Q35 - Classifying Crossover Events from Linear Tetrad Data

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Linkage, Recombination & Genetic Mapping

Centromere position can be mapped using linear tetrads in some fungi. If there is no cross-over between gene E and the centromere, four spores are arranged in a sequence of eeEE or EEee (Fig.Q.35A). If there is a cross-over between gene E and the centromere, four spores are arranged in a sequence of eEeE or EeEe (Fig.Q.35B). There are many types of crossovers involving 2, 3 and 4 strands of chromatids, as illustrated in Fig.Q.35C. Strain ab was crossed to strain a+ b+ and 100 linear tetrads were isolated. Those tetrads were divided into six classes as shown in the following table:

	Class 1	Class 2	Class 3	Class 4	Class 5	Class 6
Spore 1	ab	ab	ab	ab	ab+	ab
Spore 2	a+b	ab+	ab	a+b+	a+b	a+b+
Spore 3	ab+	a+b	a+b+	ab	ab+	ab+
Spore 4	a+b+	a+b+	a+b+	a+b+	a+b	a+b
Count	15	29	47	2	2	5

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. Locus a and b are located on the same arm of chromosome.

B. Class 4 involves a 4-strand double crossover.

C. Class 6 involves a 3-strand double crossover, with one between gene a and the centromere and between the centromere and gene b.

D. Class 5 involves a 2-strand double crossover.

← Previous: Q34 Next: Q36 →

Question reproduced from IBO 2016, Theoretical Paper A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)