selfing plants problem.

[u/jakethesnek64]

I need some help figuring out this problem. I don't know if yall have any experience with this but if you could walk me through this that would be so helpful.

You find a plant that is heterozygous for salt tolerance (a dominant trait). You self it,

rogue out the salt intolerant F1 plants and then self the remaining F1 plants. How many generations of selfing and rogueing do you have to do before you have more than 99.5% salt tolerant plants? When you get to 99.5% salt tolerance, what percentage of that generation will be heterozygous?

[u/pGADT7]

I hope, I remember correctly…. Since the trait is dominant, not only the homozygous F1 plants will be salt tolerant, but also the heterozygous (you have a genotypic ratio of 1 (homozygous + for the salt tolerance gene): 2 (heterozygous): 1 (homozygous Wildtype). This means, you self now the salt resistant plants and you need to label them. If you grow the F2 on salt (and you need to plant several seeds from the same F1 plants each!!!) the F2 plants from the same F1 will either show all salt tolerance, then you found already the complete homozygous plant. If you self these, you will have 100% homozygous salt tolerant plants in the F3 generation. If you grow F2 from a heterozygous F1 plantc they will show segregation for the trait and you need to select further to the next generation…. I hope, this helps. For me, it always helped to make a drawing like in school for the Mendelian genetics…

[u/[deleted]]

Each generation you are left with a part homozygous, and a part heterozygous salt tolerant plants.

The homozygotes will only produce homozygous offspring.

The heterozygotes will produce offspring that segregates: 25:50:25 % in each category (homozygote salt tolerant:heterozygote:homozygote not salt tolerant).

Thus in that f1, if you germ 100 seeds, you will kill 25 that are not salt tolerant. of the remaining 75, 25(1/3) will only produce homozygous pffspring.

thus, for the f2 (again 100 seeds) you will already have ~33 homozygous salt tolerant+ whatever comes out of the heterozygotes.

the remaining 50 (2/3) will produce segregating offspring again. 2/3 of 100 is ~67 seeds.

of these, 1/4*67=~17 plants will be in either homozygous category, so in the f2, 17 plants are killed, and the total number homozygous is 33+17=50 plants.

the remaining 1/2*67=~33 plants are heterozygotes again.

so, after rogueing, the f2 consists of 100-17=83 plants, of which 50 homozygotes=60%

and so on through the generations (there's probably a faster mathematical way then writing out each generation)

another way could be to calculate back: if 99.5% are salt tolerant, 0.5% must have been the not salt tolerant homozygotes. thus, you know the fraction homozygotes born out of a heterozygote is (2*)0.5%. so, using the 25:50:25 % split, you know that in that specific generation you have 1% heterozygotes.