A certain X-linked dominant disease in humans occurs when there are more than 36

Question

A certain X-linked dominant disease in humans occurs when there are more than 36 repeats of a CAG trinucleotide in a particular gene. Size changes in any trinucleotide repeat region strand slippage by DNA polymerase during DNA synthesis. tends to happen due to " is the outline of a pedigree for the disease described above, and a The DNA corresponding to each individual is directly below his or her pla sex and phenotype (affected vs. unaffected) of all individuals except II-9 (save that one for representation of a repeats. gel showing PCR-amplified DNA fragments detecting the number of CAG ce in the sed on the information you have been given, fill in the pedigree to show the the next question). . 36 repeats ?26 repeats

Explanation / Answer

(A)Shown below is the outline of a pedigree for the disease described above, and a representation of a gel showing PCR-amplified fragments detecting the number of CAG repeats. The DNA corresponding to each individual is directly below his or her place in the pedigree. Based on the information you have been given, fill in the pedigree to show the sex and phenotype (affected vs. unaffected) of all individuals except II-9 (save that one for the next question). (10 pts).

Anyone with two alleles (two bands on the gel) is female, and anyone with single band is male (only one X chromosome). This is dominant disorder, so anyone with an allele that has more than 36 CAG repeats will be affected.

(B) What is unusual about individual II-9? Suggest two mechanisms that could explain the genotype of II-9, stating whether that individual would be phenotypically female or male in each case. (6 pts)

The individual appears to have just a single copy of the polymorphic locus, so we'd predict it was an XY male -- except that the allele appears to be derived from the father (26 repeats), and the father should be contributing only a Y to his son.

Several possible explanations:

(A)Shown below is the outline of a pedigree for the disease described above, and a representation of a gel showing PCR-amplified fragments detecting the number of CAG repeats. The DNA corresponding to each individual is directly below his or her place in the pedigree. Based on the information you have been given, fill in the pedigree to show the sex and phenotype (affected vs. unaffected) of all individuals except II-9 (save that one for the next question). (10 pts).

Anyone with two alleles (two bands on the gel) is female, and anyone with single band is male (only one X chromosome). This is dominant disorder, so anyone with an allele that has more than 36 CAG repeats will be affected.

(B) What is unusual about individual II-9? Suggest two mechanisms that could explain the genotype of II-9, stating whether that individual would be phenotypically female or male in each case. (6 pts)

The individual appears to have just a single copy of the polymorphic locus, so we'd predict it was an XY male -- except that the allele appears to be derived from the father (26 repeats), and the father should be contributing only a Y to his son.

Several possible explanations:

• Non-disjunction in the mom gave an egg with no X chromosome; the sole X chromosome is derived from the dad; the individual is XO female.
• The person is an XY male, with a change in one of the alleles from the mom (either the 21-repeat allele changed to 26-repeats, or the >36-repeat allele shrank to 26 repeats).
• The person is an XX female, but a mutation in the primer-binding site prevented the PCR primer from hybridizing to its target -- so that the allele is there, it just couldn't be amplified by PCR.
• The person is XX female, but the disease allele expanded even further, to the point that the PCR product was so large that it was outside the range visualized on this gel.
• The person is XX {26,26} female -- one of the alleles in the mom changed to a 26-repeat allele; both alleles are present, but since they are the same size, only one band is seen on the gel.

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