HOMEWORK SET #5 — KEY

30 pts total

  1. (6 pts total)

    2. a. (2 pts) 1:1

    b. (2 pts) 11:1

    c. (2 pts) 35:1

  2. (12 pts total)

    1. (3 pts) Mr. Lambert is heterozygous for a pericentric inversion of chromosome 6. One of the breakpoints is within the fourth light band up from the centromere, while the other is in the sixth dark band below the centromere. Mrs. Lambert’s chromosomes are normal.

    2. (3 pts) When Mr. Lambert’s number 6 chromosomes paired during meiosis, they formed an inversion loop that included the centromere. Crossing over occurred within the loop, and gave rise to the partially duplicated, partially deficient chromosome 6 that the child received.

    3. (3 pts) The child’s abnormalities stem from having three copies of some, and only one copy of other, chromosome 6 regions. The top part of the short arm is duplicated, and there is a deficiency of the distal part of the long arm in this case.

    4. (3 pts) The inversion appears to cover more than half of the length of chromosome 6, so crossing over will occur in this region in the majority of meioses. In the minority of meioses where crossing over occurs outside the loop, and in the cases where it has occurred within the loop but the child receives an uncrossed-over chromatid, the child can be normal. There is significant risk for abnormality, so monitoring of fetal chromosomes should be done.

 

  1. (12 pts total)

    1. (3 pts) Mr. Denton has normal chromosomes. Mrs. Denton is heterozygous for a balanced reciprocal translocation between chromosomes 6 and 12. Most of the short arm of chromosome 6 has been reciprocally translocated onto the long arm of chromosome 12. The breakpoints appear to be in the thick, dark band just above the centromere of 56 and in the third dark band below the centromere of 12.

    2. (3 pts) The child received a normal 6 and a normal 12 from his father. In prophase I of meiosis of Mrs. Denton, chromosome 6 and 12 and the reciprocally translocated 6 and 12 paired to form a cruciform-like structure. Segregation of adjacent, nonhomologous centromeres to the same pole ensued, so that the child received a gamete containing a normal 6 and one of the translocation chromosomes.

    3. (3 pts) The child has a normal 6 and a normal 12 chromosome from Mr. Denton. The child also has a normal 6 chromosome from Mrs. Denton. However, the child also has one of the translocation chromosomes from Mrs. Denton. With this chromosome, it is partially trisomic as well as partially monosomic. It has three copies of part of the short arm of chromosome 6 and only one copy of most of the long arm of chromosome 12. This abnormality in gene dosage is the cause of its physical abnormality.

    4. (3 pts) Segregation of adjacent homologous centromeres to the same pole is relatively rare, and alternate segregation is more common. If alternate segregation occurs, the gamete will have a complete haploid set of genes, and the embryo should be normal. However, half of the gametes resulting from alternate segregation will be translocation heterozygotes.