Part I

1. Non-disjunction means that two heteroallelelic chromosomes go to the same pole at Anaphase I.
   
   
2. Heteroallelic chromosomes are the result of a crossover. Portions of chromatids that were formerly sister chromatids are now located on homologous chromosomes.
   
   
3. Double-reduction means that two identical by descent alleles end up in the same gamete. Identical by descent alleles are the result of the replication of a chromatid.
   
   
4. Reductional division means that identical chromatids which were derived from the same chromosome go to the same pole. This occurs when that particular chromosome has not participated in a crossover event.
   
   
5. Equational division means that a crossover event occurred which resulted in an exchange of a segment of two chromatids. As a result of the crossover, non-identical chromatids now share a common centromere and these non-identical chromatids pass to the same pole at Anaphase I.
   
   
6. The four events that must occur for double reduction to occur are:
   a) a multivalent must be formed at Prophase I of
   a) meiosis;
   b) a crossover must occur;
   c) two heteroallelic chromosomes go to the same
   c) pole at first division;
   d) at second division, a portion of two chromatids
   d) that originally shared a common centromere
   d) go to the same pole.
   
   
7. Alpha (a) is the probability of double reduction. Alpha can reach a maximum of 1/6. Alpha is maximum when there is 100% multivalent formation (q=1) and there is 100% probability of a crossover event between the centromere and locus of interest (e=1). The probability of Adjacent I segregation is 1/3. This is because there are three ways for quadrivalent segregation to occur and each way is considered equally probable. When q=1, e=1, a=1/3, then in one-half of the gametes double reduction will occur. Then alpha = {(1)(1)(1/3)}/2 = 1/6.
   
   
8. Alpha can be less than 1/6. When the locus of interest is too close or too far from the centromere, the probability of exactly one crossover between the locus and centromere will be less than 100%. When the locus is too close to the centromere, the probability of crossover is less than 100%. When the locus is too far from the centromere, the probability of 2-strand double crossovers increases and recombination will be less than 50%. Another reason that alpha could be less than 1/6 is that quadrivalents may not always form. Bivalents may sometimes form. When bivalents form, double reduction cannot occur.