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Post by Alla on Thu Aug 07, 2008 1:59 am

chousta wrote:hey all.

im havin some troubles with a question from an exercise, and it doesn't have a solution, but ive made an attempt, hoping its right and someone could lead me the right way for the next part of it.


Genetics-silkworms 96874872oa6

a) I addressed this question talking about each phenotype rather than 2 variables?


genetic hypothesis 1:
A single pair of alleles determine haemolymph colour, the allele producing yellow colour being dominant (since yellow is most frequent in F2). Therefore, the F2 should show a ratio of 3 yellow: 1 white. Any deviation from this ratio is due by chance.

This is the case

genetic hypothesis 2:
A single pair of alleles determine surface pattern, the allele producing lined surface pattern being dominant (since lined surface is most frequent in F2). Therefore, the F2 should show a ratio of 3 lined: 1 plain. Any deviation from this ratio is due to chance alone.


Phenotype Lined Plain
Observed 374 106
Expected 360 120

X^2=Sigma ((O-E)^2)/E
= 2.18

Determining Factor=2-1=1

Probability statement:
From above, the probability of obtaining as great deviation between expected and observed, by chance alone, is greater than 0.05, therefore we accept that the deviation from 3:1 is attributed to chance

Biological statement:
Therefore we do not reject the single dominant gene hypothesis

B)No not ALL, would have the same phenotype, because this would not highlight the nature of genetics. I.e. dominant and recessive.

No idea for the rest.
Im taking a second year genetics course, but im actually a first year if you could look past my lack of understanding, which is probably all very trivial to you.

thank you very much for your time.

First, you are right in saying that yellow and lined are dominant phenotypes. Your calculations are correct but I would suggest doing a proper calculation for both variables as they will show significant difference if two genes are liked answering your questions d and e.

Now, to make sure you understand the rest of the questions let’s start at the beginning.

1. Your first cross between true breeding yellow/plain and white/lined means that both are homozygous. Let’s say that Y is dominant yellow allele; y is recessive coffering white; L is dominant lined; and l is recessive plain.
Then the cross is between YYll x yyLL (F0).

2. This cross will produce ALL heterozygous F1 progeny of same genotype and phenotype: YyLl – yellow/lined.

3. The cross YyLl x YyLl should produce:

9 Yellow/Lined: 3 Yellow/Plain: 3 White/Lined: 1 White/Plain
That means that expected (observed) numbers are:

Yellow/Lined: 270 (279)
Yellow/Plain: 90 (81)
White/Lined: 90 (95)
White/Plain: 30 (25)

4. When you do the chi-squared test of this cross you will find whether or not your genes are liked. You will notice that the only thing that I am “guessing” from the beginning is which genes are dominant and recessive and you will see that expected numbers are relatively close to the observed for F2 so the hypothesis of which are dominant and which are recessive alleles must be correct and any statistically significant deviations from the expected are due to the linkage between two genes.

5. Now, if you back cross F1 to white/lined F0 you will get:

YyLl x yyLL – (do your cross for genotypes) — 50% Yellow/Lined and 50% White/Lined

6. Lastly, if you cross F0 White/Plain to Yellow/Lined you would expect the same F1 and F2. The only technical problem with this cross is that you will not be able to distinguish between Yellow/Lined parent and F1 progeny based on phenotype.

Good luck!


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