In the tomato, red fruit (R) is dominant over yellow fruit (r), and yellow flowe
ID: 54289 • Letter: I
Question
In the tomato, red fruit (R) is dominant over yellow fruit (r), and yellow flowers (F) are dominant over white flowers (f). A cross was made between true-breeding plants with red fruit and yellow flowers, and plants with yellow fruit and white flowers. All F1 offspring are heterozygotes for both traits. The F1 female plants were then test crossed to males with yellow fruit and white flowers ( homozygous recessive for both traits). The following results were obtained: 333 red fruit, yellow flowers 64 red fruit, white flowers 58 yellow fruit, yellow flowers 345 yellow fruit, white flowers Total of 800.
1. What are the phenotypes of the recombinant offspring of the F2 generation?
2. The recombinant offspring of the F2 generation in the above cross were produced as a result of crossing over. When do you think that crossing over has occurred? Is it during male gamete formation or female gamete formation? Explain why? ( Hint what are the genotypes of the F1 female and male plants)
3. Conduct a Chi square analysis to see if the two genes are linked ( Check a solved problem S1, page 148 text book for an idea)…follow the same steps)
a. Propose a hypothesis to be tested
b. Find the expected (predicted) number of offspring ( Hint what is the ratio of test cross results?
c. Calculate and Interpret the Chi –square result
d. If the two genes (fruit color and flower color) are linked, calculate the map distance between the two genes
4. Set a sample test cross, Explain the rationale behind a testcross, why is it necessary for one of the parents to be homozygous recessive for the genes of interest?
Explanation / Answer
Based on the given data,
Here, the cross was made between true-breeding phenotypes (either homozygous dominant or recessive genotypes only) of red fruit and yellow flower (RR FF) with yellow fruit and white flowers (rr ff).
Thus, the cross is: RR FF x rr ff and the F1 zygote of this cross is: Rr Ff. This F1 is crossed with one of his parent yellow fruit and white flowers (rr wfwf). Thus, this cross is made between the: Rr Ff x rr ff.
It gives the following data,
1) The phenotypes of the recombinant offspring of the F2 generation are: Red white and Yellow and yellow.
2) The corssing over takes place during both male and female gamate formation.
3)
a) Hypothesis to be tested: whether the genes are assorted indepndenty or not.
b and c)
Chi-square Test:
The Chi-square value (2) = 386.87
The degrees of freedom (DF) = 4-1 = 3
The P-Value is < 0.00001
The result is significant at p < 0.05
Since, P < 0.05, the null hypothesis is rejected, which means that there is a statistically substantial variance among the experiential and predictable data. So, the variance among the experiential and predictable data is not exclusively due to chance.
d)
The map distance between the two genes:
Calculate the recombination frequency values, form heterozygotes (i.e. recombinants) of R and F. Thus, the recombinant products of meiosis must be R.f and r.F.
We note from the list that there are 64+58 = 122 of these types; therefore, RF = (122/800)*100 = 15.25 m.u. This frequency tells us that these loci must be linked on the same chromosome, as follows: