitalia, accurate determination can sometimes be difficult depending on the fetus's position or degree of growth.

By contrast, our company seeDNA's next-generation fetal sex testing allows determination from as early as 7 weeks into pregnancy, in the first trimester. This means a determination roughly 9 weeks earlier than with ultrasound.

How next-generation DNA analysis technology works

This is the latest technology, using a proprietary genetic testing method that employs a Next Generation Sequencer (NGS), to check the fetus's sex chromosomes from the mother's blood during pregnancy and determine sex.

During pregnancy, a small amount of DNA fragments called cell-free DNA (cfDNA), originating from the fetus, circulates in the mother's blood via the placenta. This cfDNA is present in maternal blood from early pregnancy, and its proportion increases as the pregnancy progresses. In next-generation fetal sex testing, this cfDNA is analyzed with high sensitivity, and sex is determined based on whether DNA sequences derived from the Y chromosome are detected.

1. Blood is collected from the pregnant woman using a blood collection kit
2. Cell-free DNA (cfDNA) is extracted from the blood sample
3. High-precision analysis is performed using a next-generation sequencer (NGS)
4. The presence or absence of Y-chromosome-derived DNA sequences is checked
5. The sex determination result is reported

Thanks to this technology, accurate sex determination is possible even in early pregnancy, when the fetus's genitalia cannot yet be confirmed by ultrasound. Because there is no need to wait for the external genitalia to develop, highly reliable results can be obtained at a very early stage [ref:2].

Why seeDNA's next-generation fetal sex testing is chosen

seeDNA is the only DNA testing provider in Japan that can determine sex from 7 weeks of pregnancy. This early determination is made possible by highly sensitive next-generation sequencing technology combined with a proprietary analysis algorithm built on extensive clinical data.

What's more, we are currently running a special campaign offering next-generation sex testing at half the regular price.

For details, please check the page below.
[Next-generation Fetal Sex DNA Testing Campaign Information]

This is a limited-time campaign, so please take advantage of this great opportunity to apply.

Summary

A person's biological sex is determined by the combination of sex chromosomes, the X chromosome and the Y chromosome. A human fertilized egg is, by default, set to differentiate into a female, and it differentiates into a male upon receiving a signal from the SRY gene on the Y chromosome. This elaborate mechanism of sexual differentiation is a complex process involving multiple hormones, including testosterone and anti-Müllerian hormone.

With a typical ultrasound examination, one must wait until around 16 weeks of pregnancy to confirm the sex, but with seeDNA's next-generation fetal sex testing, highly accurate sex determination is possible from as early as 7 weeks of pregnancy by analyzing fetus-derived cfDNA in the mother's blood. If you would like to know your baby's sex from early in pregnancy, please consider seeDNA's next-generation fetal sex testing.

Frequently Asked Questions

Q1. What are sex chromosomes?

A. Sex chromosomes are the one pair, out of the 23 pairs of chromosomes humans have, that is involved in determining sex. There are two types, the X chromosome and the Y chromosome; females have the combination XX, and males have XY. The remaining 22 pairs are called "autosomes" and are not directly involved in sex determination.

Q2. Why does a fertilized egg basically differentiate into a female by default?

A. The default setting for sexual differentiation in a human fertilized egg leans toward female; without a specific signal from the SRY gene on the Y chromosome, the undifferentiated gonad develops into ovaries and the individual differentiates as female. Only when the SRY gene is present and expressed do testes form, and differentiation toward male proceeds through the action of male hormones — which is why it is described as "default female" (the default setting is female).

Q3. Why is next-generation fetal sex testing possible from 7 weeks of pregnancy?

A. During pregnancy, fetus-derived cell-free DNA (cfDNA) circulates in the mother's blood via the placenta. This cfDNA can be detected from early pregnancy, and because seeDNA uses a highly sensitive next-generation sequencer (NGS) to analyze whether Y-chromosome-derived DNA sequences are present, sex determination is possible from 7 weeks of pregnancy without waiting for the external genitalia to develop.

Q4. What is the SRY gene?

A. The SRY gene (Sex-determining Region Y) is a sex-determining gene located on the short arm of the Y chromosome. When this gene is expressed, it activates a transcription factor called SOX9, initiating a signaling cascade that differentiates the undifferentiated gonad into testes. The SRY gene is considered the most important "master switch" in mammalian sex determination.

Q5. What is the difference between ultrasound examination and next-generation fetal sex testing?

A. Ultrasound examination is a method that uses sound waves to visually confirm the shape of the fetus's external genitalia, and it can be used for determination from around 16 weeks of pregnancy. Next-generation fetal sex testing, on the other hand, is a method that analyzes fetus-derived DNA (cfDNA) contained in the mother's blood at the genetic level, and can be used for determination from 7 weeks of pregnancy. Both are non-invasive tests, but next-generation fetal sex testing allows you to learn the sex about 9 weeks earlier.

Q6. Is a baby's sex determined by the father or the mother?

A. A baby's sex is determined by whether the father's sperm carries an X chromosome or a Y chromosome. Since the mother's egg always carries an X chromosome, fertilization by an X-bearing sperm results in XX (a girl), while fertilization by a Y-bearing sperm results in XY (a boy). In other words, genetically, the sex chromosome from the father is the determining factor in the baby's sex.

[References]