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. Why does this parent have a normal phenotype?

Verified step by step guidance

1. Understand that cri-du-chat syndrome is caused by a deletion on chromosome 5, specifically a terminal deletion.
2. Recognize that a karyotype analysis can reveal structural changes in chromosomes, such as deletions, duplications, or translocations.
3. Consider that a parent with a normal phenotype might have a balanced chromosomal rearrangement, such as a balanced translocation, which does not affect their phenotype.
4. Balanced translocations involve the exchange of segments between two chromosomes without any genetic material being lost or gained, thus maintaining a normal phenotype.
5. In this case, the parent might have a balanced translocation involving chromosome 5 and another chromosome, such as chromosome 12, which could lead to unbalanced gametes and result in disorders like cri-du-chat syndrome in offspring.

Key Concepts

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 deletion leads to various developmental issues, including distinctive facial features, intellectual disability, and a high-pitched cry resembling a cat's meow. Understanding this syndrome is crucial for analyzing the genetic implications for the child and the parents.

Karyotype Analysis

Karyotype analysis is a laboratory technique used to visualize an individual's chromosomes, allowing for the identification of chromosomal abnormalities such as deletions, duplications, or translocations. In this case, karyotyping can help determine whether the parents carry any chromosomal anomalies that could explain the child's condition or their history of spontaneous abortions.

Phenotype vs. Genotype

The phenotype refers to the observable characteristics or traits of an individual, while the genotype is the genetic makeup that contributes to these traits. A parent may have a normal phenotype despite carrying a genetic mutation or chromosomal abnormality if the mutation is recessive or if they possess a normal copy of the gene that compensates for the abnormality. This distinction is essential for understanding why one parent appears healthy despite the child's genetic disorder.