Question

According to Lyon's hypothesis, the patchwork pattern on a calico cat can be explained by which of the following? a. One of the \(X\) chromosomes is converted to a Barr body in somatic cells of female mammals. b. One of the \(X\) chromosomes is converted to a Barr body in all cells of female mammals. c. Both of the X chromosomes are converted to Barr bodies in somatic cells of female mammals. d. Both of the X chromosomes are converted to Barr bodies in all cells of female mammals. e. One of the X chromosomes is lost in the somatic cells of female mammals.

Short answer

Hence, one of the X chromosomes is converted to a Barr body in somatic cells of female mammals.

Step-by-step solution

Understand the Lyon's Hypothesis

According to Lyon's Hypothesis, the patchwork pattern on a calico cat is due to random X-chromosome inactivation. Since female mammals have two X chromosomes, one X-chromosome is randomly inactivated and becomes a Barr body, during early embryonic development in cells.

Defining a Barr Body

A Barr body is a dense structure in the nucleus of cells, representing an inactivated X chromosome. In female mammals, one of the two X chromosomes in each cell is randomly inactivated in this way.

Correlate with Lyon's Hypothesis Definition

The process of X-chromosome inactivation applies to somatic cells. This means it is not a phenomenon occurring in all cells, but mainly those cells forming the body of an organism or cells which are not germ cells.

Conclude

Therefore, based on Lyon's Hypothesis and the process of X-chromosome inactivation and conversion to Barr bodies, the most accurate answer would be 'One of the X chromosomes is converted to a Barr body in somatic cells of female mammals.'

Key concepts

X-chromosome inactivation

Every female mammal has two X chromosomes in every cell. Yet only one is needed to function. To maintain balance, one X chromosome in each cell is randomly inactivated. This phenomenon is known as "X-chromosome inactivation." Think of it as a way to prevent the duplication of genetic information that an extra active X chromosome would provide.

During early development, each cell inactivates one of its X chromosomes at random. This ensures that some cells express genes from one X, while others express genes from the other X. This random selection process becomes permanent and is passed on to all descendant cells when they divide, contributing to cellular diversity.

• Occurs to prevent overexpression of X-linked genes.
• Random, making a mosaic pattern of gene expression throughout the organism.
• Permanent for each cell line once established.

In the case of calico cats, this inactivation creates patches of different colors, each controlled by different X chromosomes active in different parts of the body.

Barr body

A Barr body is the name given to the condensed, inactive X chromosome visible under a microscope in the nucleus of a cell. This structure is a key indicator of X-chromosome inactivation and can be found in female mammals. It's essentially nature's solution for maintaining the right number of active genes in a cell, as females have two X chromosomes compared to males, who have just one.

Here’s what makes a Barr body:

• It is inactive and tightly packed away, effectively removing its influence from the cell’s activity.
• Visible as a dark spot in the nucleus in female cells during interphase.

The process leading to Barr body formation is crucial in creating balanced gene expression between male and female organisms. While male cells naturally have only one X chromosome, female cells need to silence one of their two X chromosomes to avoid doubling the gene products.

Calico cat patchwork pattern

The distinct patchwork pattern of a calico cat’s fur is a wonderful demonstration of X-chromosome inactivation in action. This patchwork pattern occurs because calico cats primarily inherit two different color-genes on their X chromosomes, such as one for black and another for orange. Since the gene responsible for determining these fur colors is located on the X chromosome, their random activation leads to patches of different colors.

This is how the colored patches form:

• In each fur cell, one of the X chromosomes inactivates randomly as a Barr body.
• If the active X chromosome carries the gene for the black color, that area of fur turns black.
• If the active X chromosome carries the gene for the orange color, that area turns orange.

Therefore, the random inactivation results in a mosaic of black and orange colors across the cat’s body, creating the beautiful calico pattern. This anatomical marvel illustrates the dynamic process of genetic inheritance and expression.