I thought this was interesting to pin as well get towards stabilizing new crosses-especially newbies. It is from breeder Fred Hempel of Artisan Tomatoes from another forum.
The rule of thumb is that True Breeding is approximated at F7.
"What happens, after a cross is first made and then subsequent selections are made by harvesting from "selfing" plants, is that you lose half of the heterozygous gene pairs with each generation of single plant selection.
Let's assume that all gene pairs are heterozygous (not true, because many gene pairs are already homozygous, or functionally equivalent) in a hybrid, because for some genes, both parents have identical, or functionally equivalent, copies.
But, if we assume that all gene pairs are heterozygous that would mean that 100% of the genes are in a heterozygous state in an F1 hybrid -- because each parent contributes their versions of each of the genes in the genome.
In the F2 (based on Mendel's laws of segregation) one half of the genes, however become homozygous -- are represented by two copies from one of the original parents. So, at F2 you have lost half of the heterozygosity (only 50% of the genes are heterozygous)
You lose another half of the heterozygous gene pairs at F3 -- Down to 25%
F4 -- 12.5%
F5 -- 6.25%
F6 -- 3.125%
F7 -- 1.5625% So at F7 over 98% of the gene pairs are homozygous.
This means that you are not actually true breeding at F7, but you are close, and as long as the plants are uniform, you can assume that the lines are pretty much stable. It is a numbers game. And since you start out with both parents having some equivalent gene pairs right from the start, you are almost assuredly MORE homozygous than the number calculated assuming that you start out heterozygous at each gene pair."
The rule of thumb is that True Breeding is approximated at F7.
"What happens, after a cross is first made and then subsequent selections are made by harvesting from "selfing" plants, is that you lose half of the heterozygous gene pairs with each generation of single plant selection.
Let's assume that all gene pairs are heterozygous (not true, because many gene pairs are already homozygous, or functionally equivalent) in a hybrid, because for some genes, both parents have identical, or functionally equivalent, copies.
But, if we assume that all gene pairs are heterozygous that would mean that 100% of the genes are in a heterozygous state in an F1 hybrid -- because each parent contributes their versions of each of the genes in the genome.
In the F2 (based on Mendel's laws of segregation) one half of the genes, however become homozygous -- are represented by two copies from one of the original parents. So, at F2 you have lost half of the heterozygosity (only 50% of the genes are heterozygous)
You lose another half of the heterozygous gene pairs at F3 -- Down to 25%
F4 -- 12.5%
F5 -- 6.25%
F6 -- 3.125%
F7 -- 1.5625% So at F7 over 98% of the gene pairs are homozygous.
This means that you are not actually true breeding at F7, but you are close, and as long as the plants are uniform, you can assume that the lines are pretty much stable. It is a numbers game. And since you start out with both parents having some equivalent gene pairs right from the start, you are almost assuredly MORE homozygous than the number calculated assuming that you start out heterozygous at each gene pair."
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