When you think about the term biological sex, you may only think of male and female. However, biological sex is not binary. Though male and female, or XY and XX are the most commonly presenting biological sex assignments, biological sex is more complicated.

It might be helpful to think about biological sex in two ways: genetic sex identification and morphological sex identification. The genetic sex of a person is their chromosomes. For example, XX and XY represent female and male chromosomal sex. Morphological sex represents reproductive organs, external genitalia, and commonly associated sexual characteristics. During pregnancy, an expecting parent may wish to find out the biological sex of their baby. Typically, sex is confirmed through a visual identification via ultrasound. This process may confirm morphological sex, but genetic testing would be required to confirm the chromosomal makeup of the fetus.

While it is most common to find XY chromosomes presenting with external male genitalia and XX presenting with external female genitalia, this is not always the case. In some instances, the formed genitalia do not align with the chromosomal sex. Broadly, this is referred to as intersex. The prevalence of intersex individuals is somewhat controversial. Some research suggests that intersex individuals make up 1.7 percent of the population, but other research suggests that they make up only 0.018 percent of the population (Sax, 2002). However, research results are dependent on how the data was collected, the inclusion criteria, and how representative the sample is, among many other factors. In later chapters, we will include information on the physical and sexual development of intersex individuals.

Chromosomal Abnormalities

Chromosomal abnormalities refer to alterations in the structure or number of chromosomes in an individual’s cells. These abnormalities can occur during the formation of reproductive cells (eggs and sperm) or during early development after fertilization. Chromosomal abnormalities can have various effects on an individual’s health and development, ranging from mild to severe.

Types of Chromosomal Abnormalities

Structural abnormalities occur when parts of chromosomes are missing, duplicated, inverted, or rearranged:

• Deletions: Portions of a chromosome are missing.
• Duplications: Sections of a chromosome are repeated.
• Inversions: A segment of the chromosome breaks off and reattaches in reverse orientation.
• Translocations: Segments of one chromosome break off and attach to another chromosome.

Numerical abnormalities involve changes in the number of chromosomes and include the following:

• Aneuploidy: An abnormal number of chromosomes, such as trisomy (an extra chromosome) or monosomy (a missing chromosome). Examples include Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).
• Polyploidy: The presence of additional sets of chromosomes beyond the usual diploid number (46 chromosomes in humans). Examples include triploidy (three sets of chromosomes) or tetraploidy (four sets of chromosomes).
• Mosaicism: This occurs when an individual has cells with different chromosomal compositions. For example, some cells may have a normal chromosomal complement, while others have an abnormal one.
• Ring chromosomes: In this abnormality, a chromosome forms a ring structure due to loss of genetic material at both ends and subsequent fusion.
• Marker chromosomes: These are small, additional chromosomes that lack a distinct banding pattern and often contain genetic material of uncertain origin.
• Uniparental disomy (UPD): In UPD, an individual inherits both copies of a chromosome from one parent and none from the other. This can occur when both chromosomes are inherited from one parent or when there is a duplication of one parent’s chromosome and the absence of the other parent’s chromosome.

Chromosomal abnormalities can have a wide range of effects, from causing no noticeable symptoms to resulting in severe developmental disabilities or health problems. The specific consequences depend on factors such as the type of abnormality, the chromosomes involved, and the extent of the alteration.

Optional resource:

Interested to learn more about chromosomal variations? Watch Alteration of Chromosome Number and Structure [Streaming Video].

Licenses and Attributions for Determining the Sex of the Child

“Determining the Sex of the Child” by Terese Jones is licensed under CC BY 4.0.