Expert: Florence M Rollwagen - 10/23/2008

Question
QUESTION: Hi, I'm reading through my text, and I'm having a difficult time understanding some parts, which is basically causing holes in my understanding of crossing over and recombination and linkage mapping. I've posted several of my questions below:


1.  Textbook excerpt:  "The percentage of tetrads involved in an exchange between two genes is twice as great as the percentage of recombinant gametes produced. When two linked genes are more than 50 map units apart, a crossover can theoretically be expected to occur between them in 100% of the tetrads and the genes behave as if they are not linked."  -- I dont understand the first sentence and my book does not furthur explain it.

2. Is linkage map the same thing as chromosomal map?

3. What are reciprocal classes?  My book gives sort of an example and vaguely says "*they* are called reciprocal classes". My glossary also lacks a formal definition. They use "reciprocal" throughout the section and I dont know what it means so i obviously dont understand what the rest of the section is explaining. My book mentions it being something like one chromosome having one mutant allele and the second two other mutant alleles..??? Would whatever they're talking about be applicable if a three point cross was made with one parent having one allele and the other having the other two mutant alleles?  -- sorry if my question is unclear.. my understanding of it is all muddled up and so my questions may be muddled up. lol.



ANSWER: Hi Sungee:  Thanks for your detailed and difficult questions! [grin]

Not that I'm putting you off, but you CAN ask your teacher!  I just love it when my students ask questions.  It gives me a chance to talk about my favorite topic - biology!

So, in answer, here goes:

The percentage of tetrads involved in an exchange between two genes is twice as great as the percentage of recombinant gametes produced.

I think this is a numbers game.  In meiosis (where recombination occurs) four gametes are produced.  Two of them will be recombinants, and two will be parental (assuming a single crossover point).


Is linkage map the same thing as chromosomal map?

A linkage map is a genetic map of a species or experimental population that shows the position of its known genes and/or genetic markers relative to each other in terms of recombination frequency, rather than as specific physical distance along each chromosome.

A chromosomal map is generally called a karyotype and refers to all the chromosomes and their genes.

Are you sure you’re not confusing a genetic map with a linkage map?


What are reciprocal classes?

Reciprocal classes are the pairs of chromosomes that were produced by a crossover in the same location.

Hope this helps.  Write back if you have more questions.

FM Rollwagen, PhD





---------- FOLLOW-UP ----------

QUESTION: Unfortunately, I believe my teacher is more interested in research than teaching. S/he gives us this belittling expression when we have questions that she thinks isnt up to her standards - which i understand to some extent, but some of the questions that the students ask seem like genuine misunderstandings or need of clarification.


2. It said in my book that Sturtevan constructed a *chromosome map* of the three genes on the X chrom, setting 1 mu to 1% recombination.. which is why i was confused. So a chromosome map is a karyotype - without the genes and their locations on the chromosomes, and a linkage map shows the chromosome and the genes and their sequence/distance relative to one another, is this correct?  I want to make sure I understand this.
Because I like visuals, I've found pictures of what i think each of them are, but i'm not sure:

Right - Linkage map?
http://aggie-horticulture.tamu.edu/rose/pictures/rmap.GIF

Genetic map:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/Z/Zea_9.gif

Pictures of correction would be great!


3.  Sorry.. could you provide a more expansive explanation with examples and details and all?  Thanks!


Answer
Sorry to hear that your teacher is unresponsive.  If you are a biology major, you will have to learn to be more assertive.  It is the teacher's responsibility to answer your questions.  Just keep pushing!

Another linkage map: http://www.gene.affrc.go.jp/images/fig/marker_azuki01.png


Genetic map:
http://www.bionet.nsc.ru/booklet/images/imagesLaboratories/Serov1Big.gif

As for reciprocal classes.  The format of this site doesn't allow me to draw anything, so here are some more links:

http://books.google.com/books?id=KGXjKuOJm5YC&pg=PA135&lpg=PA135&dq=genetic+%22r

http://www.cod.edu/people/faculty/fancher/genetics/Notation.doc

http://www.qimr.edu.au/davidD/Course/part1.html

Hope this helps!  Write back if you have more questions.

FM Rollwagen, PhD