Question

Which of the following definitions is INCORRECT? A. Trisomy-the zygote has three copies of a chromosome B. Nondisjunction-failure of homologous chromosomes or of sister chromatids to separate C. Translocation-the event by which a chromosomal fragment joins with its homologous D. Inversion-the event by which a chromosomal fragment joins with its original chromosome, but in reverse position

Short answer

Option C is incorrect because translocation involves a fragment joining a non-homologous chromosome.

Step-by-step solution

Understand Each Term

Read each term and its definition carefully. These terms relate to chromosomal abnormalities and processes.

Evaluate Option A: Trisomy

Trisomy refers to a zygote having three copies of a chromosome instead of the usual two. This definition is correct.

Evaluate Option B: Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This definition is also correct.

Evaluate Option C: Translocation

Translocation involves a fragment of a chromosome becoming attached to a non-homologous chromosome. The given definition states that a chromosomal fragment joins with its homologous chromosome, which is incorrect.

Evaluate Option D: Inversion

Inversion refers to a chromosome fragment reattaching to its original chromosome, but in the opposite orientation. This definition is correct.

Identify the Incorrect Definition

Based on the evaluations, the incorrect definition provided is Option C.

Key concepts

Trisomy

Trisomy occurs when a zygote, the initial cell formed when a sperm and an egg merge during conception, ends up with three copies of a particular chromosome instead of the standard two. This happens because an extra chromosome is inherited. Trisomy is a common form of chromosomal abnormality.

For example, Down syndrome is a kind of trisomy, specifically called Trisomy 21, because individuals with this condition have three copies of chromosome 21. Having this extra chromosome alters the typical development and function of the cells, leading to the characteristics and health problems associated with the disorder.

Other types of trisomy include:

• Trisomy 18 (Edwards syndrome): characterized by severe developmental delays
• Trisomy 13 (Patau syndrome): associated with profound intellectual disability and physical abnormalities

Understanding trisomy is crucial in genetics and prenatal diagnoses, as it can help predict certain developmental issues.

Nondisjunction

Nondisjunction is a process failure during cell division where chromosomes do not separate properly. This can occur during two types of cell division: meiosis or mitosis. When nondisjunction happens, it leads to cells with an abnormal number of chromosomes.

During meiosis, which is the process that creates sperm and egg cells, nondisjunction can create gametes that have either too many or too few chromosomes.
If such a gamete is involved in fertilization, it can result in offspring with a condition like trisomy.

• Meiosis I nondisjunction: Homologous chromosomes fail to separate.
• Meiosis II nondisjunction: Sister chromatids fail to separate.

Mitotic nondisjunction occurs in somatic (body) cells and can lead to mosaicism, where different cells within the same individual have different genetic makeups. This can lead to various medical conditions depending on which cells are affected and how widespread the nondisjunction is.

Translocation

Translocation happens when a piece of one chromosome breaks off and attaches to another non-homologous chromosome. This can lead to various genetic disorders depending on what genes are disrupted or moved to new locations.

There are two main types of translocations:

• Reciprocal Translocation: segments from two different chromosomes have been exchanged.
• Robertsonian Translocation: one chromosome with very small p arms (short arms) attaches to another with similar small p arms.

Since chromosomes are not lost or gained in balanced translocation events, individuals may not show any symptoms.
However, they can have fertility issues or increased risk of having children with unbalanced translocations, which may cause genetic disorders.
Translocation should not be confused with recombination events which happen naturally during meiosis.

Inversion

An inversion occurs when a chromosome segment breaks off and reattaches in the opposite orientation. This rearrangement can disrupt the function of genes at or near the inversion breakpoints.

There are two types of inversions:

• Paracentric Inversion: occurs within one arm of a chromosome and does not include the centromere.
• Pericentric Inversion: includes the centromere and can involve breaks in p (short) and q (long) arms of the chromosome.

Inversions can be invisible in terms of phenotype (the outward characteristics) if no critical genes are disrupted during reorientation. However, misalignment during meiosis might cause complications.
This might lead to the production of gametes with duplications or deletions, potentially resulting in genetic disorders in offspring.