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[NOTE: This posting has been edited and updated several times during the course of this discussion to reflect criticisms and new information offered by the various contributors. This may impact the relevance of your comments and responses. We suggest you review this initial posting before proceeding with any comment or contribution. We may have already responded to, or corrected the subject of your contribution.]
The subject of the genetics of the red color form of Grammostola rosea, the Chilean rose tarantula, recently came up (for the umpteen gazzillionth time) on the Chat forum. That prompted me to compose a lengthy essay on it that eventually got so long and involved that I decided to post it as a separate thread on this forum instead so it could be referenced easily later. I'd be interested in reading critiques and criticisms in the hopes that I could correct or improve it.
Have at it gang. I already have my asbestos suit on in anticipation of the flame war!
Is that enough warning for you NOT to immediately try to turn this into God's truth and write it up in another witless care sheet, misquoting me in the process?
There are a lot of nit-picking conditions included in the statements that follow, and each detail is important. So, I would recommend that you read these paragraphs over at least twice to try to understand them better. Similarly there are a lot of assumptions made here that will be important to those with a background in genetics. But, this is not written for those people. They can write their own essay.
Lastly, we have purposely gone out of our way to not use a bunch of technical terms (e.g., homozygous, heterozygous, allele, genotype, phenotype, and a lot more). We wanted to keep this simple. This is for the novice who merely wants to expand their limits a tad. If you're interested in wading into the deep stuff, Google any of the technical terms in this essay. Or, get a good genetics book from a college library. (Hint: Good genetics books are usually those that DON'T use 90% of their bulk to itemize the 1,000,001 inheritable defects known to occur in the human race.)
I would now like to introduce you to several concepts. First, each adult Chilean rose carries two copies of each gene. It inherited one from its mother and one from its father. And, when our Chilean rose reproduces, regardless of its sex, it passes a copy of either one or the other of that pair to each of its offspring. Thus, each of its offspring ends up with a pair of that particular gene, just like each of its parents.
This becomes particularly interesting if a given gene occurs in two or more states, e.g., if there is a difference in the way that it can function. Thus, the gene of the pigment melanin exists in two states: functional and nonfunctional. It either works, or there is something wrong with it and it doesn't work. Thus, it would be reasonable to expect that any individual might inherit both functional genes, one functional and the other nonfunctional, or neither gene functional depending on what genes were available in their parents, and the "luck of the draw."
There are a number of different kinds of inheritance, and I won't discuss them all here. However, the two most common kinds are:
1> Dominant/recessive. In this scenario one gene will mask the expression of the other if they occur in the same individual. The masking gene is said to be dominant, and the gene that is masked is said to be recessive. Thus, an individual that inherits one functional gene and one nonfunctional gene for melanin will still have black pigment. But, if it inherited one nonfunctional gene from its mother and another from its father, i.e., both genes were nonfunctional, it would lack the black pigment melanin entirely.
2> Incomplete dominance. In this scenario, neither of the two states of the gene are masking their partner, and an individual that inherits one of each appears to be an intergrade or an "average" between the two "pure" states. Thus, for example, some flowers may be pink instead of red or white.
[EDIT: I have removed co-dominance from the preceding paragraph and what follows because of a particularly lucid explanation given by BioTeach, below.]
Now we come to a discussion of a construct used by geneticists called a Punnett square. Basically it's a graphical way of sorting out the inheritance of a particular character, and helps the budding geneticist (i.e., you) understand what's happening.
In its simplest form a Punnett square is merely a very simplistic checkerboard composed of two rows and two columns.
Either the rows or the columns may be ascribed to the mother, and the other orientation ascribed to the father. We did it like so. If you prefer the other way, we encourage you to do your own at home.
We will assume that the red pigment that makes an RCF Chilean rose so intensely colored is controlled by a gene that can be either functional or nonfunctional. Functional will be represented by an uppercase "R," and nonfunctional will be represented by a lowercase "r."
It should be patently obvious that either parent can have either both genes the same, i.e., RR or rr; or they can have each of the two genes different, i.e., Rr. (The combination rR is the same as Rr.) And, it should also be patently obvious that if you cross an RR mother with an RR father you're only going to get RR babies. (The same is true if they're both pure rr.)
But, if one is RR and the other is rr, you'll get all Rr babies. That is demonstrated in a Punnett square by ascribing the RR condition to the father, and the rr condition to the mother. And, yes we could have done it the other way around, but we would have obtained the exact same results in the offspring.
Now fill in the blank squares. Each square represents one set of offspring, and its contents are supposed to indicate the gene combination inherited by that set of offspring.
And, here is the scorecard for that particular square.
We got all hybrid for the red gene. And quite predictably, we got no all red (RR) or un-red (rr) individuals.
But, what happens if you mate two individuals that are both hybrid for the red gene?
And, here is the scorecard for that square.
Now we have all three color forms. Or do we?
If R masks r (i.e., R is dominant), RR will show the red condition for sure. But, so will Rr! You won't be able to tell the difference. So in fact, we SEE only two states: Red and un-red.
If however, R is incompletely dominant we will see a third color form, probably intermediate between the other two: Red, partly red (pink?), and un-red.
