Sexual reproduction happens when there is the involvement of two parents to give rise to the offsprings.
For the production of offsprings, each of the parents produces a haploid gamete and it is by the fusion of these two haploid gametes that a diploid organism is produced.
In the given situation, there are 19 chromosomes present in the haploid gametes this means that a diploid individual will have 2X19 = 38 chromosomes.
Now the underground stem cells are not gametes but somatic cells and the somatic cells are diploid and hence, they will have 38 chromosomes.
There would still be a 50% chance for the offspring to be Heterozygous (Hybrid), but then there would be another 50% chance for the offspring to be Homozygous recessive (True-breeding short plants)
