Agriculture Reference
In-Depth Information
that express as RNA in roots, shoots or callus and which have correspondences
with genes of known function in other organisms (Kuhl,
et al
., 2004; see
Genomics, below).
The majority of traits, including most of those important for crop
productivity, are controlled by the combined effects of a number of genes that
influence the trait, each of which has a similar, small, 'quantitative' influence.
For example, yield, maturity date and ease of bolting are each conditioned by
the additive effects of several genes. Thus, crosses between extreme types for
maturity date give hybrids intermediate to the parents but, from the hybrids a
continuous range, rather than a few distinct classes, of maturity dates are
derived in later generations. The aim of breeding is to combine together in the
complement of genes, or 'genotype' of a variety, alleles that are favourable for
desired quantitative traits. For example, the genotype should contain all the
favourable alleles for the genes that determine high yield either homozygously
or, if these alleles are dominant over complementary alleles conditioning lower
yields, the presence of the favourable dominant as a heterozygote is equally
good.
A high level of heterozygosity exists in onion populations sustained by
generations of random outcrossing. This allows deleterious recessive alleles to
perpetuate in the genetic pool. Self-pollination results in homozygosity for a
high proportion of these recessive alleles, thus manifesting their effect and
causing the inbreeding depression described above. Prominent among the
deleterious recessives are those causing chlorophyll deficiency. In one survey
20-30% of plants were heterozygous for a chlorophyll deficiency recessive
allele and approximately 20 such gene loci were estimated to be present in
onion populations (Berninger and Buret, 1967).
Because leeks are tetraploid and therefore carry four alleles for each gene,
there is even more scope for deleterious recessive alleles to be carried in
heterozygous genotypes than with a diploid such as onion. Between seven and
14 different chlorophyll deficiency genes have been found in leeks and, in an
open-pollinated population, it was estimated that more than 60% of plants were
carrying two copies of such deficiency alleles in one or more such genes
(Berninger and Buret, 1967). Leeks are subject to severe inbreeding depression.
Following a cross between two parents carrying contrasting alleles at a range of
gene loci, the inheritance of the alleles for each locus can be followed in subsequent
generations. If two characteristics conditioned by two different genetic loci are
inherited independently they are said to be 'unlinked', and the gene loci can be
assumed to be located on different chromosomes. If, in contrast, the inheritance of
the two characteristics shows some correlation and not just random association,
their gene loci are said to be 'linked' and they can normally be assumed to be on the
