A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.
What term best describes this kind of chromosome abnormality?

Verified step by step guidance

1. Understand that Down syndrome (trisomy 21) is caused by the presence of an extra copy of chromosome 21, resulting in three copies of this chromosome instead of the usual two.
2. Note that the boy has 46 chromosomes, which is unusual for someone with trisomy 21, as individuals with Down syndrome typically have 47 chromosomes.
3. Observe that the parents and sisters have 45 chromosomes, which is fewer than the normal 46 chromosomes in humans. This suggests a structural chromosomal abnormality rather than a numerical one.
4. Recognize that the most likely explanation for this situation is a Robertsonian translocation, a type of chromosomal rearrangement where two acrocentric chromosomes fuse at their centromeres, leading to a reduction in chromosome number without loss of genetic material.
5. Conclude that the term 'Robertsonian translocation' best describes this chromosomal abnormality, as it explains both the reduced chromosome number in the parents and sisters and the inheritance of an unbalanced form leading to trisomy 21 in the boy.

Key Concepts

Chromosome Number Abnormalities

Chromosome number abnormalities occur when there is a deviation from the normal diploid number of chromosomes, which is 46 in humans. These abnormalities can manifest as aneuploidy, where there is an extra or missing chromosome, or polyploidy, where there are multiple sets of chromosomes. In the case of Down syndrome, the presence of an extra chromosome 21 results in trisomy 21, leading to the characteristic features of the condition.

Aneuploidy

Aneuploidy is a specific type of chromosome number abnormality characterized by the presence of an abnormal number of chromosomes in a cell. This can include conditions such as monosomy (one less chromosome) or trisomy (one extra chromosome). In the context of the question, the boy with Down syndrome has trisomy 21, indicating he has three copies of chromosome 21 instead of the usual two, while his family members have a normal chromosome count.

Phenotype vs. Genotype

Phenotype refers to the observable physical and biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. In this scenario, the boy with Down syndrome has a distinct phenotype due to his genetic makeup, while his parents and sisters, despite having a normal phenotype, have an unusual chromosome count of 45, suggesting a potential genetic anomaly that may not be phenotypically expressed.