Genetics 1) ( 14 point question) Many people love colorful and tasty tomatoes an

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Genetics 1) ( 14 point question) Many people love colorful and tasty tomatoes and an ambitious student in horticulture wants to breed such a tomato variety. Luckily, he searched all the available tomato genetic resources, and found the two genetic strains he needs. One strain produces the tomatoes with a colorful red-green-yellow mosaic pattern, and the other strain is very tasty, which make you remember it forever if you have a bite.

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

i.) In the F1 generation, all the ripe tomatoes have regular red color and regular taste. The student tasted over 1000 tomatoes from 40 individual plants, and was sick with tomatoes with regular taste.

ii.) In the selfing F2 progeny, the student sampled 1000 tomatoes: 498 regular red-color tomatoes with regular taste, 248 colorful tomatoes with regular taste, and 254 red-color tomatoes with great taste. He was so disappointed that he did not find any colorful tomatoes with great taste. He so believes in what he has learned from his genetic classes. He sampled another 2000 tomatoes from the F2 progeny: 998 regular red-color tomatoes with regular taste, 511 colorful tomatoes with regular 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 his experimental results, he realized that the genes r and t are recessive and linked. To facilitate your analysis, the colorful strain carries a mutation in the gene known as r, and the tasty strain carries a mutation in t. He wants you to help him answer the following questions:

a) (4 points) What is the rough distance between r and t? Please write down your strategy to ESTIMATE the distance.

b) (4 points) How can you design a testcross for further redefining the genetic distance between the two genes (graw out the genotypes of the parents)? Indicate the ratio of progeny that you'd expect (describe phenotypes) from this cross based on the map distance you propose for part a).

c) (6 points) The tomato genome has now been sequenced and genes on the chromosome are in the following order OPQRSTU. The respective genetic distances on the genetic map between these seven genes is very similar. But we now know that there is a 'cold spot' between genes ST 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.

Explanation / Answer

The gene for tasty is ‘t’ and colorful is ‘r’. The gene for regular taste is ‘T’ and regular color is ‘R’.

The initial cross was ttRR (tasty) X TTrr (colorful)

Result: RrTt (regular)

F2 cross is: RrTt X RrTt

Result: Out of 2000 tomatoes from the F2 progeny:

998 regular red-color tomatoes with regular taste (R_T_) = parental

511 colorful tomatoes with regular taste (rrT_) = recombinants

488 red-color tomatoes with great taste (R_tt) = recombinants

3 colorful tomatoes with great taste (rrtt) = parental

= (511+488)/2000 *100 = 49.95mu

For linkage to occurs, distance between two genes should be less than 50 mu. Here, distance is ~50mu. So, very less linkage will occur here.

From this test cross 1:1:1:1 ratio should be obtained. Out of 1000, 250 tomatoes should be tasty and colorful.

Hot spots for recombination are ‘PQ’ and ‘TU’. Cold spots is ‘ST’

OQPRSUT

Now, distance between ‘r’ and‘t’ is 50mu. And all genes are at equal distances.

So, the new physical map will become:

So, distance between two genes will be (50/3) = 16.67mu

O-----16.6-----Q-----16.6-----P-----16.6-----R-----16.6-----S-----16.6-----U-----16.6-----T

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