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
meiosis; |
| |
b) |
a crossover must occur; |
| |
c) |
two heteroallelic chromosomes go to the same
pole at first division; |
| |
d) |
at second division, a portion of two chromatids
that originally shared a common centromere
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. |
Copyright
2000©, Ted Helms |
