Concepts - Part I

Mathematical Symbols

q = the frequency of quadrivalent formation.
a = the frequency of double reduction = (qea)/2
e = the frequency of equational constitutions or
q = the frequency of heteroallelic chromosomes
q = - this requires a crossover between the
q = centromere and locus. Also, 2p = crossover probability
a = the frequency of genetic non-disjunction or
a = the frequency of two heteroallelic chromosomes
a = which go to the same pole at anaphase I.

* Equational division can occur with bivalents, but non-disjunction can only occur with multivalent formation.

The value of e depends on the map distance between the locus and the centromere. e=2p for short map distances. When there is one crossover event for each quadrivalent formation e=1. e<1 when the locus is close to the centromere.
see Doyle. 1973 TAG 43:140

When loci are located very close to the centromere, the probability of crossovers between the centromere and that locus is small. The segregation ratio at these loci is random chromosome. Double reduction gametes are not expected to occur.

For loci located far from the centromere, crossovers between the centromere and those type of loci are frequent. Double reduction gametes are expected. Double reduction gametes increase the relative frequency of homozygous genotypes when compared to random chromsome segregation.

Simplex - Aaaa genotype. This is a heterozygous genotype with one dominant alleleomorph. With random chromosome segregation we could have the following combinations in gametes.

Thus with no crossover event, we would have 1/2Aa:1/2aa gametic ratio for the simplex.