Genetics is the study of how traits, or characteristics, are inherited, or passed from parents to offspring. The Greek word genesis means “born of’ or “produced by.” Genes are units of heredity, located on chromosomes inside a cell’s nucleus. For species that reproduce sexually, offspring receive half of their genetic information from each parent.
Gregor Mendel (1822-1884) worked in a garden in a monastery. He was not content raising vegetables. He was interested in the variations, or slight differences, that he observed in plants.
Mendel observed that some of his pea plants grew yellow peas, while others grew green peas. Some pea plants grew tall, while others grew short. Mendel conducted careful experiments on thousands of pea plants. Then he proposed a model that explained his evidence. Today, Mendel is recognized as the father of modern genetics. Mendel’s work in genetics applies not only to pea plants but also to all plants and animals, including humans.
Purebred and Hybrid
Mendel investigated the seven traits of pea plants shown in the chart. Each trait has two forms. The color of the pea flower, for example, is either purple or white. The shape of peas is either round or wrinkled.
To begin his experiments, Mendel obtained several purebred strains of pea plants. Pea plants that are purebred tall always produce tall plants. Pea plants that are purebred short will produce short offspring.
Mendel crossed, or mated, a purebred tall pea plant and a purebred short pea plant. The seeds formed by this cross are called hybrids. A hybrid has parents from two different purebred strains. Mendel planted the hybrid seeds. He expected that they would grow into plants of medium height. To his surprise, all the hybrid seeds grew into tall plants. It was as if the trait for shortness had disappeared! Mendel then continued his experiment. He crossed the hybrid pea plants with one another, then planted their seeds. This time, the trait for shortness returned. One-fourth of the new pea plants were short. The remaining plants were tall.
Genes and Alleles
Mendel used the term factors to explain how plants control traits. Today, Mendel’s factors are called genes.
As Mendel concluded, a pea plant receives two copies of each factor, or gene. One copy comes with each parent. There are two forms of each gene. The forms are called alleles. For the trait of height in pea plants, one allele codes for tallness. Another allele codes for shortness. A plant that is hybrid for height has one allele of each kind.
Recall that Mendel observed only tall pea plants in his first round of hybrids. He explained this observation by proposing that for each trait, one allele is dominant over the other allele. For the trait of plant height, the allele for tallness is dominant. The allele for shortness is masked, so it is called recessive.
Not all traits, especially in humans, are expressed as Mendel described. Nevertheless, the idea of dominant and recessive alleles helps explain much about human inheritance.
For example, to determine eye color in humans, the allele for brown eyes (B) is dominant over the allele for blue eyes (b). This means that a person that has both alleles (Bb) has brown eyes. But this person could pass the allele for blue eyes to a son or daughter.
Chromosomes and DNA
The nucleus of every living cell carries genetic material-the chromosomes-that determines the traits of that organism. Chromosomes are made of many different proteins and a substance called deoxyribonucleic acid, or DNA. Each molecule of DNA consists of millions, even billions of tiny units linked together in a chain. The chain resembles a twisted ladder. Each small section of DNA, which we call a gene, forms part of the rung and rail of that ladder.
The gene is like an instruction manual for assembling specific molecules of the body. Some genes determine eye or hair color. Each trait is determined by genetic information.
The Genetic Code
How is DNA able to code for all the traits of the human body? The answer lies with the tiny units, called nucleotides.
DNA is made of only four nucleotides. They are adenine (A), guanine (G), cytosine (C), and thymine (T). Each “rung” of the DNA ladder consists of pairs of these four nucleotides repeated over and over again in a specific order. The order of the nucleotides is the way that DNA codes for genes.
Surprisingly, almost all DNA is identical from person to person. Only small changes in the DNA of certain genes account for the differences that make each person unique.
In 2003, scientists completed the Human Genome Project. This project identified all of the genes in human DNA. It also identified the order of all the nucleotides in these genes. The number amounted to about three billion.
The human body is made of trillions of cells. With a few exceptions, each of these cells contains exactly the same DNA. How is this possible? When a cell divides to make new cells, its DNA undergoes a copying process. This process is called DNA replication.
Recall that the structure of DNA is like a twisted ladder. The two halves of the ladder are joined by their nucleotides, also called bases. Notice that the bases always pair in the same combination. Adenine (A) always pairs with thymine (T). Cytosine (C) always pairs with guanine (G).
During DNA replication, the two halves of the ladder “unzip,” or separate. The exposed bases then pick up and bind to new bases from their surroundings. In this way, each half of the unzipped DNA serves as a template, or model, to build a new half. The result is two new DNA molecules, each an exact copy of the original.
In most human cells, DNA and genes are divided among 46 chromosomes. DNA replication is part of the process that copies chromosomes when new cells are made.
Two of the 46 chromosomes are different from the others. These are the sex chromosomes, also called the X and Y chromosomes. The sex chromosomes determine gender. Cells in men have one X and one Y chromosome. Cells in women have two X chromosomes.
All 46 chromosomes are present in the first cell of a developing human baby. Half of the chromosomes, including either an X or a Y, come from the father. These 23 chromosomes were carried by a sperm cell. Sperm are male reproductive cells. The other 23 chromosomes, including one X chromosome, come from the mother. They are carried by an egg cell, the female reproductive cell. A sperm and egg cell combine to form a new individual.
Usually, the replication of DNA is perfect, and two identical chromosomes result. Sometimes, however, a mistake occurs in the instructions. We say the gene has mutated-or changed its instructions. This change can affect the way the organism develops. If the organism survives the mutation, that mutated gene may be inherited by future generations. Many inherited mutations are harmful. Inherited conditions include muscular dystrophy, hemophilia, and sickle-cell anemia.
DNA Transcription and Translation
How is DNA decoded? The process involves two important steps. The first step is called transcription. In this step, DNA serves as a template, or model, to make a molecule of RNA. RNA is short for ribonucleic acid. Unlike DNA, a molecule of RNA can leave the nucleus and travel to other cell parts. Away from the nucleus, RNA acts as a model to make proteins. This process is called translation. The cell uses proteins to help control all of its chemical processes.
In any cell, only certain genes are ever transcribed and translated. Active genes differ among body cells. In certain stomach cells, for example, some of the active genes help make stomach acid. In nerve cells, active genes help make the chemicals that nerves use to send messages.
DNA and Living Things
Scientists have identified and studied DNA in a huge number of organisms. As they have discovered, DNA controls traits in plants, animals, and even fungi and bacteria. Viruses contain either DNA or RNA.
With only a few exceptions, the DNA in every living thing functions just as it does in humans. Whether you are investigating the DNA of a parrot or a pineapple, you will find that it uses the same four nucleotides and the same genetic code. It also uses the same processes of transcription and translation.