In this blog post, we’ll examine the characteristics and roles of basosomes formed during X-chromosome inactivation in mammals, as well as the principle behind the varying coat colors of mottled cats.
You’ve probably either owned a cat or dog yourself or seen friends and family members raising them. When these cats or dogs have litters, you can observe some interesting phenomena: for example, the kittens’ coat colors may differ from their parents’; even identical twins may have different coat colors; or a single individual may exhibit a mottled pattern where the color varies depending on the body part. Furthermore, you can observe beautifully harmonious color patterns in nature, such as the beautiful markings on carp or the vibrant colors of corn kernels hanging from the eaves of a rural home. However, not all of these diverse colors and patterns are caused by Barr bodies; in particular, certain markings seen in cats are influenced by coat color-related genes located on the X chromosome and by X-chromosome inactivation. So why do these phenomena occur primarily in females?
It is precisely because of the “Barr body.” A Barr body is a condensed, inactivated X chromosome found in the somatic cell nuclei of most female mammals; it is a mass of heterochromatin that stains darkly during interphase of cell division. In other words, a Barr body can be described as a condensed form of the inactivated X chromosome. So why is the Barr body formed? Generally, female mammals have two X chromosomes, while males have one. If both of a female’s X chromosomes were expressed at the same level, the gene products present on the X chromosomes could be produced in excess compared to males. To prevent this and balance the levels of X-chromosome gene expression between females and males, one of the two X chromosomes is inactivated in female cells. This phenomenon is called X-chromosome inactivation, and the key hypothesis regarding this process is known as the “Lyon hypothesis.” The process by which an X chromosome is inactivated is also referred to as “Lyonization.” However, this process of X-chromosome inactivation cannot be simply explained as a mechanism for preventing chromosomal number abnormalities such as Down syndrome. Down syndrome is a disorder caused by trisomy of chromosome 21—the presence of an extra copy of chromosome 21—and is a chromosomal abnormality distinct from X-chromosome inactivation. The primary purpose of X-chromosome inactivation is to regulate gene expression on the X chromosome, thereby compensating for differences in gene dosage between the sexes. The process is as follows:
First, X-chromosome inactivation begins in a region called the X-inactivation center (XIC), located near the kinetochore—the region where the first constriction forms on chromosomes during cell division. The XIC contains several genetic elements that regulate X-chromosome inactivation, among which XIST and TSIX play key roles. The XIST gene does not encode a protein but produces a long non-coding RNA; this RNA covers the X chromosome to be inactivated, recruiting various chromatin-regulating proteins and repressive complexes to suppress gene expression on that X chromosome.
Subsequently, a series of epigenetic changes—including histone modifications, DNA methylation, and chromatin condensation—occur sequentially, gradually transitioning the X chromosome into an inactivated state and ultimately leading to the formation of a condensed body called the X body. TSIX is a long non-coding RNA transcribed in the opposite direction of XIST and is known to be involved in regulating XIST expression. Therefore, X chromosome inactivation is not achieved through a single process in which a specific gene directly methylates DNA, but rather through the sequential action of various epigenetic regulatory mechanisms centered on XIST.
While this process leads to the formation of the basosome, how is the X chromosome to be inactivated determined? Although the regulatory mechanism for which of the two X chromosomes is inactivated varies depending on the cell, species, and developmental stage, in many mammalian somatic cells, one of the two X chromosomes is randomly selected and inactivated during early development. Once it is determined which X chromosome is inactivated in a given cell, the daughter cells derived from that cell generally maintain the inactivation of the same X chromosome. Therefore, the inactivated X chromosome may differ from cell to cell in a single female individual, and phenotypic variations may be observed due to differences in X chromosome expression between cells. This is the secret behind the mottled pattern in cats, which is related to the coat color genes located on the X chromosome.
Some genes that determine coat color are located on the X chromosome. For example, the gene associated with a cat’s orange coat is located on the X chromosome. Therefore, in females with different alleles, one X chromosome may be active in certain cells, resulting in one coat color, while another X chromosome may be active in other cells, resulting in a different coat color. As these cells form distinct areas on the skin, a pattern emerges where multiple coat colors are mixed on a single individual’s body. This is also why it is rare to find male cats with these X-chromosome-related mottled patterns. Typical XY males have only one X chromosome, so it is unlikely for them to develop a mosaic pattern caused by X-chromosome inactivation, in which the two types of X-chromosome alleles are selectively expressed in different cells. However, even in males, this pattern can appear under certain chromosomal configurations—such as when two or more X chromosomes are present—or in specific genetic circumstances. Furthermore, since X-chromosome inactivation can vary between individuals and even within cells, the inactivation patterns of the X chromosomes may not be completely identical even among identical twins. In other words, even if individuals have genetically very similar chromosomes, differences in which X chromosome is active can lead to variations in phenotypic characteristics. A similar example in humans is X-linked recessive genetic disorders, of which color vision deficiency is one. Some genes associated with color vision deficiency are located on the X chromosome, and mutations in these genes can affect the function of the retinal cone cells. In males, since there is only one X chromosome, if a mutation related to color vision deficiency is present on that X chromosome, the effect is likely to manifest directly. However, since females have two X chromosomes, if they carry both a color vision deficiency-related allele and a normal allele, which X chromosome is active in each cell may vary depending on X-chromosome inactivation.
In this case, a mosaic phenomenon may occur, where the allele associated with color vision deficiency is expressed in some cells, while the normal allele is expressed in others. Therefore, the color vision function of female carriers may vary depending on their genetic makeup and X-chromosome inactivation pattern, and some may exhibit subtle differences in color vision. Some X-linked traits classified as recessive in expression conceal these characteristics of X-chromosome inactivation.Basosomes are rarely used in practical applications. In the past, the method of collecting oral epithelial cells to check for the presence of basosomes was used as one way to distinguish gender in sports competitions, and they were also utilized to study sex chromosome abnormalities or verify the chromosomal status of cells. However, since there are limitations to accurately determining an individual’s sex or chromosomal composition based solely on the presence or absence of basosomes, using basosome testing alone for these purposes is no longer considered appropriate today. Nevertheless, basosomes remain important because they provide crucial clues for understanding the complex biological mechanism of X-chromosome inactivation. In particular, X-chromosome inactivation is not simply a process of completely “turning off” a single chromosome, but rather a complex regulatory process involving the combined action of various molecules, including XIST RNA, and epigenetic mechanisms. If we can gain a more precise understanding of the process by which the X chromosome is selected and inactivated, as well as the mechanisms that maintain this state, it could help in the development of new therapeutic strategies to regulate the expression of genes associated with genetic diseases located on the X chromosome.
