Question

A 38-year-old woman in her 15th week of pregnancy undergoes ultrasonography that reveals an increased area of nuchal transparency. Amniocentesis is recommended and performed at 16 weeks' gestation. The amniotic karyotype is 46,XYadd(18)(p.11.2), indicating additional chromosomal material on the short arm of one chromosome 18 at band 11.2. All other chromosomes are normal. What is the most likely cause of this fetal karyotype?

A. A balanced reciprocal translocation in one of the parents
B. A balanced Robertsonian translocation in one of the parents
C. An isochromosome 18i(p) in one of the parents
D. Nondisjunction during meiosis 1 in one of the parents
E. Nondisjunction during meiosis 2 in one of the parents

Answer 1

The fetal karyotype with extra chromosomal material on chromosome 18 likely results from a balanced reciprocal translocation in one of the parents.

Step-by-step explanation:

The most likely cause of the fetal karyotype showing additional chromosomal material on the short arm of one chromosome 18 at band 11.2 is a balanced reciprocal translocation in one of the parents (Option A). This genetic anomaly occurs when two different chromosomes swap segments without any loss or gain in the genetic material. Reciprocal translocations are typically balanced, meaning the amount of genetic material is normal, but it's rearranged between the chromosomes. This scenario contrasts with nondisjunction, which results from the failure of chromosome pairs to separate properly during meiosis, leading to a gain or loss of chromosomes, as in trisomies or monosomies.