Question

1) The following sketch shows a region of the mais genome subdivided into 10 zones. Seven deletions have been mapped to the zones as shown. The DNA region between the parentheses is removed by the deletion.

    | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

D1 -------------(           )------------------------------------

D2 -------------------(                 )------------------------

D3 -(           )------------------------------------------------

D4 -------------------------------(           )------------------

D5 -------------------------------(                       )------

D6 -------------------(           )------------------------------

D7 -------(                       )------------------------------

a) Plants carrying a point mutation located in zone 4 (called mutation 4-1) are mated separately with the 7 deletions mentioned above. Give a general justification and then indicate by yes or no whether pseudodominance occurs in the following crosses:

1: D1 X 4-1, 2: D3 X 4-1, 3: D4 X 4-1, 4: D6 X 4-1.

b) A frameshift mutation results in pseudominance effect when crossed with D2, D4, and D5 only. In which zone is this mutation located? Justify briefly.

c) Is there a gene in the zone identified in question b)? If yes explain your reasoning. (hint: rerad question above, and think mutation type)

Answer 1

1a) Pseudodominance occurs when the recessive allele gets to express itself in a dominant fashion, as for example when the dominant allele is absent. If the deletions mentioned above,(D1-7), encompasses the region of the genome corresponding to zone 4, then there is a possibility that the point mutation 4-1 will express itself when crossed with those particular deletions. Deletions D1, D4, D6 do not have deletions in zone 4, therefore point mutation 4-1 won't express, but the wild type dominant allele in those deletion lines will be expressed. On the contrary, D3 has a section of zone 4 deleted, which indicates that the protein encoded by this region will be incomplete and non-functional, with high chances that it may not be encoded. So the gene with the point mutation 4-1 will be expressed from the mutant line (summarised below)

Cross	Zones deleted	Pseudodominance
D1 X 4-1	5-7	no
D3 X 4-1	1-4	yes
D4 X 4-1	10 onwards	no
D6 X 4-1	7-8	no

1b) Frameshift mutations involve insertion or deletion of nucleotides that shifts the tranlation mechanism from one reading frame to another. In D2,D4 and D5, the dominant allele corresponding to mutation 4-1 is already present. So it will be expressed anyway. So if an insertion in zone 3 can frameshift the mutation 4-1 to the zones corresponding to the zone 10, which is common zone deleted for all the deletion mutants, then there is a possibitlity for pseudodominance of 4-1.

1c) There is possibly a gene encompassing zone 3 and another encompassing zone 4 (as suggested by the point mutation). In part b) I have suggested that an insertion in zone 3 can frameshift 4-1 to the deletion zones corresponding to D2, D4, and D5. This will obviously disrupt proper translation of the gene in zone 3, but on the contrary the proper copies of the same gene in the deletion lines D2/4/5 will encode the protein.