Question

Which of the following mutations would be least likely to have any discernable phenotype on the individual? (A) Translocation of the last 1,000 base pairs of chromosome 1 onto chromosome 2 (B) Nondisjunction of chromosome 7 to produce trisomy7 (C) Nondisjunction of chromosome 8 to produce monosomy-8 (D) Deletion of 400 base pairs, resulting in the loss of an enhancer

Short answer

The mutation least likely to have any discernable phenotype on the individual is (A) Translocation of the last 1,000 base pairs of chromosome 1 onto chromosome 2. This mutation is less likely to disrupt any essential genes, and while it might cause some disturbance in gene regulation, the impact is likely to be minimal compared to the other mutations given.

Step-by-step solution

Understand each type of mutation

First, let's understand each mutation given in the exercise. (A) Translocation is a type of chromosomal mutation where a segment of DNA is transferred from one chromosome to another. This can disrupt gene function if the translocated segment includes a gene or if it disrupts the reading frame for a gene. (B) Nondisjunction is an error during cell division that can result in cells with extra or missing chromosomes. Trisomy is a form of aneuploidy, where an individual has three copies of a particular chromosome rather than two. This mutation could disrupt gene balance and lead to a significant phenotype. (C) Monosomy is also a form of aneuploidy, where an individual has only one copy of a particular chromosome rather than two. Similar to trisomy, this could disrupt gene balance and lead to a significant phenotype. (D) Deletion of an enhancer region refers to the loss of a DNA sequence that typically helps in controlling the expression of genes. Enhancers can be located near or far from their target genes. A deletion of an enhancer could affect gene regulation and produce a phenotype, depending on the importance of the enhancer and the genes it regulates.

Determine the mutation with the least impact on phenotype

Now that we have understood each mutation, we can analyze which mutation will have the least impact on the phenotype. (A) Translocation of only the last 1,000 base pairs of chromosome 1 onto chromosome 2 is less likely to disrupt any essential genes. While it might cause some disturbance in gene regulation, the impact might be minimal overall. (B) Trisomy 7 would likely produce a significant phenotype, as it disrupts gene balance by creating three copies of all chromosome 7 genes. (C) Monosomy-8 would also likely produce a significant phenotype, as it disrupts gene balance by having only one copy of all chromosome 8 genes. (D) Deletion of an enhancer could produce varying phenotypes, depending on the specific genes it regulates and their importance. In some cases, it might lead to no discernable phenotype, but in others, it could produce a significant phenotype. Based on this analysis, we can conclude that:

Select the correct answer

The mutation least likely to have any discernable phenotype on the individual is (A) Translocation of the last 1,000 base pairs of chromosome 1 onto chromosome 2. While it might cause some disturbance in gene regulation, the impact is likely to be minimal compared to the other mutations given.

Key concepts

Chromosomal Mutations

Chromosomal mutations refer to any alteration in the structure or number of chromosomes within a cell. This can include changes like translocations, deletions, duplications, inversions, or even more complex rearrangements. These mutations can have a variety of effects on an organism's phenotype, ranging from harmless to fatal, depending on which genes are affected and how they alter gene expression or chromosome function.

For instance, a mutation may interfere with the normal arrangement of genes on a chromosome, which can lead to misregulation of those genes or create a new combination of gene effects. This is the case with the translocation mutation question in the exercise, where the transfer of a small piece of chromosome could have negligible effects if essential genes are not disrupted. However, other chromosomal mutations might involve larger segments and can significantly affect the organism's development and health.

Nondisjunction

Nondisjunction occurs when chromosomes fail to separate properly during cell division, specifically during meiosis, leading to gametes with an abnormal number of chromosomes. This error can result in reproductive cells - eggs or sperm - that have one extra or one fewer chromosome than the typical count.

When such a gamete is fertilized, the resulting zygote has an imbalance in chromosome number, known as aneuploidy. This imbalance is notable in the development of certain syndromes, such as Down syndrome, which is caused by trisomy of chromosome 21. In the problem presented, both trisomy 7 and monosomy 8 are forms of aneuploidy that could lead to serious developmental issues, contrasting with the often less impactful result of a simple translocation of a small chromosome segment.

Trisomy and Monosomy

Trisomy and monosomy refer to types of aneuploidy, where trisomy indicates the presence of an extra chromosome (three instead of the normal two), and monosomy indicates a missing chromosome (one instead of two).

Health conditions associated with trisomy or monosomy typically have significant symptoms because having an incorrect number of chromosomes disrupts the delicate balance of gene dosage in cells. For example, trisomy 21 leads to Down syndrome, characterized by intellectual disability and physical features such as a flattened facial profile. Similarly, monosomy, like Turner syndrome (where a female has only one X chromosome), results in distinct physical traits and health issues. The presented exercise suggests that trisomy 7 and monosomy 8 would each likely cause noticeable health issues because this chromosome imbalance has a profound impact on the body's development and function.

Gene Enhancer Deletion

Gene enhancers are regions of DNA that help regulate gene expression, often by interacting with specific proteins that can increase the likelihood of transcription. These enhancers do not directly code for proteins but play a critical role in controlling when and where genes are activated. Deletion of a gene enhancer can therefore lead to a change in the expression levels of a target gene, which may alter an organism's phenotype.

An enhancer deletion may have no discernible phenotype if the affected genes are not crucial for visible traits or if other regulatory elements compensate for the loss. In some cases, redundant or overlapping enhancers ensure that gene expression remains relatively stable even when one enhancer is deleted. Consequently, enhancer deletions can have varied impacts, which may explain why, in certain contexts like in the textbook problem, their deletion may not lead to any apparent physical change in the organism.