Terms

[Carol_A]

What are "segregating hybrids"?  I keep coming across this term.

[joseph]

If plants of two different varieties or species are crossed that makes a hybrid. If we use consistent parents to make the hybrid, then the children are consistent. The first hybrid generation are called "F1 hybrids".

If the seeds of a hybrid are saved and replanted, then things can get really interesting: Because the genes are sorting themselves out into lots of new combinations. We call that rearrangement of the genes "segregation". The rearrangement occurs most noticeably in the grandchildren of the cross, often called the "F2". But it continues in varying degrees through about the 5th generation.

Here is an example with dry beans:

The mother of an F1 hybrid looked like this:

Mother of cross. Bush bean.


Hybrid beans: F1. Father of the cross unknown. Probably speckled. Probably dark colored. Definitely a pole bean.


Segregating hybrid: F2. Grandchildren of the cross. About 1/4 bush beans, and 3/4 pole beans. Saved seed only from bush beans.


Segregating hybrid: F3. Great grandchildren of the cross. Only bush beans.


Rather than growing these as a mixed population, I could pull individual colors out to grow as a pure variety.

[Carol_A]

So you're not adding new genetic material to the mix, instead just selecting, and letting new combinations develop. 

[joseph]

So you're not adding new genetic material to the mix, instead just selecting, and letting new combinations develop.

Correct for "segregating hybrids". 

Segregating hybrids in which all of the offspring are descended from the same parent are also called clades. All the seeds in the population are cousins or siblings.

My dry bush beans in the 2015 growing season included 3 clades. During the 2016 growing season, I am intending to grow 6 or 7 clades.

I grow the clades separately. Then I add seed from the clades to the general population. I also add new varieties to the general population as I trial new varieties and find some that work well here.

When I find new hybrids, I grow them separately for a year or two, while I select out of them the types that work for me, then I add them back into the general population.

[Carol_A]

Are all the seeds produced from a given plant identical genetically?

[Michael]

Each individual seed arises from an individual ovule and sperm, so the answer is no, but...

For an inbred line the seeds will be mostly genetically identical due to selection for homozygosity (the same alleles on each chromosome) and with sufficient inbreeding they should be homozygous for just about every trait.  F1 hybrid seed derived from two inbred lines will also be very nearly identical, since whatever combination of alleles they get, they will end up with chromosomes essentially identical to each parent.

Segregation in terms of Mendelian inheritance simply means the homologous chromosomes the parent has are separated into haploid ova and/or pollen. For two copies of a given chromosome, each sperm or egg gets one.  In the process of duplicating those chromosomes, segments are exchanged and recombined.  Simply put, two chromosomes, one from dad and one from mom, become four chromosomes, all with some bits from dad and other bits from mom. The chromosomes are then independently assorted into the gametes.

Beginning with the F2 generation random combinations of traits will come about through the combined effect of these processes.  If you encourage segregation by selection, isolation, selfing, and culling you will select for homozygosity and eventually end up with 99% genetically identical seeds (I really do not know what the actual figure is.)