So here we have two heterozygous organisms that will mate. You can put them into a Punnet square, just to be sure of your answers and to find all possible genotypes :
R r
R RR Rr
r Rr rr
So you have RR, Rr and rr as your possible genotypes.
A sudden shift in the tectonic plates of the earth
This would be probably true if the assumption that all possible genotypic variations would be equally distributed (so we would have 25% HH, 25% hh and 2x 25 Hh). If this distribution would be true and Huntingtons disease really was a single gene dominant trait diesase, then yes, we could expect such a distribution in the population.
In meiosis? Tetrads maby? When two homologus chromosomes do crossing over they're called tetrads so..
