From gene to trait

A trait has a cause

In the last course, you saw that Mendel's tall pea plants could hide a factor for shortness. Mendel never saw these factors. Today we know what they are made of, and how they work.

A trait is a feature that we can see or measure, such as the height of a plant or the colour of a flower. The question of this lesson is simple. How does information in a cell end up as a trait?

DNA carries the information

The nucleus of every cell holds DNA. DNA is the information source for making proteins. Proteins do most of the work in a cell.

• Some proteins build the body, such as the protein in hair and muscle.
• Many proteins are enzymes. An enzyme speeds up one chemical reaction in the cell.
• Some enzymes make hormones, which carry messages and control growth.

A section of DNA that holds the information for one protein is called a gene. A cell has thousands of genes, one for each protein it can make.

How a gene makes a plant tall

Think about the height of a pea plant. Plants make growth hormones, and the amount of hormone decides how tall the stem grows. Each hormone is made with the help of an enzyme.

1. The gene holds the information for the enzyme.
2. If the gene is the efficient version, the enzyme works well.
3. A well-working enzyme makes plenty of growth hormone.
4. With plenty of hormone, the plant grows tall.

Now suppose the gene for that enzyme is altered a little. The enzyme it makes works less well. Less hormone is made, and the plant stays short. So a small change in a gene can change a trait.

At $100$ per cent the enzyme makes $40$ units of hormone and the stem grows to $90$ cm. Lower the rate below $50$ per cent, and the plant becomes short. The numbers are made up, but the chain is real: gene, enzyme, hormone, trait.

Two copies of every gene

Each of us began as one cell, the zygote. It formed when a sperm joined an egg. So half of our DNA came from the father and half from the mother.

• For each gene, a child gets one copy from each parent.
• So a body cell has two copies of every gene.
• The two copies may be the same version, or two different versions.

This is why Mendel found two factors for each trait. The factor for tallness and the factor for shortness are two versions of the same gene. When one version is dominant, a single copy of it is enough to show the trait.

Both parents shape a trait

A child of a tall parent and a short parent does not simply copy one of them. Each parent passes on one copy of each gene, so both parents contribute to every trait.

Which version shows depends on how the two copies act together. In Mendel's peas, one copy of the tall version was enough. Other traits, such as human height, depend on many genes at once. That is why children of the same parents can look so different from each other.

Try it in Python

The program follows the chain from gene to trait for two versions of the height gene.

You should see $40.0$ units and a tall plant for the efficient version, and $10.0$ units and a short plant for the altered one.

Practise with new numbers

Common mistakes

• Thinking a gene is the trait itself. A gene is information in DNA. The trait appears through the protein it makes.
• Thinking only one parent passes on a trait. Each parent gives one copy of every gene.
• Thinking a cell has one copy of each gene. Body cells have two copies, one from each parent.
• Thinking every trait depends on a single gene. Many traits depend on several genes together.

Remember these

• DNA is the information source for making proteins.
• A gene is a section of DNA that holds the information for one protein.
• Enzymes and hormones are made through proteins, and they control traits such as height.
• A child gets one copy of each gene from each parent, so both parents contribute to every trait.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a gene?

2. How does an enzyme help to make a pea plant tall?

3. A gene is altered and the enzyme it makes works poorly. What happens to the height of the plant?

4. How many copies of each gene does a body cell have, and where do they come from?

5. Why can children of the same parents look different from each other?