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trait controlled by 1 or 2 genes? dom or rec problem

Genetics as it applies to evolution, molecular biology, and medical aspects.

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trait controlled by 1 or 2 genes? dom or rec problem

Postby virlomi_stone » Sat Mar 04, 2006 6:00 am

I have been working on this problem for an hour and I finally think I have the correct answer but my solutions manual does not have this problem in it. If anyone could comfirm the answer or tell me what I did wrong I would greatly appreciate it. Oh and here's a pre-emptive sorry for the length but I have to type out all 4 parts of the question for my answer to make sense. :D

Here goes....(my answers in itallics)

Two true-breeding white strains of the plant Illegitimati noncarborudum were mate and the F1 progeney were all white. When the F1 pants were allowed to self-fertilize, 126 white flowered and 33 purple-flowered F2 plants grew.




a.) How could you describe the inheritence of flower color? Describe how specific alleles influence eachother and therefore affect phenotype.

Flower color is determined by 2 genes. In one of the genes the white mutation is due to a dominant allele. If at least one of these alleles are present the flower will be white no matter hat alleles are at the second locus. The second gene gives a white mutant when recessive at the second locus, even if the first locus is Wild-type (does not have the mutant dominant allele)

b.) A white F2 plant is allowed to self-fertilize. Of the progeny 3/4 are white and 1/4 are purple. What is the genotype of the white F2 plant?

Nomenclature that I am using....
w+--wild-type at w gene locus (need 2 w+ to be wild-type because of dominant mutant allele)
wd--dominant white mutant (only need 1 wd to be a white mutant)
i+--wild-type at i gene locus (i+i+ or i+i will be wild-type)
i--white mutant (need 2 i's to be mutant)

To get purple the genotype must be w+w+ i+i+ or i+i, the white F2 that is self crossed must be w+wd i+i......Right?

c.)A purple F2 plant is allowed to self-fertilize. Of the progeny, 3/4 are purple-flowered and 1/4 are white-flowered. What is the genotypes of the 2 white F2 plants?

The purple F2 plants are w+w+ i+i and w+w+i+i

The question didn't ask this but the progeny from this cross would be:
Purple---w+w+ i+i+ or i+i
White---w+w+ii......Right?


d.)Two white F2 plants are crossed with eachother. Of the progeny, i/2 are white-flowered and 1/2 are purple-flowered. What are the genotypes of the two white F2 plants?

White 1---w+w+ ii
White 2---wdw+ i+i+
and therefore the progeny would be:
1/2 w+w+ i+i (purple)
1/2 wdw+ i+i (white)


The question also did not ask this but I figured the original parental whit flowers and F1 progeny's genotypes and I just want to confirm theses as correct.

White parental 1---wdwd i+i+
White parental 2---w+w+ ii
F1---wdw+i+i

To anyone who actually made it this far Thank-you, Thank-you, Thank-you!!!!! It all seems to make sense but I worked on it so long I just want to be sure that it is correct(so I don't screw up my exam this Monday)
Last edited by virlomi_stone on Wed Mar 08, 2006 10:15 am, edited 2 times in total.
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Postby Khaiy » Sat Mar 04, 2006 4:41 pm

Well I'm not 100% sure that I followed all of your answers properly, but a couple of things that I saw:
You're calling the white phenotype a white mutation, which I don't really understand the reason behind. You're saying that the dominant white gene causes a white mutation, which is incorrect. A gene only causes a phenotype. There could be a mutation of the genes to produce a different phenotype, but that isn't the same thing as what you wrote.

Second, I might be missing a part of the question, but I didn't see where it said that white was a mutation and purple was wild type. But either way you want to look at it, the parental generation was true breeding for white flowers, which means that there wasn't any recessive gene for color in its genome. The same would thus be true for the F1 generation. I seems like there was a mutation of the color gene in the F1, which would allow for a recessive allele that codes for the purple phenotype.

This makes sense to me, please post again or PM me if I was unclear (or just wrong, which is also possible :p)
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Postby virlomi_stone » Sun Mar 05, 2006 4:22 am

A mutation is a phenotype that is represented in less than 1% of the population(as a whole not just the progeny of one specific cross) and a wild-type phenotype is a phenotype that is represented in greater than 1% of the population. For a particular trait there can be many mutant phenotypes as wells as many wild-type phenotypes. Dominance and recessiveness is not the same as wild-type and mutant. Dominance/recessive refers to the how a particualr allele is expressed versus the 2nd allele at the same locus. Wild-type/mutant refers to the phenotype only. A wild-type phenotype can come from dominant alleles or from recessive alleles depending on the trait and organism in question, same for mutant.
A mutation can occur because of a dominant allele. For example huntington's disease in humans is casued by a dominant allele. If a person carriers at least 1 of the Huntington alleles they will be affected with the disease. In order to not be affected the person must have two of the "wild-type" alleles which are inherited as recessive alleles. If the person has 1 Huntingtons alleles and one wild-type allele the person will still be affected with the disease. (ie..Huntingtons is a mutation)

The part you were refering to as missing part of the questions that said white was mutant and purple was wild-type was partially my fault. I should have stated that this was a complemation test, however when the question asks how the trait is being inhertited it is infered. a complemation test can help explain if a particualr trait is caused by mutation in the dame gene or different genes. To do a complementation test you would cross two mutants from different stocks (ie.. not related)that have the same phenotype. If in the F1and F2 all you get is white then the mutation is in the same gene. If in the F1or F2 another phenotype arises then you know that the mutation is in 2 genes. This is because at one locus the plant is mutant and the other locus it is wild-type. The second plant is mutant at the locus that the first is wild-type and wild-type where the first is mutant. Usually the F1 are all thewild-type phenotype because they recieve a wild-type allele for each locus because between the parents a wild-type allele is present at each locus in the F1. In the case of my problem the F1 was all white because one of the loci had a dominant mutation. The wild-type phenotype could not be observed until the F1 were self-crossed and the progeny were able to recive 2 of the wild-type(purple alleles) The moral of the story is... From the first cross being 2 white flowers it is assumed that that is the mutant phenotype.

Sorry to write so much again. I really appeciate your input. This actually helped me to understand this concept better by putting it in to words. As for the problem I would greatly appreciate more input from anyone who cares to work it out themselves. Thanks again!!!
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getting nervous

Postby virlomi_stone » Mon Mar 06, 2006 2:25 am

I see that a lot of people have views this post but only 1 reply(thank-you again) I would really appreciate some more insight so I can confirm my logic is correct...I have an exam tomm @11:30 am pacific time and I just want to make sure that I didn't learn this incorrectly. My prof doesn't have office hrs before the exam so I can't ask her. Help please :D
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