Using segregation to recover traits and create new varieties

Let's now look at how one can use that segregation from the previous page to select and create new true breeding lines from any hybrid.

The F1 hybrid is self pollinated and creates a segregating F2 generation. 










phenotypic ratio  3:1  of 4 possibilities, 3 regular leaf plants and 1 potato leaf plant

One in every four plants should be a recessive potato leaf. If all one wanted was a potato leaf plant, one would just select for the plant(s) that show the trait and that would be it . Recessive traits are easy to recover because as soon as you see that trait you know it is "fixed" because the only outcome it can have in the next generation (assuming self  pollinating only) is another recessive. Like as stated on  the first page:





  Once a recessive trait is recovered, that recessive gene will be in all of that individual's offspring.

Fixing or recovering a dominate trait in a population is not as easy. This is because when one sees the dominate trait in the F2 (and for the next few generations) one can't be sure if the genotype is heterozygous or homozygous.

Does the

regular leaf





To recover a dominate trait it will require one to select for it and grow out till at least to the F7/F8 generation for the reasons illustrated on the page about segregation. This will process rids the population of ambigous heterozygotes so that the line will eventually breed true (by becoming homozygous).

Though it may be easy to recover a recessive quickly, one must remember that it isn't just that one recessive gene segregating but many others which may or may not be desirable (like plant size, foliage cover, yield, etc..). For this reason one should continue to carry out that line with the desired recessive trait (through continuous selfing each generation) to the F8 to ensure that all other gene pairs are fixed by removing most, if not all, of the heterozygotes.

Using Segregation

Now let's consider how one can use predictable segregation patterns to help purposely "create" a new line.

Because one can estimate what the potential outcome will be in a given F2 population, one can also predict how many plants one needs to find a given number of recessive traits.

Lets say one has two plants that each have separate characteristics and would like to combine both of those characteristics into a new line. One plant is a potato leafed variety with red fruit. The other plant is a regular leaf variety with yellow fruit. How many plants do we need to find one plant with both traits - a potato leafed with yellow fruit?

Potato leafed is recessive and from other lists know that yellow fruit is recessive to red fruit.


Remember that in an F2 one can expect that 1 in every 4 plants will be recessive.  If we are trying to recover 2 recessive traits in the same plant, there is a 1 in 4 chance for each trait. A 1 in 4 chance for a potato leaf type and a 1 in 4 chance for a yellow leaf (4 and 4). It will take16 plants to find one plant with both traits.


From the 4 chances for each gene 16 types are found.
Only one plant (bottom right corner) is both yellow fruited and potato leafed.

So to get just one plant with both one needs to grow at least 16 plants. However, one may want to select more than just one plant with both traits. So, by growing out 32 plants, one should find at least two plants (potato leaf and yellow fruit). Then one could select for other traits - like plant habit, foliage cover, yield, taste - among those two lines and have more opportunities to select from the best of those other segregating traits till stable.

What if one wants to look at more than just 2 genes?

# of recessive traits
for each
# of plants needed
to find one (with all)
4 x 4
4 x 4 x 4

If one is looking to recover even more traits simply use the calculator below to determine how many plants will be needed to find one individual with the number of traits entered.

Calculate Required Number of Plants Needed to Recover n Recessive Genes

Use the "+" and "-" buttons to toggle the number of genes

  • one will see the most segregation in the F2 and F3 lines. These are the best generations to try to recover traits desired. Some of it will be serendipity but by applying the know potential outcomes and selecting the desired traits early one can easily develop a new line
  • always take careful notes each year about what was observed in the various lines. Since this will occur over a matter of years pictures can help as well.

De-Hybridizing a Hybrid to make a stable true-breeding line.

Let's say you want to take a hybrid and make a reasonably close form of it so that one can save seeds of it and have it come "true" year to year. The process is really quite simply. Just use the segregation method and at the F2 begin selecting for the plants that best represent the original parent. Continue to do this for 6 or 7 generations and one will have a line that will be stable and true-breeding and somewhat like the hybrid parent. I say "somewhat" because it will be impossible to retain any heterozygous traits expressed in the hybrid that may have had made it unique and with hundreds of potential gene combinations, it is nearly impossible to grow enough plants to best represent the original hybrid (just try running that gene calculator up to the number "50" and see how many plants it's going to take just to get one that has all 50).

Other Information

These pages only discussed simple inherited dominance. There are other several other forms of inheritance I have not covered (such as incomplete dominance and co-dominance). This technique mentioned above is just one form of cultivar/variety development that lends itself well to working with an inbreeder like tomato. See these for  more information about other kinds of inheritance or breeding strategies.

Selection Methods in Plant Breeding Based on Mode of Inheritance
Selection Methods Relevant to Tomato
Self Fertilizing Crops
Selection Methods for the Development of Pure Breeding Cultivars from Crosses
Dominance Relationships

Resources for determining inheritance of genes:

Specific Fruit Genes
TGRC: List of Gene Names and Symbols

Return to Gene Segregation Patterns
Return to Tomato Gene Basics

back to the tomato page