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How Genes Work

People like to compare babies to their parents and siblings. You may have heard statements like, "He has his mother's eyes," or "She has curly hair just like her brother." These statements refer to a baby's traits. Traits are notable characteristics or qualities. Why do people share some traits with other people in their families? The answer is in their genes.

For hundreds of years, people have realized that certain traits are passed down from parents to offspring. Farmers used this idea to breed stronger animals and to create plants that produced better fruit. However, until the mid-1800s, no one was quite sure why or how this type of breeding worked.

In 1865, a monk named Gregor Mendel began studying how traits are passed from generation to generation. He worked with pea plants. He studied seven traits that each occur in two possible ways. For example, seeds can be green or yellow, pods can be puffed or pinched, and peas can be wrinkled or smooth. We'll focus on seed color.

Mendel started with plants that had purely yellow seeds and plants that had purely green seeds. When he crossed yellow-seed plants with yellow-seed plants, the new plants produced yellow seeds. Crossing green-seed plants with green-seed plants gave new plants that produced green seeds.

What do you think happened when Mendel crossed pure green-seed plants with pure yellow-seed plants? You might guess that the colors blended, making plants that produced greenish-yellow or yellowish-green seeds. However, Mendel found that all the new plants produced yellow seeds. The green color seemed to have vanished! When Mendel crossed these new plants, the green seeds came back. About one plant in every four produced green seeds.

Mendel found similar results with the other traits. In each case, one form of the trait disappeared when he crossed purebred plants of different types. Then the trait reappeared when he crossed the new plants. Mendel's experiments led him to form a theory of how traits are passed from parents to offspring.

Mendel said that each plant had two genes for each trait, one from each parent. For seed color, a plant can have two yellow-seed genes, two green-seed genes, or one gene of each type. If we use Y to stand for the yellow-seed gene and y to stand for the green-seed gene, then we can say that a plant has one of these pairs: YY (both yellow), yy (both green), or Yy (one yellow, one green).

Plants with YY pairs produce yellow seeds, and plants with yy pairs produce green seeds. What about plants with Yy pairs? Mendel found that the Y gene was dominant over the y gene. In other words, when both genes are present, the yellow gene takes over because it is stronger.

When two plants are crossed, each offspring gets one gene from each parent. The combined pair determines how the trait appears in the offspring.

We can use these ideas to explain what happened in Mendel's experiments.
Purebred yellow-seed plants have YY pairs of genes. When two of these plants are crossed, each gives a Y gene to the offspring, so all the offspring have YY pairs and produce yellow seeds. In the same way, the offspring from crossing purebred green-seed plants have yy pairs and produce green seeds.
When a purebred yellow-seed plant is crossed with a purebred green-seed plant, the offspring get a Y gene from one parent and a y gene from the other, so all have Yy pairs. Because Y is dominant, all of these plants produce yellow seeds.
When two yellow-seed plants with Yy pairs are crossed, each gives either a Y gene or a y gene to each offspring. So, an offspring can have a YY, yy, or Yy pair. Those with YY or Yy pairs produce yellow seeds, and those with yy pairs produce green seeds.

Humans pass on genes in the same way pea plants do. Here are a few examples:
Hitchhiker's thumb : When you extend your thumb, is it straight or does it bend way back? The ability to bend the thumb back, known as hitchhiker's thumb, is determined by your genes. The straight-thumb gene, S, is dominant over the hitchhiker's thumb gene, s. If you can bend your thumb back, then you must have an ss pair. If you can't, then you have either an SS or Ss pair.
Earlobes : Do your earlobes hang or are they attached to your head? The gene for hanging earlobes, H, is dominant over the gene for attached earlobes, h.
Widow's peak : Does the hairline in the middle of your forehead form a downward point? This is called a widow's peak . The widow's peak gene, W, is dominant over the gene for a straight hairline, w.

In humans, most traits are determined by several pairs of genes. Environment and experiences also play a role in determining our traits. For example, a person born with genes for dark hair may actually have light hair if he or she spends a lot of time in the sun. A person born with genes for heart disease may be able to avoid the disease if he or she leads a healthy lifestyle.

Mendel's discoveries raised many questions: What exactly are genes? Where are they located in our bodies? How do they get passed on?

Beginning in the 1900s, scientists were able to study the structures inside cells. They believed that thread-like structures, called chromosomes, in the nucleus of a cell carry genetic information.

In the 1940s and 1950s, scientists figured out that genes were part of long, ladder-like molecules called DNA, found in chromosomes. Your DNA contains all of the information needed to build your body and keep it running.

Although you inherited your DNA from your parents, you might be surprised to know how similar your DNA is to people you are not even related to. In fact, 99.9% of your DNA is the same as the DNA in every other human being on the planet. This means that only 0.1% of human DNA is responsible for the amazing diversity in the human population!




1.

CONNECT

Do you share traits with other people in your family? Explain.



2.

INTERPRET

How do you think farmers used the idea that traits are passed down to help them breed stronger animals?



3.

MONITOR COMPREHENSION

Reread this paragraph and explain Mendel's findings in your own words.



4.MONITOR COMPREHENSION Reread this paragraph until you understand what dominant means. If the gene for smooth peas is dominant over the gene for wrinkled peas, what type of peas would a plant with one gene of each type produce?



5.

VISUALIZE

Suppose a plant with a YY pair is crossed with a plant with a Yy pair. What gene pairs are possible for the offspring? What color seeds are possible?



6.

INTERPRET

Why do you think a bent thumb is called a hitchhiker's thumb?



7.

CONNECT

Which type of earlobes do you have? What does this tell you about the pair of genes you have for this trait?



8.

INTERPRET

Why might spending time in the sun cause a person to have blonde hair?



9.

RESPOND

What types of things can a person do to minimize the risk of heart disease?



10.

INTERPRET

Why do you think scientists couldn't look inside of cells before the 1900s?

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