Has anyone ever said to you, "You look just like your father!" or "Your grandma had the same curly hair!" or "You've got your mother's eyes!" or "You got your freckles from your grandpa!" Why do children look like their parents? Why do brothers and sisters look similar, but different from each other? Why do you look like you do?
The answer is heredity. Heredity means that traits are passed from parents to their children. This is true not only of people, but of animals, plants, and all living things. How does this process work? The science of studying heredity is called genetics, and scientists who study how traits are passed from one generation to the next are called geneticists. It's a fairly new but fascinating science. Let's find out more about what makes you "YOU."
Inherited traits are those characteristics that are passed from parents to their children. These are traits you are born with. You may hear people say that a certain trait, such as red hair, "runs in the family." In humans, these traits include eye color, hair color, height, curly or straight hair, and facial features like dimples, freckles, cleft chin, or nose shape. A special ability for sports or music might also be inherited. In animals, inherited traits might be fur color, presence of feathers, an instinct for herding, and size or speed. In plants, inherited traits might include being tall or short, having fruits that are small or large, and flower color.
In the past, people had always known that traits are passed down. They knew that the young of living things are like, but not exactly like, their parents. Yet no one really understood how it worked until Gregor Mendel, an Austrian monk, began to experiment with pea plants in his garden. From 1856 to 1864, Mendel grew thousands of pea plants, observing the parent plants, their offspring, and generations that followed. Why would two parent plants, one with green peas and one with yellow peas, produce only plants with yellow peas? Why would two of those new plants, both with yellow peas, then produce a plant with green peas? He kept careful notes on traits such as pod color and shape, inner pea color and shape, flower position and color, and stem length.
After much experimentation, Mendel learned some important facts that came to be known as the Laws of Inheritance:
Only one form of a trait shows up in the offspring plant; inherited traits do not mix. Some traits are dominant (stronger) over other traits, and if present, will show up in the offspring. But recessive (weaker) traits can still be carried inside the offspring and show up in the next generation.
Even though a plant may carry two forms of the trait, each offspring receives only one unit of inheritance from each parent.
Different traits are inherited independently. In peas, the trait for pod shape may have nothing to do with the trait for stem length.
Even though Mendel's work was not widely known until 30 years after his death, his discoveries remain the foundation of genetic science. Learn more about the "father of genetics."
Chromosomes, Genes, and DNA
Mendel never used the words "genes" or "genetics" because those words did not yet exist. He explained how genes work without understanding what they were. Today we understand that heredity takes place inside the cells of living organisms.
Every living thing, including all plants, animals, and people, is made up of cells. All human beings start out as one single cell and grow into trillions of tiny cells. Humans have skin cells, brain cells, bone cells, and much more. Different cells have different jobs to help our bodies function. Each cell contains a nucleus, and within the nucleus of every cell are 46 chromosomes, arranged in 23 coiled pairs. Every person receives half their chromosomes from their father and half from their mother. Other plants and animals have different numbers of chromosomes.
These chromosomes inside the cell nucleus are made up of an amazing molecule called deoxyribonucleic acid, better known as DNA. DNA is shaped like a spiraling double helix or a long, twisted ladder. DNA is like a computer program within the cell; it contains all the instructions that tell the cell how to grow and what to do. DNA contains all the information needed to make a fish a fish, a rose a rose, and you YOU. Each person's DNA is like a unique code that is found in every cell of his or her body.
Sections of DNA called genes contain the information for each person's traits, like whether they are short or tall, whether they have blue or brown eyes, and whether they are a boy or a girl. You can think of genes as "recipes" for making proteins that are necessary for all life. Humans have about 25,000 genes. Often several genes work together to determine traits. Learning which trait is controlled by which genes is the work of genetic scientists.
DNA is made up of nucleotides of four different types: G (guanine), C (cytosine), A (adenine), and T (thymine). These four bases, found in every living thing, combine in different ways to create the instructions for the cells. For example, a section of the code might look like this: ATC TGA GGA AAT GAC CAG. There are billions of different combinations. It's the order of the letters in the code that makes each kind of organism different.
All humans have the same genes arranged in the same order. About 99.9% of everyone's DNA sequence is the same. It's that 0.1% of the genetic code that accounts for all the differences between people. Those few differences between us make each one of us unique. We also share about 96% of our DNA with chimpanzees and 85% with mice.
Scientists first identified the structure of DNA in 1953. In 2001, scientists described the sequence of the entire human genome - all the DNA in a cell, about 3 billion letters! It would take 200 1000-page books to contain all of that information for a single person. They have identified the genomes of other species as well. The major work of understanding genomes still lies in the future. Learn more about genes and DNA.
Patterns of Heredity
Since each new organism receives half of its chromosomes from its mother and half from its father, how is it determined which traits show up in the child? Why do two sisters look different, if they received their genes from the same two parents? Genes are inherited in certain patterns. Gregor Mendel figured out patterns of heredity in his pea plants, and scientists continue to study those patterns today.
Inside the DNA molecule are sections of information called genes that tell the cell how to make proteins that lead to traits. But it is important to remember that each child inherits two copies of every gene, one from each parent. For each gene, which copy gets passed along from the parent is random, and it may be different for each child. The two copies that the child receives may be the same, or they may be different. The specific pattern in a gene is called an allele. For example, one specific allele of the hair color gene causes the hair to be black.
Some forms of genes are more dominant than others. For example, the brown-eyed allele is dominant, and the blue-eyed allele is recessive. If a child receives a brown-eyed gene from the father and a blue-eyed gene from the mother, he or she is said to be heterozygous (two different alleles) for that trait. The dominant version overpowers the recessive version, and the child will have brown eyes. In order to have blue eyes, the child must receive two recessive blue-eyed alleles and is said to be homozygous (two same alleles) for that trait.
