Biology Reference
In-Depth Information
3.1 Introduction
The chromosomal theory of inheritance was proposed in the early years of
the 20th century and supported by a large number of experiments in fruit fly
(
Drosophila
) in the 1920s. The theory states that genes controlling different
phenotypes are carried on the chromosomes and transmitted from parents
to offspring. Now we know that in eukaryotic species, each chromosome
contains a long piece of DNA that harbors numerous genes. The genes on
the same chromosome tend to be transmitted together (linkage). However,
crossing over between two chromatids in meiosis produces gametes of
recombinant genotypes. The likelihood of crossing over depends on the
distance between the two genes. Thus, the distance between two linked
genes can be estimated by observing the frequency of recombinant
phenotypes among the segregating progeny derived from a cross. The
segregating data of three linked genes in a population permit estimation of
the distance between genes and establishment of an order for the three genes.
This distance between genes is called a genetic map unit (mu), or more
commonly, a centiMorgan (cM). The success of constructing a linkage map
for
Drosophila
with morphological markers
stimulated gene mapping in
various species. Among agricultural crops, corn (
Zea mays
) and tomato
(
Solanum lycopersicum
, earlier known as
Lycopersicon esculentum
) were the
first to have complete linkage maps with the number of linkage groups
equal to the number of chromosome pairs.
A genetic linkage map provides knowledge of the organization of the
genes of the species. This information eventually leads to the isolation
(cloning) of the gene (by map-based or positional cloning) for structural and
functional characterizations. It also helps breeders efficiently manipulate
the genetic variation to create new varieties with improved quality and
increased productivity via marker-assisted selection (MAS). This chapter
reviews the history, techniques and strategies, achievement and challenges
of the sunflower genome mapping efforts.
3.2 Brief History of Mapping Efforts: Morphological and
Biochemical Markers
3.2.1 Morphological Markers
Although sunflower has been cultivated as a crop for a long time, the genetic
study of sunflower lagged behind other major crop plants. Working towards
improving this crop, individual researchers have observed and recorded
many morphological variations in leaf, flower, and seed characteristics, as
well as resistance to numerous diseases. Miller and Fick (1997) summarized
the research and identified about 40 morphological traits in sunflower, prior
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