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What Are Genes? - Definition, Types & Function

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  • 0:00 What Is a Gene?
  • 1:47 DNA & Chromosomes
  • 2:48 Trait
  • 3:17 Monogenic and Polygenic Traits
  • 4:45 Alleles & Gene Therapy
  • 5:51 Lesson Summary
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Lesson Transcript
Instructor: Thomas Higginbotham

Tom has taught math / science at secondary & post-secondary, and a K-12 school administrator. He has a B.S. in Biology and a PhD in Curriculum & Instruction.

It's in the genes. Height, sense of humor, or athletic skill are all traits we've heard as being in the genes. But what are genes really, and how do they influence who we are, what we look like, and how we function?

What Is A Gene?

What is this thing that can so hugely influence so much of our and other animals' and plants' very essence? Most simply, a gene is a specific area of DNA on a chromosome that codes for the production of certain proteins that influence a particular trait (e.g., hair color). However, to understand what a gene is, some understanding of basic genetics is necessary, including what DNA, chromosomes, and traits are. To do that, we'll take a look at the story of a baby girl named Emma.

Emma started off as a healthy and vibrant infant. However, after six months, she began crying inexplicably. Her parents wrote it off as a baby being a baby. But then, she started showing a yellowish hue. Her hands and feet started feeling cold. She cried more and more often, and more intensely. Her parents began to get worried; this seemed like much more than a baby being a baby. Emma's pediatrician ran a simple blood test that confirmed Emma suffered from a disease called sickle cell anemia.

''Sickle-cell anemia?!? How do we get rid of it?'' her mom asked.

''I'm sorry,'' the doctor said. ''This is a genetic disease. It's not caused by germs. We can only treat the symptoms.''

''What? Why?''

''Because Emma's DNA tells every one of her cells to make a protein that causes her red blood cells to not work correctly.''

''Well, fix it!''

''I'm so sorry. DNA just does one thing. It tells the body's cells which proteins to make. And in Emma's case, those instructions tell her to make the protein that causes sickle-cell anemia. And since every one of her cells has these instructions, there is not much we can change.''

By looking at Emma's story, we can better understand what genes actually are.

DNA & Chromosomes

Deoxyribonucleic Acid (DNA) is a chemical compound found in all eukaryotic cells that tells the cell's machinery to produce specific proteins. It has a 'code' that gets translated into proteins. Every one of Emma's cells has the exact same DNA as every other cell.

Each one of Emma's cells has long strands of DNA. The structure by which her DNA is organized is a chromosome. Different organisms have different numbers of chromosomes. For example, humans (including Emma) have 23 pairs of chromosomes (human chromosomes number 1-23), while onions have eight pairs of chromosomes (onion chromosomes number 1-8). Emma got half of each chromosome pair from her mother and the other half from her father. Scientists have figured out which chromosomes code for particular traits. You may have guessed that the areas on chromosomes that code for particular trait-influencing proteins are what we're talking about here: genes!

Trait

A trait is simply a characteristic of an organism. Some of Emma's traits are easily identifiable and entirely heritable (e.g., hair color and sickle cell anemia), while some are only partially heritable (e.g., body weight or predilection for heart disease). Emma's genes are the specific sections of specific chromosomes that entirely or partially determine Emma's heritable traits by coding for the production of those proteins that influence the expression of her traits.

Monogenic and Polygenic Traits

It is estimated that humans have between 20,000 and 25,000 genes in their 23 chromosome pairs. Interestingly, less than 2% of the DNA on chromosomes are part of identified genes. The Human Genome Project is an attempt to map those genes so that their impact can be better understood.

Human chromosome 11 provides a good example to describe the relationship between genes, traits, and chromosomes. It is also where Emma's sickle cell anemia comes from. It is estimated that human chromosome 11 contains nearly 1,500 genes, coding for the proteins that influence many different traits, including sickle-cell anemia and many of the olfactory (smell) receptor genes in humans.

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