A healthy couple with a history of three previous spontaneous abortions has just had a child with cri-du-chat syndrome, a disorder caused by a terminal deletion of chromosome 5. Their physician orders karyotype analysis of both parents and of the child. The karyotype results for chromosomes 5 and 12 are shown here. Which parent has an abnormal karyotype? How can you tell? What is the nature of the abnormality?

Verified step by step guidance

1. Step 1: Understand the genetic basis of cri-du-chat syndrome. This syndrome is caused by a terminal deletion on the short arm of chromosome 5 (5p). The deletion results in the loss of genetic material, leading to the characteristic symptoms of the disorder.
2. Step 2: Analyze the karyotype results of the child. Look for evidence of the terminal deletion on chromosome 5. Specifically, compare the size and banding pattern of chromosome 5 in the child’s karyotype to a normal chromosome 5.
3. Step 3: Examine the karyotype results of both parents. Check for any structural abnormalities in their chromosomes, particularly chromosome 5 and chromosome 12, as these are mentioned in the problem. Look for signs of a balanced translocation or other structural rearrangements.
4. Step 4: Determine which parent has the abnormal karyotype. If one parent has a balanced translocation involving chromosome 5, this could explain the child’s terminal deletion. A balanced translocation means that no genetic material is lost or gained in the parent, but it can lead to unbalanced gametes during reproduction.
5. Step 5: Conclude the nature of the abnormality. If a parent has a balanced translocation involving chromosome 5, the child likely inherited an unbalanced form of this translocation, resulting in the terminal deletion on chromosome 5. This explains the child’s cri-du-chat syndrome and the history of spontaneous abortions, as unbalanced translocations often lead to nonviable pregnancies.

Key Concepts

Karyotype Analysis

Karyotype analysis is a laboratory technique that involves the examination of an individual's chromosomes to identify structural abnormalities, such as deletions, duplications, or translocations. It provides a visual representation of the chromosome number and structure, allowing for the detection of genetic disorders. In this case, analyzing the karyotypes of the parents and child will help determine if there are any chromosomal abnormalities contributing to the child's cri-du-chat syndrome.

Cri-du-chat Syndrome

Cri-du-chat syndrome is a genetic disorder caused by a deletion of a portion of chromosome 5, specifically at the 5p15.2 region. This condition is characterized by distinctive features, including a high-pitched cry resembling that of a cat, developmental delays, and various physical anomalies. Understanding this syndrome is crucial for interpreting the karyotype results and assessing the genetic implications for the parents and child.

Chromosomal Abnormalities

Chromosomal abnormalities refer to changes in the normal structure or number of chromosomes, which can lead to genetic disorders. These abnormalities can be numerical, such as aneuploidy (extra or missing chromosomes), or structural, such as deletions, duplications, or inversions. Identifying the nature of the abnormality in the karyotype results will help determine which parent may carry a genetic risk factor that could have contributed to the child's condition.