Chromosomal Rearrangements: Translocations: Videos & Practice Problems

Translocations involve the movement of chromosomal segments between chromosomes, with two main types: reciprocal and Robertsonian. Reciprocal translocations can lead to viable or non-viable gametes depending on how chromosomes segregate during meiosis. In contrast, Robertsonian translocations, often linked to familial Down syndrome, involve the fusion of acrocentric chromosomes, resulting in a loss of short arms but maintaining genetic integrity. This can lead to trisomy 21 in offspring, highlighting the importance of understanding chromosomal mutations in genetics.

Reciprocal Translocation

Translocations refer to the movement of chromosomal segments from one chromosome to another, which can occur between completely separate chromosomes or within the same chromosome. Understanding translocations is crucial, especially in the context of inheritance, as they can lead to chromosomal aberrations and mutations. There are two primary types of translocations: reciprocal translocations and non-reciprocal translocations. This summary will focus on reciprocal translocations, which involve the exchange of acentric fragments—segments of chromosomes that lack a centromere.

When reciprocal translocations occur, the resulting gametes can be sorted into three categories based on how the chromosomes segregate during meiosis. The first category is adjacent-1 segregation, where gametes contain one normal chromosome and one translocated chromosome. This type of segregation results in non-viable gametes because they do not contain a complete set of chromosomes. The second category is adjacent-2 segregation, which also produces non-viable gametes, as it involves homologous chromosomes that do not provide a full set of genetic information. The third category is alternative segregation, where gametes consist of either two normal chromosomes or two translocated chromosomes. This type of segregation is viable because it maintains a complete set of genes.

To illustrate these concepts, consider a scenario where two chromosomes undergo a translocation. Initially, there are two normal chromosomes, each containing distinct genes. After the translocation, the chromosomes can be represented as having exchanged segments, leading to the formation of translocated chromosomes. During meiosis, these chromosomes replicate, resulting in four copies of each chromosome. During metaphase, homologous chromosomes align, allowing for various pairing combinations. The gametes formed from these pairings will determine the viability based on whether they contain a complete set of genes.

In adjacent-1 segregation, for example, if a normal chromosome pairs with a translocated chromosome, the resulting gametes will lack certain genes, rendering them non-viable. In contrast, alternative segregation allows for the formation of gametes that retain all necessary genes, ensuring their viability. Thus, understanding the mechanisms of translocation and the resulting gamete formation is essential for grasping the implications of chromosomal mutations in inheritance.

Robertsonian Translocations

Robertsonian translocation is a specific type of chromosomal rearrangement that can lead to familial Down syndrome, a rare form of the condition. This translocation occurs when there are breaks at the short arms of two non-homologous acrocentric chromosomes, typically chromosomes 14 and 21. The result is the fusion of the long arms of these chromosomes, forming a single chromosome while the short arms are lost. This process leads to a total of 45 chromosomes instead of the usual 46, as the small short arms do not contain significant genetic information and are often lost during cell division.

There are two forms of Robertsonian translocation: the balanced form and the unbalanced form. In the balanced form, a parent may carry the translocation without any phenotypic effects, as they possess the necessary genetic material from both chromosomes. This individual has the long arms of chromosomes 14 and 21, along with the short arms, but appears normal because they have the complete set of genes required for normal function.

In contrast, the unbalanced form typically occurs in the offspring, leading to chromosomal imbalances. For instance, during gamete formation, a parent with a balanced Robertsonian translocation can produce gametes that may contain either the normal chromosomes or the translocated chromosome. This can result in various combinations of chromosomes in the offspring, including the possibility of having three copies of chromosome 21, which is characteristic of Down syndrome (trisomy 21).

When gametes from a parent with a Robertsonian translocation combine with those from a normal parent, several outcomes are possible. The offspring may inherit a normal chromosomal arrangement, a carrier status for the translocation, or, in the case of Down syndrome, an extra copy of chromosome 21. The likelihood of these outcomes is influenced by the specific gametes produced during meiosis, where the presence of the translocated chromosome can lead to the formation of gametes with an abnormal number of chromosome 21.

