Chromosomal Rearrangements: Deletions - Online Tutor, Practice Problems & Exam Prep

Deletions are chromosomal rearrangements resulting in the loss of chromosome segments, typically requiring double-stranded breaks. Types include enterogenic, multigenic, terminal, and intercalary deletions. A key phenotype is pseudodominance, where a recessive allele expresses its phenotype due to the absence of a dominant allele. Deletion loops facilitate proper segregation during meiosis. An example is Cri du Chat syndrome, caused by a deletion on chromosome 5, leading to a distinctive cat-like cry in affected infants. Understanding deletions is crucial for genetics and medical applications.

Deletions

So deletions are a type of chromosomal rearrangement, and this results in a loss of a part of a chromosome. Deletions typically require double-stranded breaks, so two breaks to cut out that segment of the chromosome. And the deleted chromosomal segment usually contains no centromere and is lost after cell division. Once the deletion is made, it is just sort of floating around, and because it doesn't have a centromere, the microtubules creating those daughter cells will not attach onto it, and so it'll just be degraded and lost during cell division.

Now, there are many types of deletions, including Enterogenic Deletion. These are deletions within a gene, which is going to completely obliterate that gene. You also have Multigenic Deletions, which involve multiple genes, getting rid of a ton of genes more than one. There are also terminal deletions which get rid of an end of a chromosome, and intercalary deletions. Deletions can cause a couple of important phenotypes.

The first and probably most important phenotype is called Pseudodominance. If we consider these two chromosomes with dominant alleles on one and recessive alleles on the other, and a region gets deleted, the normal expression of dominant and recessive, which leads to a dominant phenotype, is altered. When this region gets deleted, the genotype of the organism changes such that you have this one recessive allele, and because there is no dominant allele, this recessive shows its phenotype, which is called Pseudodominance.

In order to create the deletion, a structure forms called a deletion loop, which is created between the normal homolog and the partner containing the missing segment. This allows for proper segregation during meiosis. An example of a deletion loop might look like this: you have genes labeled a, b, c, and d, and there’s your loop. The deletion will be cut out, and at the end, you're going to get the centromere and gene d.

An example of a deletion in humans is Cri-du-chat Syndrome, and it is actually a deletion in chromosome 5. It results in an interesting phenotype where babies who have it sound like cats meowing when they cry. That's how it gets its name, Cri-du-chat, translating to "cry of the cat" in French. This is a relevant example in the medical world for all future doctors.

With that, let’s now move on.

An intragenic deletion is a deletion found where?

• A. In a centromere
• B. In a gene
• C. In multiple chromosomes
• D. In multiple genes

Deletions can cause what type of phenotype?

• A. Leaky allele
• B. Psudodominance
• C. Intercalary
• D. Faulty

Which of the following genetic diseases is an example of a chromosomal deletion?

• A. Down syndrome
• B. Familial down syndrome
• C. Cri du Chat syndrome
• D. Klinefelter's Disease

Here’s what students ask on this topic:

What are chromosomal deletions and how do they occur?

Chromosomal deletions are a type of chromosomal rearrangement that results in the loss of a segment of a chromosome. They typically occur due to double-stranded breaks in the DNA, which cut out a segment of the chromosome. The deleted segment usually lacks a centromere, causing it to be lost during cell division as it cannot attach to the spindle fibers. This loss can lead to various genetic disorders and phenotypic changes, depending on the genes located within the deleted segment.

What are the different types of chromosomal deletions?

There are several types of chromosomal deletions, including:

• Enterogenic Deletions: These occur within a single gene, often rendering the gene nonfunctional.
• Multigenic Deletions: These involve the loss of multiple genes, which can have more severe effects.
• Terminal Deletions: These involve the loss of the end of a chromosome.
• Intercalary Deletions: These occur within the interior of a chromosome, removing a segment between two breaks.

What is pseudodominance and how is it related to chromosomal deletions?

Pseudodominance is a phenomenon where a recessive allele expresses its phenotype due to the absence of a dominant allele. This occurs when a chromosomal deletion removes the dominant allele, leaving only the recessive allele. For example, if a dominant allele (A) and a recessive allele (a) are present, the dominant phenotype (A) will be expressed. However, if the segment containing the dominant allele (A) is deleted, only the recessive allele (a) remains, and its phenotype will be expressed.

What is a deletion loop and what role does it play in meiosis?

A deletion loop is a structure that forms during meiosis between a normal homologous chromosome and its partner containing a deletion. This loop allows for proper alignment and segregation of chromosomes during meiosis. The loop forms because the segment of the chromosome that is missing in one homolog cannot pair with the corresponding segment in the other homolog, causing the remaining segments to loop out. This ensures that the chromosomes segregate correctly, despite the deletion.

What is Cri du Chat syndrome and how is it related to chromosomal deletions?

Cri du Chat syndrome is a genetic disorder caused by a deletion on the short arm of chromosome 5. The name of the syndrome, which means 'cry of the cat' in French, comes from the characteristic cat-like cry of affected infants. This cry is due to abnormalities in the larynx and nervous system. The deletion leads to various developmental issues, including intellectual disability, delayed development, and distinctive facial features. Understanding this syndrome helps illustrate the significant impact that chromosomal deletions can have on human health.