Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
Genetics question
K....,
I don't read Orchid Digest but from your post, I can see where you are thinking the wrong thing. I'll attempt to sort this out but I'll probably make it messy! Starting at the beginning (I just KNOW that you know this at least as well as I do)... genes are packaged in chromosomes. Lets pretend an orchid has 20 total chromosomes (numbers in the 50s are common for Oncidiums). There would be 10 different kinds of chromosomes because they come in matches pairs. 2n = 20 and n = 10 in this pretend orchid. Lets start with species A, B, C, and D. Breed A and B together and there are are 10 chromosomes from each parent. Now breed C and D together and there is another plant with 10 chromosomes from each parent. Now lets go to F2. Breed the 2 new hybrids together. There are still only 10 pairs of chromosomes but 4 species involved. At this point the AVERAGE plant had 5 chromosomes from each grandparent. Even if ALL got 5 from each parent, all the chromosomes don't do the same thing. Some may have genes that affect color or size of flowers and some chromosomes do not. That's not really the point, however. The point is that with hundreds of plants, some will just happen to combine with chromosomes only from original plant A and C or B and D. MANY will get only one or 2 chromosomes from A or B or C etc. Since there are only 10 kinds of chromosomes (in this made up group of plants) and the plants only gets 2 of each, only 2 grandparents get represented in each kind of chromosome. Some plants will just happen to get none of the 10 from one ancestor or another. Go on for several more generations and many plants will have no genetic material from some of the early ancestors. K...., I know I didn't tell you one thing about genetics that you didn't already know. Somewhere you took Helmut Rohrl to mean something more than he did, I think. Last of all, I know genetics gets more complicated with genes having ways to migrate to different chromosomes so that one chromosomes will actually contain genes from more than one parent. There, also are the plants that end up as 4n or 6n; multiple copies of each chromosome and more potential to carry genes from more ancestors. That doesn't matter much. The above is still valid. I just hope I didn't confuse everyone. I'm no teacher! Steve (in the Adirondacks) K Barrett wrote: Anyone else subscribe to the Orchid Digest Magazine (not the Orchid Guide Digest email list)? In the most recent edition Helmut Rohrl gives a brief discussion on genetics and inheritance in complex oncid hybrids on page 40 thru 41. He makes the point that it 'was a widely held belief that an orchid hybrid cultivar could be precisely described in terms of the percentage of genetic material contributed by each species ancestor, however remote.' He goes on to say that in the F2 generation 'we know that for progeny which are more than one generation removed from the species parent (ie the F2 or grandchild generation) calculating percentages for genetic inheritance is meaningless. For these hybrid progeny it is a virtual certainty that some of these cultivars carry no chromosomal genetic material from a given grandparent, or older ancestor.' I'm having trouble agreeing with that. I *think* I'm having trouble because I'm used to animal genetics where a limited number of ovae are fertilized and offspring without genes from a grandparent might not survive past the blastula stage (*G*). However in orchids, hundreds of thousands of eggs can potentially be fertilized. Therefore the mathematical realm of possibilities (bell curve or gene segregation ) can be demonstrated. I still think, however, that in practice the lack of genetic material would lead to the decreased fertility seen in complex hybrids (ie its not unususal for some plants to be poor parental stock or for some crosses to yeild a minute number of seed or flasks). And that therefore there would indeed be genetic material from all parental stock in the exisiting offspring , no matter how far removed..... Am I wrong? Or am I right in why I'm wrong? I expect to be wrong because Rohrl is much more brilliant than I am. And when I was in school genetics was taught simplistically to us undergrads and DNA theory was in its infancy (the one gene one trait idea is now out the window, for example) Thanks in advance K Barrett |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Principles of Genetics 6th Ed. on sale | Plant Science | |||
Biotech & Genetics Industry Almanac 2003-2004 | Plant Biology | |||
Postdoc: Plant Evolutionary Genetics | Plant Biology | |||
GENETICS OF AMERICANS | Ponds | |||
GENETICS OF AMERICANS | Gardening |