So now, here is where the proverbial rubber meets the road:
1> What do we see in Chilean rose coloring? Answer: We see at least three color forms!
2> Which of the two preceding conditions does that match more closely? Answer: The second situation with red pigmentation being incompletely dominant.
So, based on our (newly acquired) background in genetics, and our observations, we can hypothesize that the red coloring in Grammostola rosea, the Chilean rose tarantula, is probably an incompletely dominant type of heredity. And that, men, women, boys, and girls, if it's true would neatly tie up the question of the inheritance of the different color phases in one big, internally and externally, consistent theory without calling into question the issues of hybrids between species or multiple species being mistaken for one.
But how do we prove it? We could start an elaborate breeding program, carefully noting the outward appearance of each individual Chilean rose, who it was bred to, and what the resulting babies looked like, but that will surely take the better part of this century. But, we may have a better alternative. Enough Chilean roses have been bred in captivity that we probably could merely compile the appropriate data about what the father and mother in each crossing looked like, and what the babies looked like (with the numbers of each color form produced as well, a topic I intentionally skirt until the very end, below.). A sufficiently clever amateur geneticist should be able to sort out the genetics of red pigmentation merely by making such a population study without a 75 year commitment.
But hey! If you want to go take the high road, go for it!
EDIT, 2012-Jan-03: I failed to include the following paragraphs in the original post. I am appending them herewith. Sorry for any confusion.
Thus, by this hypothesis we can define the three colors reported most often as:
(Click on the thumbnails if you need a larger version of the graphics. All graphics are uploaded with ImageShack.us)
(Appearance = Acronym = Genetic Makeup)
Red (copper) color form = RCF = RR
Pink color form = ??? = Rr (Help me with the acronym for this one, somebody!)
Grey color form = NCF = rr
And, the outcomes from interbreeding these various color forms are inferred in the scorecards above and itemized here:
RR x RR = 100% RR, 0% Rr, 0% rr
RR x Rr = 50% RR, 50% Rr, 0% rr
RR x rr = 0% RR, 100% Rr, 0% rr
Rr x Rr = 25% RR, 50% Rr, 25% rr
Rr x rr = 0% RR, 50% Rr, 50% rr
rr x rr = 0% RR, 0% Rr, 100% rr
Thus, the relative proportions of the various color forms in any single eggsac might hold important clues about the genetic makeup (e.g., RR, Rr, rr) of the parents.
The subject of the genetics of the red color form of Grammostola rosea, the Chilean rose tarantula, recently came up (for the umpteen gazzillionth time) on the Chat forum. That prompted me to compose a lengthy essay on it that eventually got so long and involved that I decided to post it as a separate thread on this forum instead so it could be referenced easily later. I'd be interested in reading critiques and criticisms in the hopes that I could correct or improve it.
Have at it gang. I already have my asbestos suit on in anticipation of the flame war!
[size=+1]WARNING: THE FOLLOWING IS MERELY CONJECTURE!
CONSIDER IT AN HYPOTHESIS ONLY!
IT NEEDS TO BE PROVEN BY ACTUAL EXPERIMENT!
DON'T QUOTE THIS AS FACT![/size]
CONSIDER IT AN HYPOTHESIS ONLY!
IT NEEDS TO BE PROVEN BY ACTUAL EXPERIMENT!
DON'T QUOTE THIS AS FACT![/size]
Is that enough warning for you NOT to immediately try to turn this into God's truth and write it up in another witless care sheet, misquoting me in the process?
There are a lot of nit-picking conditions included in the statements that follow, and each detail is important. So, I would recommend that you read these paragraphs over at least twice to try to understand them better. Similarly there are a lot of assumptions made here that will be important to those with a background in genetics. But, this is not written for those people. They can write their own essay.
Lastly, we have purposely gone out of our way to not use a bunch of technical terms (e.g., homozygous, heterozygous, allele, genotype, phenotype, and a lot more). We wanted to keep this simple. This is for the novice who merely wants to expand their limits a tad. If you're interested in wading into the deep stuff, Google any of the technical terms in this essay. Or, get a good genetics book from a college library. (Hint: Good genetics books are usually those that DON'T use 90% of their bulk to itemize the 1,000,001 inheritable defects known to occur in the human race.)
I would now like to introduce you to several concepts. First, each adult Chilean rose carries two copies of each gene. It inherited one from its mother and one from its father. And, when our Chilean rose reproduces, regardless of its sex, it passes a copy of either one or the other of that pair to each of its offspring. Thus, each of its offspring ends up with a pair of that particular gene, just like each of its parents.
This becomes particularly interesting if a given gene occurs in two or more states, e.g., if there is a difference in the way that it can function. Thus, the gene of the pigment melanin exists in two states: functional and nonfunctional. It either works, or there is something wrong with it and it doesn't work. Thus, it would be reasonable to expect that any individual might inherit both functional genes, one functional and the other nonfunctional, or neither gene functional depending on what genes were available in their parents, and the "luck of the draw."
