Genetics Question

I have a Genetics final Monday, and I’m about to tear all my hair out in frustration. I’m hoping someone can explain the ideas behind parental and recombitant gametes to me, because I sure as hell am not getting it. It’s either because I’m a bit slow or because the writers of my textbook are intentionally making this impossible. Here’s the paragraph in question:

Here’s my question about that. I thought if you mated

AA BB X aa bb

the only possible offspring would be Aa Bb (They’d get A and B from the first parent, a and b from the second, with no other possibilities.) It looks to me like Aa and Bb are entirely recombitant traits; that is, they are wholly different from either AA, BB, aa or bb, so they are not parental. I know that there can be no more than 50 percent recombination (although I don’t understand why; isn’t the above example one of 100 percent recombination?) I don’t see how you’d ever get Ab or aB gametes from such a cross, as the above quote claims.

Anyway, I’m really, really confused, and this chapter is a big part of the final. Any help would be welcome.

Argh, I’m really sorry, I could have sworn I posted this to General Questions, not Great Debates. Could a moderator please move this? Thanks.

Its been a long time since I’ve had Genetics, but this website may help. I just did a search in google and pulled up a ton of them.

http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/linkage/linkage1.htm

oop, posted the wrong link.

http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/linkage/linkage2.htm

This paragraph deals with the results of a testcross of the progeny, that is AaBb x AaBb. We are looking to see what gametes these individuals would produce in such a testcross. There are four kinds of gametes produced here, because the alleles independently assort: AB, Ab, aB, and ab. If the parents of the double heterozygote (AaBb) were AABB and aabb, then the parental gametes (produced by the double heterozygote) are AB and ab. Likewise, if the parents were AAbb and aaBB, then the parental gametes would be Ab and aB. The recombinant gametes are the other two types.

Recombinant gametes are produced by independent assortment. Mendel noted that two traits separate in reproduction-- that is, the parental traits are not preserved. So if A represented large (versus small) pods and B represented green (versus yellow) pods, AaBb plant would have large, green pods. This individual would, in a test cross, produce all four phenotypic classes (green large, green small, yellow large, yellow small). This is true even if the AaBb parents were green large + yellow small (AABB x aabb) or green small + yellow large (AAbb x aaBB).

If independent assortment weren’t happening, what we would see is only parental gametes. So if the AaBb was a result of AABB + aabb mating (green large + yellow small), the AaBb would only produce AB and ab gametes, and therefore in a test cross would only produce AaBb, aabb, or AABB individuals. So only green large or yellow small. This is called genetic linkage.

The easiest way to think about this (for me) is in terms of DNA. Genes on different chromosomes independently assort – there is no tendency for one chromsome to be matched with another during gametogenesis. Genes close together on the same chromosome are linked. Linkage varies dependent on physical proximity, with DNA recombination (“crossing over”) separating alleles on the same chromosome during meiosis.

Take fruit flies. Let’s talk dominant alleles for simplicity. Three traits – Bc (Black cell – little black spots on the abdomen of the fly), Elp (Ellipse, a roughened eye), and Pr (Prickly bristles). Bc and Elp are close together on chromosome 2. Pr is on chromosome 3. If you cross a Bc/+ Elp/+ (double heterozygote) fly out, the majority of what you will see will either be Bc and Elp or neither. The parental gametes are not separated very often. If you cross a Bc/+ Pr/+ fly out, you will see Bc, Pr, Bc and Pr, and neither flies. Independent assortment gives you both the parental and the recombinant gametes.

The last line from my initial quote is actually supposed to read:

Sorry about that.

Thank you for the link, Scarlet. It does look helpful.

So “parental gametes” are gametes that would be produced by AaBb but resemble its own parents’ gametes (AA BB and aa bb)? So in determining if gametes are parental or recombitant we don’t look to the gamete producer (AaBb in this case) but its parents (the “grandparents” of the gametes, so to speak)? I think I confused this with recombitant and parental offspring (in which you compare the offspring’s and parents’ genes to determine if they resemble its parents’ genes or are a product of recombination.)

Thank you for your help so far. I’m still not sure why the maximum rate of recombination is 50% though. AaBb can produce four types of gametes:

Ab aB AB ab

Of those, half are identical to those of AaBb’s parents (the AB and ab) which does equal 50%.

But when AA bb and aa BB mate :

Ab can be produced by AA bb

and aB can be produced by aa BB, leading to:

AaBb children – which are wholly recombitant, it seems to me, in that they are completely different from their parents’ genotypes.

Moderator’s Note: Moving to General Questions.

You are basically correct – we look at AaBb’s parents to determine what parental versus recombinant will look like. If AaBb’s parents were AABB x aabb then the parental gametes (off AaBb) will be AB and ab. The recombinant ones will be Ab and aB. If AaBb’s parents were AAbb and aaBB, then the parental-type gametes will be Ab and aB, and the recombinant ones will be ab and AB.

