Glossary Plant Breeding

same gamete. If quadrivalent formation is complete and crossing over results in 50%
recombination, we have to think in terms of chromatids (instead of chromosomes) to
predict gametic output. For instance, in a simplex (Aaaa), there are 2A chromatids and
6a chromatids. Three types of gametes are possible: AA, Aa and aa. The probability of
picking two A chromatids (out of AAaaaaaa) is 2/8×1/7 = 1/28AA. Similarly, the
probability for Aa is 2/8×6/7 + 6/8×2/7 = 12/28Aa. Obviously, the probability for aa
is 15/28. Thus the gametic output in a simplex will be in a ratio of 1AA: 12Aa: 15aa.
Similarly, gametic output in the duplex (3AA: 8Aa: 3aa) and triplex (15AA: 12Aa:
1aa) can be predicted under such conditions.
Random Chromosome Assortment (Muller 1914). A type of partition of chromatids in
which sister chromatids always go to different products of meiosis in an
autotetraploid (or in any higher order even polyploids). It happens if bivalents form
regularly during meiosis. This type of segregation also occurs, when quadrivalents
always form and the locus in question is inseparably linked to the centromere. The
effect of linkage is to cause the locus to separate reductionally at the first anaphase
and equationally at second anaphase, exactly as happens in case of bivalent formation.
In an autotetraploid, five genotypes namely AAAA, AAAa, AAaa, Aaaa and aaaa are
possible. Gametes produced by the first and the last will be AA and aa, respectively.
Gametic output in triplex (AAAa) andsimplex(Aaaa) will be 1AA: 1Aaand1Aa: 1aa,
respectively. Duplex (AAaa) produces three types of gametes in the ratio of 1AA: 4Aa:
1aa.
Random Fixation. The fixation (100% allelic frequency) of one of two alleles in a
population by chance events (e.g., genetic drift). Random fixation depends on a
number of factors such as selective value of the allele(s), mutation pressure, gene
flow, the degree of genetic relationship between the number of breeding individuals.
Random Genetic Drift. Random changes in allelic and genotypic frequencies in a small
random mating population owing to sampling errors. The genetic effect is similar to
that of inbreeding, that is, there is gradual loss of heterozygosity. The rate of this loss
is 1/2N, where N is the number of diploid individuals in the population. Thus, H t ≅
H 0 e -t/2N , whereas, H t and H 0 are proportions of heterozygotes in the t th and original
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