Mendel figured this out when he worked with green and yellow peas. The allele for yellow peas is dominant over the allele for green peas. This diagram shows what happened when he crossed a homozygous yellow pea plant with a homozygous green pea plant.
In the 2nd generation, both children were heterozygous yellow. In the 3rd generation (grandchildren), 3 had yellow peas, and one had green peas.
Can you see how brown-eyed parents may have a blue-eyed child, even though brown eyes are dominant? Remember, each parent has two genes for eye color that they got from their own parents. When we write the expression for inherited genes, we write one letter representing the gene from the mother and one representing the gene from the father. Dominant genes are written with capital letters and recessive genes with small letters. For example, the dominant brown-eyed gene is written B, and the recessive blue-eyed gene is written b. So if the father carries a brown-eyed gene (B) and a blue-eyed gene (b), his eye-color can be written as Bb. He has brown eyes, because the brown-eyed B allele is dominant. But he still carries the recessive blue-eyed gene (b), and he could still pass along that blue-eyed gene (b) to his child. If the child's mother also happens to pass along a blue-eyed gene (b), the child will have two blue-eyed genes (bb), and thus have blue eyes.
It is easier to understand these patterns using a Punnett Square. A Punnett Square shows all the possible combinations of two parents' eye-color genes. The mother is Bb, with brown eyes, and the father is Bb, with brown eyes. Yet there is a one-in-four (25%) chance that their child will have blue eyes (bb).
Because they share genes, children in a family resemble their parents and each other. But unique gene combinations give each person a unique set of inherited characteristics. Learn more about patterns of heredity.
Variation and Mutation
When two parents pass on their genes to their offspring, the mixing of genes leads to genetic variation. Variation is the difference in genes between individuals or groups. Genetic variation is important because it helps populations change over time. In any species, some individuals in a population will have variations that are more likely to help them survive. Variations that help an organism survive are passed on to their offspring. Over many generations, the successful variation may become a trait for that population.
Mutation is a natural process that causes a change in the DNA code or information. Sometimes a part of a gene is missing, or a nucleotide occurs in a different order. Changes in genes can result in changes to proteins made in the cells, which can cause changes in traits. In most cases the result is a small change that is neither bad nor good. Because of popular stories, we sometimes think of "mutant" as being something bad or scary, but in most cases a mutation is simply a difference that creates variation.
Occasionally, however, there are changes or mutations in the DNA sequence that lead to health problems. Sometimes there are altered genes that are linked to certain diseases or conditions. It is possible that a change in a single nucleotide can cause trouble. Some of these problem genes can be passed down from parents, even when the parents do not have the disease. Geneticists study DNA to learn which part of the code may be causing problems.
Genes Aren't Everything!
Every single person is unique. You are the only YOU that has ever lived or will ever live. No one else looks or acts exactly like you. One reason for that is your DNA, which is different from every other person's. Your DNA is what makes you different from a frog, a tree, or even your best friend.
But even identical twins, who have exactly the same DNA, are not exactly alike. Each twin has his or her own personality, talents, likes and dislikes. The differences between twins do not come from their genes, because DNA is not the only thing that makes you who you are. The environment you live in, the people you know, and the experiences you have all work together with your genes to make you "YOU." Scientists are working to understand more about how genes and the environment work together to affect traits.
Even inherited traits can be affected by the environment. A plant may have the genes for a certain height, but poor growing conditions or drought may prevent the plant from growing that tall. A retriever dog may inherit the instinct to chase things and bring them back, but he could be trained to roll over instead. While a person's genes may specify a certain hair color, chemicals or sunlight can change that color. A child may have inherited musical or athletic ability, but it still requires practice and effort to become a soccer player or pianist. A person may be born with an increased risk of heart disease, but eating healthy foods and exercising may reduce that risk.
Making good choices for your body and mind is just as important as what your genes give you. Your DNA may decide what you are made of and what you look like, but you get to decide what you do with your life.
Genetic Science at Work
Scientists have learned a great deal about heredity, genes, and DNA in the last 50 years. They are putting this knowledge to work to help our planet and to improve people's lives.
Because everyone has a unique DNA sequence, forensic scientists can use DNA evidence to help identify criminals. For example, a hair found at a crime scene can be analyzed to see if it is an exact match with a suspect's DNA.
DNA science can be used in agriculture to make crops more productive or resistant to diseases or pests. Although farmers have always cross-bred crops to get desired traits, scientists have now identified the genes that determine specific traits in plants, like the number and size of kernals in corn. Genetically modified organisms (GMOs) are created by inserting genes for specific traits into a genome. This leads to improved crops, but some people are concerned that genetic modification of foods could lead to unwanted side effects.
Using computers that can search through the long instruction set that makes up a person's DNA, scientists can detect mistakes in the pattern that cause certain diseases. People who have these genes can often take steps to avoid or reduce the risk of the disease. Scientists are also experimenting with gene therapy, where a faulty gene is replaced with a normal one. Continuing research may lead to new ways to treat or even prevent diseases.
Scientists can analyze the DNA in endangered animals and learn more about them. This information can then be used to help protect animals in the wild, breed them in zoos, conserve their habitat, and increase genetic variation. DNA analysis is also used to determine if an animal has been hunted illegally.
Scientists have learned how to clone certain organisms. With cloning, a new organism is an exact genetic copy of another. Cloning is used to help scientists study diseases and develop new medicines. It is fun to think about bringing extinct dinosaurs back to life by cloning, but finding well-preserved DNA from 65 million years ago is unlikely.
The future is wide open for the study and uses of genetic technology. Many discoveries remain to be made. Perhaps you will be a genetic scientist someday!