Genetic recombination

[Blue_X511]

Alright, first and foremost, I would like to apologize in advance for my haste, but there is a biology question that I really don't seem to understand. Now I understand the material, but I guess it's just the way that the question states it. Now, I was hoping that someone could either help step me through this problem or would help state it in a manner that I may understand. Now the question originates from my textbook in my Accelerated Biology (9th grade) class, under a chapter assessment. The chapter goes into detail of Gregor Mendel's work, Punnett Squares, Meiosis, Linkage, and Gene Maps. I want to thank you in advance, and the question follows:

Comparing and Contrasting: Suppose a plant geneticist uses true-breeding plants to make a two-factor cross involving genes that are closely linked on a chromosome. How would the genetic variation in the F2 generation probably differ from a cross in which two genes assort independently?

Now, I understand what it means by true-breeding plants and crossing over and such, but I guess that I just get lost somewhere. Any help would be appreciated.


EDIT: I would also like to apologize for the title--being so vague--however, I couldn't decide on a fitting title.

[mith]

Assume you had a parent with genotype AaBb(remember each person has two copies of chromosomes, one from each of their parents).

Then the chromosomes may look like this


copy 1 of chromosome looks like this

____gene A______gene B_______

and then copy 2 looks like this

____gene a______ gene b________


Now a cross over event causes something like this to be produced

_____gene A _________gene b____(copy 1)


_____genea__________gene B____(copy 2)

That's a recombination.

[Blue_X511]

Alright, thank you. I think that I understand now.

[blcr11]

What mith said is true, but I thought the question was what was the difference between the recombination frequencies in linked vs unlinked genes. If your question has been answered then the following is superfluous, I guess.

The expected 9:3:3:1 phenotypic ratios observed in the F2xF2 offspring assumes that the genes for the two traits assort independently of one another. (There is usually the tacit assumption that the traits exhibit simple dominant/recessive behavior, which need not be the case and can alter the outcomes.) This will almost certainly be true of two genes located on separate chromosomes, and will likely be true of two genes on the same chromosome but “far apart.” The closer together the two genes are to each other, the less likely it is that cross-over events will separate them; they do not assort independently as per the Mendelian assumption, and the phenotypic ratios of the offspring will be something other than the expected 9:3:3:1.