Not quite. AaBb is producing four types of gametes: AB, aB, Ab, and ab. In this case, aB and Ab are parental type, and the ab and AB are from recombination. If the genes are not linked, each of these comes out with equal frequency. If they are, we will see parental type more than recombinants.

This kind of analysis usually comes from analyzing fungal tetrads, like from yeast or Ascobolus. One cell undergoes meiosis and produces four gametes in a little pod. These can be either phenotypically parental or recombinant. If all you see are parental tetrads, then you know the genes causing your phenotypes are genetically linked. This means they are in close proximity. By calculating the percentage of recombinant gametes, one can establish a genetic distance between the two genes, which is related to their physical distance. Using a set of alleles, one can dissect out the genetics of the organism – how many chromosomes, what order the genes are on each chromosome, etc.

Again, it is easiest to think of the DNA. Imagine that A and B are on different chromosomes. Let’s draw out the meioses. Start with a 2N 2C cell – we are going to 1N 1C. Pipes are chromosomes, dashes are centromeres.



|-|   |-|
A a   B b


Then we have DNA replication to 4N 2C.



||-||   ||-||
AA aa   BB bb


Meiosis I – this halves the complement, so we have 2N 1C. This is where independent assortment happens.



||    ||      ||    ||
AA    BB      aa    bb
 Cell 1        Cell 2

or

||    ||      ||    ||
AA    bb      aa    BB
 Cell 1        Cell 2


Then meiosis II to create 4 mature gametes at 1N 1C.



|   |    |   |    |   |    |   |
A   B    A   B    a   b    a   b
cell1    cell2    cell3    cell4

or

|   |    |   |    |   |    |   |
A   b    A   b    a   B    a   B
cell1    cell2    cell3    cell4


If AaBb’s parents were AABB and aabb, then the top line represents parental type and the bottom line represents recombinant type. It is a simple matter of which chromatid pair goes where in meiosis I. There is no bias to the top versus the bottom line, so there is a 50/50 split. When there is genetic linkage, we depend on DNA recombination to generate the non-parental gametes and separate the linked alleles. I can draw this out too, but since the scope of your question didn’t go there, I’ll leave it unless you ask.

Ack my quoting brackets are a little off. The last two sentences were supposed to be outside of the quote. Makes sense that way.

This paragraph deals with the results of a testcross of the progeny, that is AaBb x AaBb. We are looking to see what gametes these individuals would produce in such a testcross. There are four kinds of gametes produced here, because the alleles independently assort: AB, Ab, aB, and ab. If the parents of the double heterozygote (AaBb) were AABB and aabb, then the parental gametes (produced by the double heterozygote) are AB and ab. Likewise, if the parents were AAbb and aaBB, then the parental gametes would be Ab and aB. The recombinant gametes are the other two types.

Recombinant gametes are produced by independent assortment. Mendel noted that two traits separate in reproduction-- that is, the parental traits are not preserved. So if A represented large (versus small) pods and B represented green (versus yellow) pods, AaBb plant would have large, green pods. This individual would, in a test cross, produce all four phenotypic classes (green large, green small, yellow large, yellow small). This is true even if the AaBb parents were green large + yellow small (AABB x aabb) or green small + yellow large (AAbb x aaBB).

If independent assortment weren’t happening, what we would see is only parental gametes. So if the AaBb was a result of AABB + aabb mating (green large + yellow small), the AaBb would only produce AB and ab gametes, and therefore in a test cross would only produce AaBb, aabb, or AABB individuals. So only green large or yellow small. This is called genetic linkage.

The easiest way to think about this (for me) is in terms of DNA. Genes on different chromosomes independently assort – there is no tendency for one chromsome to be matched with another during gametogenesis. Genes close together on the same chromosome are linked. Linkage varies dependent on physical proximity, with DNA recombination (“crossing over”) separating alleles on the same chromosome during meiosis.

Take fruit flies. Let’s talk dominant alleles for simplicity. Three traits – Bc (Black cell – little black spots on the abdomen of the fly), Elp (Ellipse, a roughened eye), and Pr (Prickly bristles). Bc and Elp are close together on chromosome 2. Pr is on chromosome 3. If you cross a Bc/+ Elp/+ (double heterozygote) fly out, the majority of what you will see will either be Bc and Elp or neither. The parental gametes are not separated very often. If you cross a Bc/+ Pr/+ fly out, you will see Bc, Pr, Bc and Pr, and neither flies. Independent assortment gives you both the parental and the recombinant gametes.

Ok, that does make sense. Thanks. You’re right, I think I confused parental/recombitant gametes with offspring.

Actually, crossing over (DNA recombination) in prophase I and calculating map distance are what I have to study next. No rest for the weary :stuck_out_tongue:

edwino, how’d you manage a 2-hour double post?

Which genetics text is this? Seems to me that this passage:

has got parental and recombinant types reversed. The parental types should be AB and ab while the recombinant types are aB and Ab. The rest, edwino has quite ably explained.

Must be the transgenic hamsters.

Actually, that’s my dumb mistake in copying the paragraph. I posted the correction above.