In early human history, where did people get
their food?
They were limited to gathering food
that nature produced.
Around 12,000 years ago, humans developed
new techniques and tools to improve the
quality of plants.
This system for producing plants
with better yield, flavor, and
nutrition became known as
agriculture.
Farmers selected plants with
preferred characteristics. Each
season, seeds from the best
plants were saved for future
plantings.
As a result, plant characteristics gradually changed. Crops
today are very different from the native varieties of the
distant past. Selective breeding techniques are still
important in modern agriculture.
Ancient corn → Modern corn
In the past 150 years, scientists have examined why and
how selective breeding works. They now recognize that
traits are inherited (passed on) from parents to offspring
through a mechanism known as heredity.
Inherited traits, such as size, shape, color, and many other
characteristics are controlled by a genetic code found in
the nucleus of an organism’s cells.
This genetic information is found on rod-like structures
called chromosomes. They are made of long, coiled
strands of DNA.
A gene is a segment or section of the chromosome’s
DNA that codes for a specific trait.
Corn plants have 32,000 genes crammed onto only
10 pairs of chromosomes. By contrast, humans have
23 pairs of chromosomes and at least 20,000 genes.
Seed color is an example of an inherited trait in
corn plants.
Eye color is an example of an inherited trait
in humans.
Because they are located on paired chromosomes,
genes also occur in pairs. The paired genes may carry
identical information or they may contain different
codes.
Gene pairs on
paired
chromosomes
These differences in genetic codes are known as alleles.
Two alleles for eye color are shown—a blue allele and a
brown allele. Eye color, like most traits, is controlled by
more than one gene.
Today’s activity models trait inheritance in corn plants.
Unlike humans (who are either male or female),
individual corn plants have both male and female parts.
The tassel is the male part.
The ear is the female part.
The tassel produces pollen grains which contain
the male sperm cells.
Magnified pollen grain
The ear contains the ovules which hold the
female egg cells.
Now that you know about corn reproductive
structures and trait inheritance, you will begin Part
1 of the activity.
You will be working with just four traits. Realize,
however, that corn is selectively bred for many
traits including resistance to drought, disease, and
insect pests.
Randomly select 4 colored paper clips representing
the traits of a corn plant and a round tag representing
the male or the female reproductive cell.
Follow the directions on the procedure sheet to
construct a “trait chain” and complete the correct
table on the worksheet. The trait chain models your
corn plant’s genotype.
Traits can be beneficial, harmful, or neutral to an organism.
• Height: Tall corn plants have more leaves resulting in higher levels of
photosynthesis and better kernel (seed) production.
• Leaf color: Green leaves have chlorophyll, a pigment necessary for
food production and life.
• Seed color: Seed color has no effect on plant health. Humans,
however, do select color for nutrition and flavor. White corn tastes
sweeter, but is less nutritious than yellow or purple corn.
• Seed texture: Smooth seeds are high in starch. Wrinkling is due to
water loss in sugar-rich seeds.
In order to produce new corn plants, the pollen from
the tassels must reach the ovules on the ear. Most
often the wind transports the pollen to the silks at the
ends of the ears.
Wind
Pollen
donor
Pollen
receiver
(ovule)
The pollen’s sperm cells travel down the silks to
fertilize the eggs in the ovules. The photo shows
pollen captured by corn silk. The silks have been dyed
for easier pollen viewing.
After fertilization, the silks detach and the eggs develop
into corn kernels. EACH kernel has a combination of the
parents’ traits, and, once planted, will grow into a plant
that expresses its unique genetic profile.
In Part 2 of the activity, you will model
pollination. The pollen groups will join the
ovule groups with their trait chains and
worksheets.
Follow the directions on the procedure sheet
to complete the first two tables on the
worksheet.
Recall that traits are controlled by the alleles an
offspring inherits from each of its parents. Some alleles
are dominant while others are recessive.
A dominant allele will always be expressed in
offspring when both parents pass it on.
Parent 1 Parent 2
A recessive allele from one parent is hidden (masked)
whenever a dominant allele from the other parent is
present.
Parent 1 Parent 2
A recessive allele can only be expressed if both
parents pass it on.
Parent 1 Parent 2
In Part 3 you will analyze the alleles from both
corn parents (pollen and ovule). This will allow
you to determine the traits found in the
kernel’s genetic code. The code determines the
physical appearance (phenotype) of the plant
the kernel grows into.
Follow the directions on the procedure sheet to
complete Table 3 on the worksheet.
In Part 4 you will find your offspring’s correct
phenotype card.
Conventional breeding techniques continue
to be important.
Click photo to play video.
Advances in technology, however, have
dramatically increased the efficiency of plant
breeding and crop yield.
The seed chipper is a recent achievement in
selective breeding technology. Developed in the
early 2000s, the chipper allows scientists to
identify the traits a plant will have at maturity.
A chip is removed opposite the kernel’s growing
point, and its DNA is analyzed. Only seeds with
a desired genetic makeup get planted. This
greatly cuts the time needed to get new plant
varieties to farmers. No resources are wasted
growing seeds with undesirable traits.
Growing point
Chip
The world faces the challenge of feeding its
rapidly growing population. Farmers must
produce more food in the next 50 years than in
the past 10,000 years combined.
Scientists are responding to the challenge
through improved selective breeding for food
crops including corn.