Familial Down syndrome accounts for approximately 5% of all Down syndrome cases, highlighting the significance of Robertsonian translocations in genetic inheritance. Understanding this mechanism is crucial for genetic counseling and assessing the risk of chromosomal abnormalities in future generations.

Which of the following represents the chromosomal segregation into gametes after a reciprocal translocation caused by adjacent-1 segregation? N=Normal chromosome T = Tanslocated chromosome

• A. (N2 T1) and (N1 T2)
• B. (N1 T1) and (N2 T2)
• C. (T1 T2) and (N1 N2)

Answer: (A)

Which of the following ways reciprocal translocated chromosomes are sorted produces viable gametes?

• A. Adjacent-1 segregation
• B. Adjacent-2 segregation
• C. Alternative segregation

Answer: (C)

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after alternative segregation? 
A B • C D E F G                        J K • L M N O P 
A B • C D N O P                       J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and J K • L M N O P

Answer: (B)

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after adjacent-1 segregation? 
A B • C D E F G                J K • L M N O P 
A B • C D N O P               J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and A B • C D E F G

Answer: (A)

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after adjacent-2 segregation? 
A B • C D E F G             J K • L M N O P 
A B • C D N O P            J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and A B • C D E F G

Answer: (C)

How many chromosomes does a person who is a carrier for familial down syndrome caused by a robertsonian translocation have?

• A. 45
• B. 46
• C. 47
• D. 48

Answer: (A)

Here’s what students ask on this topic:

What are the main types of chromosomal translocations?

There are two main types of chromosomal translocations: reciprocal and Robertsonian. Reciprocal translocations involve the exchange of segments between two non-homologous chromosomes, often resulting in viable or non-viable gametes depending on how chromosomes segregate during meiosis. Robertsonian translocations occur when the short arms of two acrocentric chromosomes break off and the long arms fuse to form a single chromosome. This type is often associated with familial Down syndrome, where the fusion can lead to trisomy 21 in offspring.

How do reciprocal translocations affect gamete viability?

Reciprocal translocations can affect gamete viability based on how chromosomes segregate during meiosis. There are three main segregation patterns: adjacent-1, adjacent-2, and alternate. Adjacent-1 and adjacent-2 segregations result in gametes with unbalanced genetic material, leading to non-viable offspring. In contrast, alternate segregation produces gametes with balanced genetic material, resulting in viable offspring. The key factor is whether the gametes contain a complete set of genes, despite the translocation.

What is a Robertsonian translocation and how does it relate to Down syndrome?

A Robertsonian translocation involves the fusion of the long arms of two acrocentric chromosomes, with the short arms being lost. This type of translocation is linked to familial Down syndrome, a rare form of Down syndrome. In this condition, a parent carries a balanced Robertsonian translocation, which can lead to an unbalanced translocation in the offspring. This results in trisomy 21, where the child has three copies of chromosome 21, causing Down syndrome.

What is the difference between balanced and unbalanced Robertsonian translocations?

In a balanced Robertsonian translocation, the individual has a normal phenotype because they have the correct amount of genetic material, despite the structural rearrangement of chromosomes. This person typically has 45 chromosomes but appears normal. In an unbalanced Robertsonian translocation, there is an abnormal amount of genetic material, leading to phenotypic abnormalities. For example, in familial Down syndrome, the child inherits an unbalanced translocation, resulting in trisomy 21 and the associated symptoms of Down syndrome.

How do chromosomal translocations contribute to genetic disorders?

Chromosomal translocations can contribute to genetic disorders by disrupting the normal arrangement and function of genes. In reciprocal translocations, the exchange of segments can lead to unbalanced gametes, causing developmental issues or miscarriages. Robertsonian translocations can result in conditions like familial Down syndrome, where the fusion of chromosomes leads to an extra copy of chromosome 21. These disruptions can affect gene expression and lead to various genetic disorders, highlighting the importance of understanding chromosomal mutations in genetics.