Problem 8 - Biology Concepts and Applications

Question

A cell with a Barr body is ________. a. prokaryotic b. from a male mammal c. from a female mammal d. infected by Barr virus

Step-by-Step Solution

Answer

The cell with a Barr body is from a female mammal.

Step 1: Understand the Context

A Barr body is an inactivated X chromosome found in the cells of female mammals. This process is a form of dosage compensation where one of the X chromosomes in females becomes inactive to ensure an equal level of X chromosome gene expression as in males, who have one X chromosome.

Step 2: Identify Relevant Information

Since Barr bodies are formed as a mechanism for dosage compensation of X chromosomes, this implies that cells with Barr bodies must have at least two X chromosomes. Typically, this occurs in females (XX), while males generally have one X (XY) and one Y chromosome.

Step 3: Analyze the Options

Let's examine the options: (a) prokaryotic (Barr bodies are a eukaryotic feature), (b) from a male mammal (males typically have one X, no Barr body), (c) from a female mammal (females usually have two X chromosomes, one of which forms a Barr body), (d) infected by Barr virus (no virus is known by this name related to Barr bodies).

Step 4: Select the Correct Answer

Based on the understanding that Barr bodies are associated with the inactivation of one X chromosome in cells that have two X chromosomes, which typically occurs in female mammals, the correct answer is (c) from a female mammal.

Key Concepts

Dosage Compensation

Dosage compensation is a crucial biological process that helps equalize the expression of genes on the X chromosomes between males and females. In mammals, males possess one X chromosome (XY) and females possess two (XX). To prevent females from having double the expression of X-linked genes compared to males, organisms have developed various mechanisms:

• Inactivation of one X chromosome in females, resulting in the formation of a Barr body.
• Up-regulating the single X chromosome in males in some species, such as Drosophila.
• Balancing gene expression without complete inactivation, seen in worms like C. elegans.

In female mammals, the mechanism of choice is the inactivation of one of the X chromosomes, ensuring that both males and females have a similar level of gene expression from their X chromosomes. This process helps maintain genetic stability and proper functioning of cells.

X Chromosome Inactivation

X Chromosome Inactivation (XCI) is a process specific to female mammals to manage genetic imbalances. Since females have two X chromosomes, one of these must be turned off, or inactivated, to prevent overexpression of X-linked genes. This inactivated X chromosome is visible in cells as a dense, dark-staining structure known as the Barr body.

The inactivation process is random in embryonic cells; either the maternal or paternal X chromosome can be inactivated. Once an X chromosome is chosen for inactivation in a cell, all the cell's descendants will inherit the same pattern of inactivation. This leads to a unique feature known as "mosaicism," where cells in a female body can express different X-linked traits depending on which X chromosome remains active.

This mechanism plays a critical role not only in normal cellular development but also in understanding genetic disorders linked to the X chromosome.

Female Mammals

Female mammals are fascinating organisms with complex genetic management due to their XX chromosomal composition. The presence of two X chromosomes means they carry two copies of each X-linked gene. The process of X chromosome inactivation ensures these genes are not overexpressed, creating a balance with their male counterparts.

This balance is achieved because only one X chromosome remains active in each cell, making it equivalent to the single X chromosome in males. The other X chromosome shrinks into a Barr body, which is not expressed. This inactivation process balances the "gene dosage" between sexes.

Additionally, since the choice of which X chromosome becomes inactive varies, females exhibit X-linked traits differently, often exhibiting a blend of characteristics from both X chromosomes. This innate variation can be seen in calico cats, where different patches of fur color demonstrate the expression from different active X chromosomes. Understanding these principles helps elucidate the genetic complexity and diversity observed in female mammals.