2) (10 point question) Many people love colorful and tasty tomatoes and an ambit

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: 1) 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. 2) 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. A) (4 points) How closely linked are the r and genes? Please explain your strategy to calculate the distance B) (2 points) A testcross with the Fl can be used to further define the genetic distance between the two genes. Indicate the ratio of progeny phenotypes that you'd expect (describe phenotypes) from this cross based on the map distance you propose for part a)

Explanation / Answer

A. R - color and T - Taste
Crossover occurs are

      r T             R    T     - 998
      R   t ----      r    t     - 3

Parents rrTT X RRtt

F2 Red and normal taste = 998
Color and normal taste = 511
Red and great taste = 488
Color and normal taste = 3

RF = 998 + 3 / 2000
   = 0.5 cM

B Testcross RrTt X rrtt
The ratio 1:1:1:1
The map distance >50cM leads to no linkage.

So ratio 1:1:1:1 not 7:1:1:7

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