A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right.

This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?

Verified step by step guidance

1. Step 1: Understand the nature of the chromosomal rearrangement. Since the woman is heterozygous for a rearrangement between chromosomes 2 and 3, it means one set of her chromosomes is normal, and the other set has a structural change involving these two chromosomes.
2. Step 2: Recognize that being phenotypically normal despite a chromosomal rearrangement is not surprising if the rearrangement is balanced. A balanced rearrangement means no genetic material is lost or gained, so gene dosage remains normal, which often results in no observable phenotype.
3. Step 3: Consider that phenotypic effects might arise if the rearrangement disrupts important genes or regulatory regions, even if balanced. This could lead to altered gene function or expression, potentially causing clinical symptoms.
4. Step 4: Identify circumstances where phenotypic effects are more likely, such as if the rearrangement is unbalanced (leading to duplication or deletion of genetic material), or if it causes position effects that alter gene expression.
5. Step 5: Also consider reproductive consequences. Even if the woman is phenotypically normal, during meiosis, the rearrangement can lead to unbalanced gametes, increasing the risk of miscarriages or offspring with genetic disorders.

Key Concepts

Chromosomal Rearrangements and Their Types

Chromosomal rearrangements involve structural changes such as translocations, inversions, deletions, or duplications between chromosomes. In this case, a rearrangement between chromosomes 2 and 3 likely refers to a reciprocal translocation, where segments are exchanged without loss of genetic material, often resulting in a balanced karyotype.

Phenotypic Effects of Balanced vs. Unbalanced Rearrangements

Balanced rearrangements typically do not cause phenotypic abnormalities because all genetic material is present, just rearranged. However, unbalanced rearrangements lead to extra or missing genetic material, often causing developmental or health issues. A heterozygous balanced translocation carrier is usually phenotypically normal.

Genetic Counseling and Reproductive Risks

Even if a carrier is phenotypically normal, chromosomal rearrangements can affect gamete formation, increasing the risk of miscarriages or offspring with unbalanced karyotypes. Genetic counseling assesses these risks and guides family planning decisions based on the type and consequences of the rearrangement.