Homework Sourse1 In the tomato red fruit R is dominant over yellow fruit r
1. In the tomato, red fruit (R) is dominant over yellow fruit (r) and yellow owers (Wf ) are dominant over white owers (wf ). A cross was made between true-breeding plants with red fruit and yellow owers, and plants with yellow fruit and white owers. The F1 generation plants were then crossed to plants with yellow fruits and white owers. The following results were obtained:
Fruit Flower
Red Yellow 333
Red White 64
Yellow Yellow 58
Yellow White 350
Answer the following questions, being sure to show your work clearly and concisely.
(a) Calculate the map distance between the two genes.
(b) How many of each type do you expect to see after examining 50 ospring from a cross between two of the F1 generation plants?
I don\'t understand the theory or what is linkage mapping to calculate the map distance so I don\'t know how to calculate for 50 offspring
Solution
Answer:
a) From the data given, it can be easily deciphered that the genes for fruit and flower are linked i.e. they are present on the same chromosome. If the genes are not linked i.e if they are present on different chromosomes then crossing between F1 generation plants and rrwfwf plants will yield offspring of equal proportions. The details are explained below:
The first crossing i.e. production of F1 generation is done as follows. RRWfWf crossed with rrwfwf.
RRWfWf X rrwfwf
All the plants in F1 generation will have the following heterozygous genotype. RrWfwf.
So, if F1 generation plants are backcrossed with rrwfwf (as given in the question), the resulting F2 generation plants will have the following genotype. Assuming that the genes are not linked i.e. independent of each other the resulting genotypes will have the following proportions. I am going to show the outcome by Punnett square.
So crossing between RrWfwf X rrwfwf.
The type of gametes present in parents are listed horizontally and vertically.
rwf
rwf
RWf
RrWfwf
RrWfwf
Rwf
Rrwfwf
Rrwfwf
rWf
rrWfwf
rrWfwf
rwf
rrwfwf
rrwfwf
You can clearly see from the square that out of 8 offspring the following phenotypes were observed.
2 – Red fruit and yellow flowers
2 – Red fruit and white flowers
2 – Yellow fruit and Yellow flowers
2 – Yellow fruit and white flowers
So all genotypes were obtained equally in the F2 offspring, which only happens if the genes are independent of each other. But this is not the case in our question as the genotypes in offspring are not of equal proportion. This gives a clue that the genes are linked.
To calculate the map distance between two genes the following formulae have to be used.
(Number of recombinants in offspring / Total number of offspring ) x 100
In this case, the number of recombinants is (Red fruit and white flowers (64) , Yellow fruit and yellow flowers (58)) 122.
Total number of offspring = 333 + 64 + 58 + 350 = 805
Map distance = (122/805) x 100 = 15.1 m.u. or 15 map units.
So, the distance between the fruit gene and flower gene is 15.1 map units.
b) The cross between F1 generation plants
RrWfwf X RrWfwf
The outcome is shown in punnett square below:
RWf
Rwf
rWf
rwf
RWf
RRWfWf
RRWfwf
RrWfWf
RrWfwf
Rwf
RRWfwf
RRwfwf
RrWfwf
Rrwfwf
rWf
RrWfWf
RrWfwf
rrWfWf
rrWfwf
rwf
rrWfwf
Rrwfwf
RrWfwf
rrwfwf
The proportion of genotypes from the punnett square is as follows:
Red fruit and Yellow flowers - 9/16
Red fruit and white flowers – 3/16
Yellow fruit and yellow flowers – 3/16
Yellow fruit and white flowers – 1/16
Since the genes are linked the frequency of recombinants will be based on map distance and not on the punnett square results. So, the number of recombinants formed in an offspring would be:
Number of recombinants = (map distance x Total number of offspring) /100
Number of recombinants = (15.1 x 50) /100 = 7.55 or 8.
Out of 50 offspring 8 will be recombinants i.e. the plants may have Red fruit and white flowers or yellow fruit and yellow flowers.
The rest offspring i.e. 42 F2 plants will have the parental phenotype (either Red fruit and yellow flowers or yellow fruit and white flowers).
| rwf | rwf | |
| RWf | RrWfwf | RrWfwf |
| Rwf | Rrwfwf | Rrwfwf |
| rWf | rrWfwf | rrWfwf |
| rwf | rrwfwf | rrwfwf |
