Coming back to the OT 'Basic Genetics Explained', does anyone have an idea on the frequency of chromosomal crossover in cannabis/hemp? I mean, this is an important part of basic genetics and we've been ignoring/neglecting it completely. I tried several times to bring that up by mentioning linked genes/traits but so far nada...
For those who don't know what chromosomal crossover means:
In diploid species like humans or cannabis, each set of chromosomes comes as a couple. The simple version is that one of each comes from the mother and the father, respectively. As you may know by now (if you read the whole thread), an F1 population is homogeneous and genetically 50:50 of the original different varieties but starts to segregate in the subsequent generations (F2 etc.). Till now, we were talking about whole chromosomes or traits/loci recombining to homozygous couples following certain rules like 9:3:3:1 segregation and such (see Mendel's rules). In reality, these numbers (given a huge enough test population) are only true if the genes responsible for the traits are located on separate chromosomes.
Theoretically, all traits defined by loci physically residing on the same chromosome would be linked (chemically bound to each other via DNA strands and hence always occur together). In practice, they are only linked to a certain degree, the linkage isn't perfect and can be broken during meiosis (i.e. once per generation). This happens through an exchange of DNA strands between the mothers and the fathers chromosomes. After a certain number of generations, an individual will no longer contain chromosomes from mother and father but a chromosome patchwork made up of pieces from the 'original ancestors'.
Simply put, the longer the physical distance between two loci/genes, the higher chances are that they get separated. The unit, an experimental or functional distance, is called map units (m.u.) or centimorgan (cM) and indicates the relative chance in % of two loci/genes getting separated during meiosis (1 cM = 1 separation each 100 generations); obviously, this also works for two separate traits getting recombined or linked. The maximum would be 50% (for a detailed explanation see for example wiki, the article if reliable). The linked genes or rather traits I'm interested in would be such with less than maybe 5% or 5 m.u. because these I regard as useful or 'manageable linked'.
Because each species has a crossover rate of its own, 5 m.u. can be a long or a short physical distance in different species. Now, I have absolutely no idea how often crossing overs occur in cannabis and hence how likely macroscopic monogenetic traits might occur. But I can't repeat myself often enough, they do occur... we just have to find them.
Why do I insist so much on that? Simply, if you breed and want to combine two traits (or separate a wanted from an unwanted one, respectively) but don't find any useful individual in the offspring, then knowing the cM might help you realise that you'd need 100'000 meioses (could be 100'000 generations, 100'000 plants, or a combination thereof like 10 generations with each 10'000 individuals) to statistically succeed and hence, that the struggles you have are normal
.
Oh, and BTW, crossovers are also one of the reasons why reverse breeding and 'rebreeding' stabilised heirloom hybrids, like Haze or Skunk #1, from scratch is as good as impossible even with the original parental lines in hand...
For those who don't know what chromosomal crossover means:
In diploid species like humans or cannabis, each set of chromosomes comes as a couple. The simple version is that one of each comes from the mother and the father, respectively. As you may know by now (if you read the whole thread), an F1 population is homogeneous and genetically 50:50 of the original different varieties but starts to segregate in the subsequent generations (F2 etc.). Till now, we were talking about whole chromosomes or traits/loci recombining to homozygous couples following certain rules like 9:3:3:1 segregation and such (see Mendel's rules). In reality, these numbers (given a huge enough test population) are only true if the genes responsible for the traits are located on separate chromosomes.
Theoretically, all traits defined by loci physically residing on the same chromosome would be linked (chemically bound to each other via DNA strands and hence always occur together). In practice, they are only linked to a certain degree, the linkage isn't perfect and can be broken during meiosis (i.e. once per generation). This happens through an exchange of DNA strands between the mothers and the fathers chromosomes. After a certain number of generations, an individual will no longer contain chromosomes from mother and father but a chromosome patchwork made up of pieces from the 'original ancestors'.
Simply put, the longer the physical distance between two loci/genes, the higher chances are that they get separated. The unit, an experimental or functional distance, is called map units (m.u.) or centimorgan (cM) and indicates the relative chance in % of two loci/genes getting separated during meiosis (1 cM = 1 separation each 100 generations); obviously, this also works for two separate traits getting recombined or linked. The maximum would be 50% (for a detailed explanation see for example wiki, the article if reliable). The linked genes or rather traits I'm interested in would be such with less than maybe 5% or 5 m.u. because these I regard as useful or 'manageable linked'.
Because each species has a crossover rate of its own, 5 m.u. can be a long or a short physical distance in different species. Now, I have absolutely no idea how often crossing overs occur in cannabis and hence how likely macroscopic monogenetic traits might occur. But I can't repeat myself often enough, they do occur... we just have to find them.
Why do I insist so much on that? Simply, if you breed and want to combine two traits (or separate a wanted from an unwanted one, respectively) but don't find any useful individual in the offspring, then knowing the cM might help you realise that you'd need 100'000 meioses (could be 100'000 generations, 100'000 plants, or a combination thereof like 10 generations with each 10'000 individuals) to statistically succeed and hence, that the struggles you have are normal
.Oh, and BTW, crossovers are also one of the reasons why reverse breeding and 'rebreeding' stabilised heirloom hybrids, like Haze or Skunk #1, from scratch is as good as impossible even with the original parental lines in hand...
