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?

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 individual has an extra copy of chromosome 21, leading to trisomy 21.

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.

Phenotype vs. Genotype

Phenotype refers to the observable physical and physiological traits of an organism, while genotype refers to the genetic makeup that determines these traits. In the scenario presented, the boy with Down syndrome has a distinct phenotype associated with his condition, despite having a normal chromosome count of 46. His parents and sisters, with 45 chromosomes, may exhibit a normal phenotype, suggesting they could be carriers of a chromosomal abnormality without expressing it.