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.