Practice question

Question

In Drosophila, balancer chromosomes are used to keep all the alleles on one chromosome together. A balancer contains multiple inversions; so that when it recombines with the corresponding wild type chromosome, no viable cross over products are formed. Balancers also carry an allele for a dominant phenotype. A Drosophila male with sepia eye color Is crossed to a female carrying a third chromosome balancer (TM&B). The allele for sepia phenotype (se) is located on chromosome 3 and is recessive to the wild type eye color. The dominant marker for TM&B is a tubby phenotype. Further, an Individual homozygous for TM&B balancer does not survive. F1 progeny with tubby phenotype Is sib-mated. The F2 progeny Is expected to have: