While I‘m at it…
Post from 9 years ago:
Self-proclaimed breeders occasionally bring up Mendel and Punnet, but what they should really be looking into is
Wilhelm Johannsen's work, the Genotype-Phenotype Distinction, his work on pure-line breeding and the discovery of hybrid vigor...
In reality it’s more complicated because traits are often the results of a combination of many genes, but, for example:
Parent 1 genotype for fat buds AA. Phenotype is fat buds too.
Parent 2 genotype for mediocre buds is aa. Phenotype is mediocre buds too.
Their (F1) offspring will all have genotype Aa. Now when A is dominant, the effect of the A gene from parent 1 has a stronger expression than ’a’ (sometimes complete overwhelming the effect of ‘a’). In that case ‘a’ is recessive. The genotype Aa will then still produce the fat buds phenotype.
Now if you cross two of those Aa genotypes into an F2, the genotype falls apart in its possible Mendelian combinations (predictable percentages that don’t apply unless you have many many plants). The genotypes and phenotypes of the F2 will be:
- Genotype 1 (homozygous) AA - Phenotype fat buds
- Genotype 2 (heterozygou) Aa - Phenotype fat buds
- Genotype 3 (homozygous) aa - Phenotype mediocre buds
To produce (F3) seeds from those plants and end up with seeds that all produce fat bud plants, you need to select two plants (or self 1) that do not only have the fat buds phenotype (what you see) but have genotype 1 AA.
If you were to select based purely on phenotype, you might pick two plants with genotype Aa again and again end up with the 3 genotypes above. Or you’d pick a fat bud pheno with genotype AA and one with Aa and end up, which would results in AA and Aa genotypes and thus all fat bud phenotypes, but then you might pick two Aa again in a next generation.
Additionally, preferably you end up with something homozygous so when you outcross it to a different something you can better predict the outcome and/or create proper hybrids.
The fact one of the parents is normally a male obviously complicates matters but it’s just an example.
That all said. Unless you can run a couple of thousand plants or can read the dna or focus on just one or two traits, it’s often not very fruitful to chase specific genotypes. Most of the good stuff is the result of crossing different varieties into a genepool and in that hunt for phenos. Clone or self the best. Or backcross to a parent/grandparent which you think is homozygous. Or do open pollination with muliple males and only use the seeds from the females with the desired traits. Read the book, then throw it out.