Hi Sandeep,
Regarding how many plants you need to generate in F2 and subsequent generation,
It depends on a)how many gene(s) control the resistance and b)type of experiment you want to do.
a)For ex. if it is single dominant gene you would expect 1:2:1 segregation of RR:Rr:rr. So if you can select plants of resistance genotype RR in F2 they wont be segregating in next generation you can just do selfing and fix the other traits. So at least 200 plants of f2 will be okay to get ateast 50 RR plants.
b)If you want to map the genomic region conferring resistance and fine map you probably need at least 200 F2 and >1000 advance generation plants (recombinant plants progenies) (considering single dominant gene/locus conferring) to decipher the genetic factor.
If you have crop like rice and if you have 10 true F1 plants at least you can harvest 10,000 seeds and at least 5000 plants at ideal germination condition (200*5*10=10,000)
For more information you may read this article
Article Identification and fine mapping of a new gene, BPH31 conferr...
Sometimes breeders tend to carry out selection for more than one characters simultaneously. In that case how many characters you are selecting for will decide the minimum population size. In order to obtain at least one individual plant with a combination of all desirable characters, go for calculation by simple Mendelian segregation pattern, e.g., 1:2: 1 for 1 character, 9:3:3:1 for 2 characters, so on and so forth. Since your desirable trait is dominant, heterozygoted will throw susceptible individuals in every generation. So, from F2 onwards you will have to identify single resistant plants with good agronomic traits and grow progeny rows/plots in subsequent generations. the homozygous dominant ones will breed true in following generations. you have to keep only these families and carry out between family selection.
If the goal is to produce a resistant and high yielding genotype, it actually depends on how you will handle the segregating population.
If it's single seed descent, you will maintain the 200 individuals all throughout the generations. Other methods include double haploid, pedigree method and bulk method.
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