In a certain plant, a dominant allele for one gene, R, produces round fruit and
ID: 54231 • Letter: I
Question
In a certain plant, a dominant allele for one gene, R, produces round fruit and the recessive allele, r, produces oblong fruit. A dominant allele of a second gene, Y, produces yellow flowers and the recessive allele, y, produces white flowers. The two genes are linked on chromosome IV. A plant ( ry / ry ) with normal chromosomes is crossed to a plant (R Y/R Y) homozygous for a reciprocal translocation between chromosomes IV and VIII. The presence of the translocation causes semi-sterility. The FI progeny are phenotypically yellow flowered with round fruit and are semi-sterile. A backcross to the parent with normal chromosomes yields the 750 progeny shown in the table below. From these data, draw a map showing all relevant map distances (in centimorgans) for the genes and the translocation breakpoint. Show your work.
Explanation / Answer
The map distance is 22 cM can be calculated from the frequency of cross overs.
Here , Total gametes=381,
Yellow & Round=3, White & oblong=295, Yellow &oblong= 73, White & Round = 10.
We know, first two are normal gametes (as they are dominant and recessive) while the rest two are cross over gametes.
Therefore,
Frequency of the cross over gametes= number of crossover gametes divided by total number of gametes= 83/381 = 22 cM
Same way for semi-fertile,
Total gamete= 369
Frequency of crossover gamete= 82/369 = 22 cM