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.
Explain how this is possible.

Verified step by step guidance

1. Understand that Down syndrome (trisomy 21) typically results from an individual having three copies of chromosome 21, leading to a total of 47 chromosomes. However, in this case, the boy has 46 chromosomes, which is unusual and requires further investigation.
2. Recognize that the parents and sisters each have 45 chromosomes, which is fewer than the normal 46 chromosomes in humans. This suggests the presence of a chromosomal rearrangement, such as a Robertsonian translocation, which can involve chromosome 21.
3. Explain that a Robertsonian translocation occurs when two acrocentric chromosomes (e.g., chromosome 21 and another chromosome) fuse at their centromeres, forming a single chromosome. This reduces the total chromosome count but does not necessarily result in a loss of genetic material.
4. Propose that one of the parents likely carries a balanced Robertsonian translocation involving chromosome 21. In this case, the parent would have 45 chromosomes but a normal phenotype because no critical genetic material is lost or duplicated.
5. Describe how the boy inherited an unbalanced combination of chromosomes: two normal copies of chromosome 21 and the translocated chromosome. This results in trisomy 21 (Down syndrome) despite the total chromosome count being 46.

Key Concepts

Chromosomal Abnormalities

Chromosomal abnormalities occur when there is a deviation from the normal number or structure of chromosomes. In the case of Down syndrome, also known as trisomy 21, an individual has three copies of chromosome 21 instead of the usual two. This can happen due to nondisjunction during meiosis, where chromosomes fail to separate properly, leading to gametes with an abnormal number of chromosomes.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This can occur during meiosis, resulting in gametes that have an extra chromosome or are missing one. If such a gamete participates in fertilization, it can lead to conditions like Down syndrome, where the resulting zygote has an abnormal number of chromosomes.

Phenotype vs. Genotype

Phenotype refers to the observable characteristics or traits of an organism, while genotype refers to the genetic makeup. In this scenario, the boy with Down syndrome has a distinct phenotype due to his extra chromosome, but his parents and sisters may have a normal phenotype despite having 45 chromosomes, possibly due to a balanced chromosomal rearrangement or a different chromosomal abnormality that does not affect their phenotype.