Gene Segregation After a Cross
What Happens to the Heterozygous Gene Pairs?



Now let's look what happens to the genes of the F1 hybrid population when allowed to self pollinate.

note- the ability for a tomato to self pollinate is critical. It prevents any other genes from being introduced and this sets up how it is that true-breeding lines can be maintained or created.

Now that each gene pair is heterozygous in the F1, when the reproductive cells reduce, they have an equal chance of contributing either a dominate or recessive gene. As these halves recombine to make a new "egg" pair,  one now gets all three types of gene outcomes and again both leaf types appear in the new F2 generation's population. The gene outcome is called genotype and the leaf appearance is called the phenotype.

X
ovule
ovule

pollen

heterozygous
homozygous
dominate

pollen
homozygous
recessive
heterozygous

genotypic ratio 1:2:1  1 dominate : 2 heterozygotes : 1 recessive
25% dominate | 50% heterozygous | 25% recessive
phenotypic ratio 3:1 3 regular leaf plants for each potato leaf plant
how many are still heterozygous?
 50% heterozygous
50% homozygous		  The amount of heterozygotes was reduced by half

The new generation is called the "F2". Notice that the amount of "unlike genes" (heterozygotes) are reduce to half of the total population compared to the F1 generation. Notice also that the population's phenotype (how it looks) is about 75% regular leaves and 25% potato leafed or a 3:1 ratio.

Of the regular leafs,  one still can't tell exactly which offspring are homozygous dominate and which are heterozygous (what their genotype is) but by the probability table above one knows that the chance is that most of the regular leaf types will be heterozygous and less than half of the regular leaf types will be completely homozygous.

In the previous page I stated that over time inbreeding populations, like that of tomato, want to become completely homozygous. Let's look at how that happens below.  I don't have that many eggs so I need to switch to other symbols to represent the eggs.

Let's start back with the F1 hybrid generation. Allow the F1 hybrid to self pollinate to create a new F2 generation. Randomly select 128 seeds to grow out the F2 population and see what happens. Then allow that F2 generation to self pollinate and randomly select another 128 seeds to grow out to create an F3 generation and so on for 7 generations. Each generation is created by allowing the plants to only self pollinate and then randomly selecting 128 seeds it produces.

note - I only choose the number 128 because it works well to make the charts below

Look what happens to the unlike paired genes over time in randomly selected populations.

F1
100%
F2
50%
F3
25%










F4
12.5%
F5
6.25%
F6
3.125%











F7
1.6%
 F8
0%





After selfing, each subsequent generation reduces the amount of "unlike genes" in half until they are virtually all removed.  Once the chance of unlike genes is gone, the populations will be all homozygotes.

Now what I have shown you is the mathematical or statistically probability. In "real life" one is not likely to see such "exact numbers".
For the example above the F2 produced 96 regular leaf types to 32 potato leaf types or a 3:1 ratio. In real life, because one randomly chooses, that actual number may be different but fairly close to the 3:1 ratio (ie you might see something like 108:20). Genetists use a test called a Chi-square test to determine if what one "sees" actually fits what one "expects". If it "fits" the test that means the inheritance is simple and as expected. If it doesn't "fit" the test that might indicate some other method of inheritance is going and further tests might be needed to determine what is going on genetically with that trait. This test is not normally necessary for amateur breeders especially when one can determine the inheritance of a given trait ahead of time (like we know about regular vs potato leafed).

Another point to make about segregation is that in a population, especially very large ones, the heterozygotes don't necessarily completely disappear. This is why I said above "virtually all gone" on the previous page concerning true breeding homozygous populations. Even in a controlled population with NO outcrossing or no random mutations, there is a chance that some "unlike genes" are still left in the population due to random assortment (real life number as opposed to theoretical probabilities). Let me use the table below totry and illustrate this point:

Again notice how in each generation the number of  the "unlike genes" is reduced in half from the generation before..

Generation
 heterozygotes
 homozygotes
F1
 100%
0%
F2
 50%
 50%
F3
 25%
 75%
F4
 12.5%
 87.5%
F5
 6.3%
 93.7%
F6
 3.1%
 96.8%
F7
 1.6%
 98.4%
F8
 0.8%
 99.2%
F9
 0.4%
 99.6%
F10
 0.2%
 99.8%
F11
 0.1%
 99.9%
F12
 0.05%
 99.95%
F13
 0.025%
 99.975%
F14
 0.0125%
 99.9875%

But notice also from this table that when one keeps taking half of something, they never conclusively reach "zero". In the "egg chart", for all practical purposes "zero" is pretty much reached by the F8. But because of random assortment, there is a small chance that some unlike genes are left over in the population. This is even more pronounced in larger populations.

Example: In an F12 population of 10,000 seeds there is a 0.05% chance of unlike gene pairs.
(0.05%/100 = 0.0005)    10,000 x 0.005 =  5 plants which will be heterozygous

This is another reason why it is important that one carry out line development of creating a new line or de-hybridizing to at least the F7 before releasing or naming.

So it ends up being a numbers game. The result is that even generally stable populations of true-breeding individuals sometimes can potentially contain off-types by random chance and this must be considered. Fortunately for amateur tomato breeders large populations are not practical and chances are if you have taken the population out to an F7 or F8 you a good chance of completely removing these - but there still is some chance.

The main points to this page are:
  1. tomato genes want to revert back to being homozygous by sorting independently
  2. to become relatively stable and true-breeding, lines should be carried out to the F7 or F8 generation and
  3. that there still exists a small chance that some heterzygotes (off-types) can be in an advanced population.

Let us now look at how we can use the segregation above to produce new and stable varieties.


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