Essential Genetics 6th Edition by Daniel Hartl

Exercise 3

Dro sophila virilis has 6 pairs of chromosomes in somatic cells, consisting of 5 acrocentric chromosome pairs and 1 tiny chromosome pair referred to as the "dot" chromosome. The closely related species D. texana has 5 pairs of chromosomes, consisting of 4 acrocentric pairs, 1 metacentric pair, and a tiny "dot" chromosome pair. Hybrids between these species have meiotic cells in which there are 4 bivalents (including the "dot") and 1 trivalent. The trivalent consists of the metacentric chromosome of D. texana paired with two of the acrocentrics of D. virilis , oriented such that the three centromeres are close together. Suggest an explanation.