A plant is homozygous at a locus when both copies of the gene are the same allele, for example AA or aa. Every gamete it makes carries that allele, so for that trait it breeds true. A line can be homozygous at one locus and heterozygous at thousands of others.
In practice
"Stable" in breeder talk means "homozygous for the traits that matter". Inbreeding (sib crossing, backcrossing, selfing) raises homozygosity generation by generation. One generation of selfing removes, on average, half of the remaining heterozygosity; full-sib mating removes it more slowly.
A plant that is homozygous for its key traits is valuable as a parent: cross it to anything and it donates the same alleles every time, which makes its offspring predictable. That is the logic behind IBLs and inbred parents for F1 seed. The price is that high homozygosity across the whole genome also exposes deleterious recessives, which is inbreeding depression.
Expected heterozygosity after n generations of selfing, starting from a fully heterozygous locus: (1/2)^n. After S1: 50%; after S3: 12.5%; after S6: about 1.6%.
Don't confuse with
- Heterozygous: two different alleles at the locus.
- Uniform: a line can look uniform while carrying hidden heterozygosity at many loci.
A locus is the fixed position on a chromosome where a particular gene or DNA marker sits; the plural is loci.A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.A plant is heterozygous at a locus when it carries two different alleles there (Aa), so its offspring can inherit either one and segregate.Inbreeding is mating between related plants, which raises homozygosity; an inbred line is the resulting population, measured by the inbreeding coefficient F.Selfing is pollinating a plant with its own pollen, in cannabis usually by reversing part of a female to make pollen; it is the most intense form of inbreeding.F1 is the first generation of offspring from crossing two parents; if the parents are inbred and different, F1 plants are uniform and often show hybrid vigour.Inbreeding depression is the loss of vigour, fertility, yield or survival that follows mating between relatives, mainly because harmful recessive alleles become homozygous.A true-breeding line produces offspring that consistently look like the parents for a given trait, because the plants are homozygous for the alleles behind it.An IBL (inbred line) is a seed line that has been inbred and selected over many generations until it breeds true for its main traits, giving uniform offspring.
