Principles of Plant Genetics and Breeding

encloses the soft starch at the center. The kernels are
smooth, hard, and usually rounded at the top. This
type of corn is grown widely in Europe, Asia, Central
America, and South America. It is less widely grown
in the US.
3 Flour corn (Z. mays amylacea). As the name implies,
flour corn consists almost entirely of soft starch,
making the kernels soft. It has the shape of dent
corn but shrinks uniformly upon drying. It is grown
in the drier sections of the USA, mainly by American
Indians, and also in the Andean region of Central
and South America. Different kernel colors exist, the
most common being white, blue, and variegated.
4 Popcorn (Z. mays everta). Popcorn is an extreme
form of flint corn. It has a very hard corneous
endosperm with only a small portion of soft starch.
The kernels are characteristically small and may either
be pointed or have a rounded tip. Different colors
exist, most corneous varieties being yellow or white.
The kernel pops upon heating as a result of the
unique quality of the endosperm that makes it resist
the steam pressure generated, until it reaches explosive
proportions.
5 Sweet corn (Z. mays saccharata). This corn is characterized
by a translucent and wrinkled appearance
upon drying, and a sweet taste when immature.
Standard sweet corn is a mutant of the dent corn with
a mutation at the sugary (sy) locus. This mutation
causes the endosperm to accumulate about two times
more sugar than field corn. New mutants have been
developed – sugary enhanced (se) and shrunken-2
(sh2) or supersweet corn. Some sweet corn varieties
are unable to convert sugar to starch. Sweet corn is
grown as a winter crop in the southern US, especially
Florida.
6 Waxy corn. Waxy corn has a uniformly dull appearance.
Instead of amylose, the starch of waxy corn
consists of amylopectin, the result of waxy (wx)
mutation. Ordinary corn consists of about 78% amylopectin
(a high molecular weight branched chain)
and 22% amylose (a low molecular weight straight
chain).
7 Pod corn (Z. mays tunicata). Pod corn has primitive
features, each kernel being enclosed in a pod or husk,
before the entire ear is enclosed in husks like other
corns. Pod corn versions of the other types of corn
(e.g., flint pod corn, dent pod corn) exist.
Similarly, corn that is indigenous to the USA (excluding
sweet corn and popcorn) may be classified in up
to nine or 10 races. Of these, the most important are
the Corn Belt dents, the southern dents, and the northern
dents.
BREEDING CORN 487
Germplasm resources
Over 13,000 accessions of corn are kept in storage at
the Centro Internationale de Mejoramiento de Maiz y
Trigo (CIMMYT) in Mexico, with duplicates of these
accession held in the US (at the US National Seed
Storage Laboratory, Fort Collins, Colorado), Columbia,
and Peru. Many heterotic populations have been
identified in maize, the most well known and exploited
including “Reid” × “Lancaster” and “Lancaster” ×
“Stiff Stalk”. Others are the European flint × dents,
“Tuxpeno” × “ETO”, and “Pantap × Suwan 1”.
Cytogenetics
Corn (Zea mays L.) is a diploid (2n = 20) and a monocot
of the family Poaceae (Gramineae), or grass family.
The genus has four species: Z. mays (cultivated corn
and teosinte), Z. diploperennis Iltis et al. (diploperennial
teosinte), Z. luxurians, and Z. perennis (perennial
teosinte). Of these four species, only Z. mays is widely
grown commercially in the US. The closest generic
relative of Zea is Tripsacum, which has seven species,
three of which are known to occur in the US. Teosinte
occurs in the wild in Mexico and Guatemala. Cultivated
corn has 10 pairs of chromosomes (n = 10). However,
plants with 1–8 sets of chromosomes have been developed
for various purposes.
In addition to the autosomes or normal or standard
chromosomes (A-chromosomes), maize has supernumerary
elements such as the B-chromosomes (also
called supernumerary chromosomes). The role of the
B-chromosomes in the cell varies from being practically
a nuisance to having some definite function, depending
on the organism. However, when the B-chromosomes
number 10–15 or more, certain abnormalities may
occur (e.g., reduced fertility, decreased vigor, aborted
pollen, defective kernels). Seed is rarely produced with
the presence of a B-chromosome number in excess of
25. Maize B-chromosomes are among those most widely
studied in plants. They are suspected to influence the
frequency of crossing over, among other roles. Thousands
of translocation events have been described in maize.
They are used for locating genes on chromosomes.
Monoploids (haploids) may arise spontaneously by
pathogenesis (the unfertilized egg develops into a plant).
Occasionally, paternal haploids develop by androgenesis.
Haploid generation via these systems has low frequency
of occurrence. The average frequency of this event in
corn is estimated at one per 1,000 kernels. These lines