Isochromosomes Have the Structure ABC $\bullet$ CBA (Where $\bullet$ Represents the Centromere and A, B, and C Represent Wild-Type

Essay

Isochromosomes have the structure ABC $\bullet$ CBA (where $\bullet$ represents the centromere and A, B, and C represent wild-type alleles of three different genes). In some cases, such isochromosomes are derived from two copies of one half of a metacentric chromosome, which has a centromere near its center. For example, a wild-type metacentric chromosome ABC $\bullet$ DEF might form two distinct isochromosomes, ABC $\bullet$ CBA and FED $\bullet$ DEF. The centromeres of such distinct isochromosomes can be homologous so that they cause the segregation of the two distinct isochromosomes from each other during meiosis.
a. A mutant of genotype ABC $\bullet$ CBA / FED $\bullet$ DEF is viable. If you cross this mutant with a wild-type individual, what would you expect to be the genotype of the offspring? Would these offspring be viable or not? Explain.
b. What would you expect if you cross this mutant with another individual of genotype ABC $\bullet$ CBA / FED $\bullet$ DEF? Explain.
c. You are given a pericentric inversion mutant in which this same chromosome is of genotype ABD $\bullet$ CEF. In an individual heterozygous for this inversion and a wild-type chromosome there is a crossover between C and the centromere. Draw the crossover event and indicate the gametes that would be generated.
d. Assume that the inversion interval is very small so that both duplications and deletions for this interval (from C to
D) do not cause inviability. Indicate the gentoypes expected in offspring of a cross between the inversion heterozygote and an individual of genotype ABC $\bullet$ CBA / FED $\bullet$ DEF.