Barr Body

Introduction

The genetic makeup of every individual is determined by the genes expressed in the body. Every trait of the body, like the eye color, height, complexion, facial features etc., is inherited from the parents to the offspring as coded by the DNA. The DNA is tightly packed around the proteins forming a thread-like compact structure called chromosomes.

Human beings consist of 22 pairs of autosomal chromosomes, meaning that the chromosomes in males or females remain the same regardless of the sex of the individual and one pair of sex chromosomes. Genes are the segment of DNA encoding a particular trait. The sex chromosomes determine the gender of the individual. Most of us are born with complete 23 pairs of chromosomes inherited from our parents, except in rare cases like turner’s syndrome and down’s syndrome.

What is Barr body?

The Canadian physician Murray Barr proposed the term Barr body. The males have one X and Y chromosome, while the female contains two X chromosomes. In the case of females, the somatic cells contain one active X chromosome, and the other X chromosome is inactive or sedentary. The Barr body is the inactivated X chromosome. It is also referred to as sex chromatin. It is found near the boundary of the nuclear membrane. This inactivation of one X chromosome is essential to prevent the unnecessary expression of genes.

Furthermore, the active X chromosome allows transcription as it is packed in the euchromatin. In contrast, the Barr body condensed and crammed in the heterochromatin does not allow the transcription machinery to occur. However, the number of genes for both the X chromosomes remains the same. It is plano-convex in shape and was discovered in the neuron of a female cat. It can be stained by swabbing the individual’s inner cheeks. When stained and observed under the microscope, the Barr bodies show darkly stained spots due to their curled-up condensed structure. Barr bodies can determine the sex of an individual.

Lyonization

Lyonization is called the process by which one of the X chromosomes is deactivated or shut down in female somatic cells leading to the Barr body. The process takes place during embryonic development. The term was named after the Geneticist Mary Lyon in 1961, who discovered this process while she was studying the effects of radiation exposure and genetic abnormalities. Therefore, the process bears her name. Inactivation is a must to prevent any diseases in the offspring. This process is not just restricted to humans but to every living being determined by the Y or W chromosome. Lyonization is an irreversible process.

Lyon’s Hypothesis

Lyon’s hypothesis states the following −

• One of the two X chromosomes in the female somatic cell remains dormant.

• This deactivation occurs during embryonic development, after two weeks of conception.

• The deactivation is arbitrary.

• This inactive X chromosome remains inactivated in all the cell generations.

What leads to the development of the Barr body?

The formation of the Barr Body is random; any one of the two X chromosomes in the female somatic cell is inactivated. The dormant X chromosome remains in the latent stage in all cellular activities and life. The same applies to the active X chromosome. Lyonization begins when the centromere serves as the origin of lyonization.

Moreover, the centromere confirms the presence of two or more chromosomes before forming a Barr Body. The centromere region where the inactivation occurs is the X inactive centre, abbreviated XiC. Out of the 12 genes of the centromere, Xist and Tist are two genes which participate in lyonization. Tist gene levels drop down in the process, and Xist gene levels rise. Xist then begins to spread all over the chromosome to be inactivated. Next, the Xist factor gradually covers the whole chromosome leading to the formation of the Barr body.

Importance of Barr body

• Barr body helps to negate the consequences of double doses of X-linked genes.

• Barr body allows accurate cell division in the progeny to prevent genetic disease transmission.

Barr Body Disorders

Klinefelter syndrome

Males have a X and Y chromosome. However, in rare cases, a male is born with an extra X chromosome (XXY), called Klinefelter syndrome. The syndrome is not an inherited disorder but an error during cell division. It is diagnosed in adulthood. Males suffering from this syndrome do not have well-developed testicles, enlarged breasts, narrow shoulders, little body hair and produce few sperm, thereby affecting sexual reproduction.

Calico cat

Nearly most of the calico cats are female. The orange-black pattern is due to some cells having dominant orange colour alleles and recessive black alleles.

FAQs

Qns 1. What are somatic cells?

Ans. Somatic cells are the cells in the body other than the germ cells, i.e., sperm and egg cells. Somatic is derived from the Greek word “soma”, which means body. Somatic cells are diploid in humans, i.e., they contain two sets of chromosomes inherited from each parent. These are responsible for forming multiple organs. If DNA mutations occur in somatic cells, the individual is affected, not their offspring.

Qns 2. What is Down’s syndrome?

Ans. Down syndrome or down’s syndrome is a genetic disorder when an individual has an extra copy of chromosome 21 in every cell.

Qns 3. What are alleles? Explain with an example.

Ans. Alleles are the variant form of the gene. They are responsible for diversity. Example − If the parental genes contain information for hair texture, one gene may code for straight hair, and the other may code for curly hair. The child may have either curly or straight hair.

Qns 4. Define heterochromatin and Euchromatin.

Ans. Heterochromatin is a condensed and tightly packed form of DNA, constituting 98% of the genome. It does not participate in transcription. At the same time, euchromatin is a loosely packed form of DNA, comprising 2% part of the genome and participating in transcription.

Qns 5. What is a centromere?

Ans. Centromere is the compressed region of the chromosome that links two sister chromatids. Spindle fibres attached themselves to the centromere