There are a number of different kinds of inheritance, and I won't discuss them all here. However, the two most common kinds are:
1> Dominant/recessive. In this scenario one gene will mask the expression of the other if they occur in the same individual. The masking gene is said to be dominant, and the gene that is masked is said to be recessive. Thus, an individual that inherits one functional gene and one nonfunctional gene for melanin will still have black pigment. But, if it inherited one nonfunctional gene from its mother and another from its father, i.e., both genes were nonfunctional, it would lack the black pigment melanin entirely.
2> Incomplete dominance. In this scenario, neither of the two states of the gene are masking their partner, and an individual that inherits one of each appears to be an intergrade or an "average" between the two "pure" states. Thus, for example, some flowers may be pink instead of red or white.
[EDIT: I have removed co-dominance from the preceding paragraph and what follows because of a particularly lucid explanation given by BioTeach, below.]
Now we come to a discussion of a construct used by geneticists called a Punnett square. Basically it's a graphical way of sorting out the inheritance of a particular character, and helps the budding geneticist (i.e., you) understand what's happening.
In its simplest form a Punnett square is merely a very simplistic checkerboard composed of two rows and two columns.
Either the rows or the columns may be ascribed to the mother, and the other orientation ascribed to the father. We did it like so. If you prefer the other way, we encourage you to do your own at home.
We will assume that the red pigment that makes an RCF Chilean rose so intensely colored is controlled by a gene that can be either functional or nonfunctional. Functional will be represented by an uppercase "R," and nonfunctional will be represented by a lowercase "r."
It should be patently obvious that either parent can have either both genes the same, i.e., RR or rr; or they can have each of the two genes different, i.e., Rr. (The combination rR is the same as Rr.) And, it should also be patently obvious that if you cross an RR mother with an RR father you're only going to get RR babies. (The same is true if they're both pure rr.)
But, if one is RR and the other is rr, you'll get all Rr babies. That is demonstrated in a Punnett square by ascribing the RR condition to the father, and the rr condition to the mother. And, yes we could have done it the other way around, but we would have obtained the exact same results in the offspring.
Now fill in the blank squares. Each square represents one set of offspring, and its contents are supposed to indicate the gene combination inherited by that set of offspring.
And, here is the scorecard for that particular square.
We got all hybrid for the red gene. And quite predictably, we got no all red (RR) or un-red (rr) individuals.
But, what happens if you mate two individuals that are both hybrid for the red gene?
And, here is the scorecard for that square.
Now we have all three color forms. Or do we?
If R masks r (i.e., R is dominant), RR will show the red condition for sure. But, so will Rr! You won't be able to tell the difference. So in fact, we SEE only two states: Red and un-red.
If however, R is incompletely dominant we will see a third color form, probably intermediate between the other two: Red, partly red (pink?), and un-red.
So now, here is where the proverbial rubber meets the road:
1> What do we see in Chilean rose coloring? Answer: We see at least three color forms!
2> Which of the two preceding conditions does that match more closely? Answer: The second situation with red pigmentation being incompletely dominant.
So, based on our (newly acquired) background in genetics, and our observations, we can hypothesize that the red coloring in Grammostola rosea, the Chilean rose tarantula, is probably an incompletely dominant type of heredity. And that, men, women, boys, and girls, if it's true would neatly tie up the question of the inheritance of the different color phases in one big, internally and externally, consistent theory without calling into question the issues of hybrids between species or multiple species being mistaken for one.
But how do we prove it? We could start an elaborate breeding program, carefully noting the outward appearance of each individual Chilean rose, who it was bred to, and what the resulting babies looked like, but that will surely take the better part of this century. But, we may have a better alternative. Enough Chilean roses have been bred in captivity that we probably could merely compile the appropriate data about what the father and mother in each crossing looked like, and what the babies looked like (with the numbers of each color form produced as well, a topic I intentionally skirt until the very end, below.). A sufficiently clever amateur geneticist should be able to sort out the genetics of red pigmentation merely by making such a population study without a 75 year commitment.
But hey! If you want to go take the high road, go for it!
EDIT, 2012-Jan-03: I failed to include the following paragraphs in the original post. I am appending them herewith. Sorry for any confusion.
Thus, by this hypothesis we can define the three colors reported most often as:
(Click on the thumbnails if you need a larger version of the graphics. All graphics are uploaded with ImageShack.us)
(Appearance = Acronym = Genetic Makeup)
Red (copper) color form = RCF = RR
Pink color form = ??? = Rr (Help me with the acronym for this one, somebody!)
Grey color form = NCF = rr
And, the outcomes from interbreeding these various color forms are inferred in the scorecards above and itemized here:
RR x RR = 100% RR, 0% Rr, 0% rr
RR x Rr = 50% RR, 50% Rr, 0% rr
RR x rr = 0% RR, 100% Rr, 0% rr
Rr x Rr = 25% RR, 50% Rr, 25% rr
Rr x rr = 0% RR, 50% Rr, 50% rr
rr x rr = 0% RR, 0% Rr, 100% rr
Thus, the relative proportions of the various color forms in any single eggsac might hold important clues about the genetic makeup (e.g., RR, Rr, rr) of the parents.
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