Question

2) (10 point question) Many people love colorful and tasty tomatoes and an ambitious student in horticulture wants to breed such a tomato variety. He searched all the available tomato genetic resources, and found the two inbred lines he needed. One line produces the tomatoes with a colorful red-green-yellow mosaic pattern (rrTT), and the other strain is 'very tasty' (RRtt).

When the student crossed the two strains together, he made the following observations:

In the F1 generation, all the ripe tomatoes have a regular red color and a regular flavor. The student tasted tomatoes from over 40 individual plants, and was sick with regular tasting tomatoes.

In the F2 progeny, the student sampled 1000 tomato plants: 498 had red-color tomatoes with normal taste, 248 colorful tomatoes with normal taste, and 254 red-color tomatoes with great taste. He was disappointed not to find any colorful tomatoes with great taste. He then sampled another 2000 F2 plants: 998 had red-color tomatoes with normal taste, 511 colorful tomatoes with normal taste, 488 red-color tomatoes with great taste, and 3 colorful tomatoes with great taste; he was so happy to find what he wanted!!!

Based on these experimental results, he realized that the alleles he needs are recessive and the two genes are linked.  

C) (2 points) The tomato genome has now been sequenced and genes on the chromosome are in the following order OPQRSTU. The respective physical distances on the chromosome between these seven genes are very similar. But we now know that there is a 'cold spot' between genes RT and 'hot spots' between PQ and TU. Draw a representation of the genetic map relative to the physical map (both maps are needed here) for this part of the tomato genome. Show the relative (approximate) locations of the genes on the two maps.

D) (2 points) What can you predict about the chromosome chromatin structure between the R and T genes?

Answer 1

2.

C) as the physical distance between genes are quite similar (given), we could expect sam length of DNA fibre between genes, while genetic map will show an increased distance due to "hot spots" (i.e. between PQ and TU, given) and closer distance due to presence of "cold spot" (i.e. between RT) . Depending on these assumptions, following maps can be concluded :

----O----------P----------Q----------R----------S----------T----------U---- PHYSICAL MAP(based on sequence distance)

----O----------P---------------Q----------R-----S-----T---------------U---- GENETIC MAP(based on recombination frequency)

D) As the recombination frequency are being expected to be lower than usual, the DNA fibre between R and T genes are need not to be in open form to facilitate cross over event. Thus, chromatin structure could be expected to be in folded state and also in interaction with accessory proteins. We could conclude as a hetero-chromatin structure to be present at that position.