Hybrid purity

​Single cross hybrids have grain production advantages because of specific combination of the genetics of the two inbred parents plus uniform plant height and pollination timing.  
 
Essential to reproducing identical hybrid seed corn is use of homozygous parent seed.  This requires inbred seed production isolated from outside pollen, which is no easy task given that corn’s basic advantage for genetic variability is its tendency for cross fertilization.  Even if the parent seed is relatively clean of contaminants, the problem continues into hybrid seed production.  Traditionally, outcrosses have been often defined as plants taller than the majority of plants in a field.  This has been misleading.  Several years ago, we intentionally made outcrosses by pollinating a female hybrid parent with pollen from hybrid plants. That seed was planted along with the correct hybrid seed for comparison.  Some of the outcrosses were taller than the correct hybrid and some were much shorter. Outcross plants varied greatly in timing of pollen and silk production as well and, significantly, were all different from each other.  This is consistent with what we see in seedlings (Seedling Growouts®), each outcross plant is different from the correct hybrid and different from each other.
 
Outcrosses that originate with pollen coming from hybrid plants are different because that pollen is the result of meiosis in the hybrid plant.  Meiosis results in only one member of each of the 10 pairs of chromosomes to be represented.  Minimum number of possible combinations of genetics is 2log10 or 1024 combinations of chromosomes among the pollen grain.  Actually, more than that are probable because of chromosome crossovers and mutations that also occur during meiosis.  Evaluation based upon taller plants is also influenced by plant height of the correct hybrid versus that of contaminating hybrid.  If the correct hybrid is produced by a tall female inbred crossed with a short male inbred, it is more likely that pollen from a tall commercial hybrid will cause more ‘talls’.  Even in that case, the pollen that happen to include more of the contaminating hybrid male chromosomes are likely to be smaller than the correct hybrid.
 
Hybrid seed with less than 1% outcrosses, tall or short, have an insignificant effect on yield. Outcrosses have become more important to those with GMO interests.  If the contaminating hybrid included a gene for a GMO from one of its parents, half of the resulting pollen would include that gene.  If the seed production field was intended to be non-GMO, then half of the outcrosses would include that gene.  If the contaminating pollen included multiple traits on separate chromosomes then the math becomes more complicated, with each of those chromosomes having a 50% chance of being included in each pollen grain.  If the trait genes were closely linked on the same chromosome then they likely would be carried together but not always because some of the linkages may break.
 
If the intended hybrid has the gene on the male parent only, then some or all the outcrosses from a non-trait hybrid would lack the gene.  Selfing of the female parent in the seed production field would also lack the gene.  These cases are most relevant to herbicide resistant traits and for that reason it is preferred that these genes are in the female parent genetics.
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Grain production involves similar dynamics except that hybrid plants tend to produce sufficient pollen to reduce the probability of contamination.  It does present apprehension about strict non-gmo rules, including sampling and testing procedures.
 
Separation of male and female flowers in corn and natural aerial distribution of pollen allowed corn to have broad genetics during the domestication of the species and adaptation of many environments.  It also carries with that a small problem when we try to maintain specific trait purity such as in specific grain characteristics (ie. white or amylose corn) or GMO traits.