Glossary Plant Breeding

Holliday Junctions. The structures formed during chiasma formation when
chromosomes exchange information during meiosis. Holliday junctions form four
double helical arms (branched DNA) flanking a branch point. The branch point can
relocate throughout the molecule by virtue of the homologous sequences.
Holocentric Chromosomes. Chromosomes having diffused kinetochores (centromeres)
with microtubules attached along the length of the chromosomes. These may be found
in certain species or strains of a species (as in some strains of maize, for instance).
Homeologous Chromosomes. Partly homologous chromosomes originating from
different but related genomes, between which there does not occur point-to-point
pairing at meiosis. The partial homology between them usually indicates some
original ancestral homology.
Homeosis. The replacement of one body part by another. It can be caused by
environmental factors leading to developmental anomalies or by mutation.
Homeostasis. The tendency of a physiological system to resist to an external disturbance
such that the system is not displaced from its normal values.
Homeotic Genes. Genes that control the fate of segments along the anterior-posterior
axis of higher animals.
Homeotic Mutations. Mutations that can change the fate of an imaginal disk.
Homodimer. A protein consisting of two (more than two in case of homomultimer)
identical polypeptide subunits.
Homogametic Sex. The sex producing the same type of gametes with respect to the sex
chromosomes. Such sex contains homologous sex chromosomes (XX, for example).
Homogamy. The preference of a mating individual for another with similar phenotype or
genotype.
Homolog. A member of a pair of homologous chromosomes.
Homologous Chromosomes. The chromosomes (or parts of chromosome) that synapse
or pair during meiotic prophase; chromosomes with point-to-point pairing between
them at meiosis. They are also defined as the chromosomes in different but related
species that have retained most of the same genes during their evolution from a
common ancestor.
112