Introduction: Genetics

Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethical and legal controversies. 

Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities in the cell. Although all humans share the same set of genes, individuals can inherit different forms of a given gene, making each person genetically unique. 

Since the earliest days of plant and animal domestication, around 10,000 years ago, humans have understood that characteristic traits of parents could be transmitted to their offspring. The first to speculate about how this process worked were Greek scholars around the 4th century bc, who promoted theories based on conjecture or superstition. Some of these theories remained in favor for several centuries. The scientific study of genetics did not begin until the late 19th century. In experiments with garden peas, Austrian monk Gregor Mendel described the patterns of inheritance, observing that traits were inherited as separate units. These units are now known as genes. Mendel’s work formed the foundation for later scientific achievements that heralded the era of modern genetics.

DNA Strands

Nucleic acids are complex molecules produced by living cells and are essential to all living organisms. These acids govern the body’s development and specific characteristics by providing hereditary information and triggering the production of proteins within the body. This computer-generated model shows two strands of deoxyribonucleic acid (DNA) and the double-helical structure typical of this class of nucleic acids.

Chromosome

This photomicrograph shows a specialized type of giant chromosomes called polytene chromosomes. Polytene chromosomes occur in many species of two-winged flies. They are formed when the strands of DNA within normal chromosomes undergo numerous rounds of replication without separating from one another.

Recessive Gene Transmissions

Some genes that cause genetic diseases interact in a dominant-recessive pattern. In these cases, two copies of the recessive gene are required for the disease to occur. A person who has just one copy of the recessive gene is termed a carrier, since he or she carries the gene but is not affected by it. In the illustration above, the dominant gene is represented in green, and the recessive in blue. For the couple on the left, the father has one copy of the dominant gene and one copy of the recessive gene. The mother has two copies of the dominant gene. Each parent can contribute just one gene to the child. The four children shown on the lower left represent the probabilities (not the actual children) for the combinations that can result from their parents. The children on the far left received the recessive gene from their father and the dominant gene from their mother, and are therefore carriers. For any child born to these parents, there is a 50 percent chance that the child will be a carrier. Since none of the children can inherit two copies of the recessive gene, none of the children will develop the disease. When both parents are carriers, however, as shown by the couple on the right, there is a 25 percent chance that any child born has the disease, a 50 percent chance that a child is a carrier, and a 25 percent chance that a child does not have the disease and is not a carrier.

Albinism

Albinism, the lack of normal pigmentation, occurs in all races. A rare condition, albinism occurs when a person inherits a recessive allele, or group of genes, for pigmentation from each parent. In this case, production of the enzyme tyrosinase is defective. Tyrosinase is necessary to the formation of melanin, the normal human skin pigment. Without melanin, the skin lacks protection from the sun and is subject to premature aging and skin cancer. The eyes, too, colorless except for the red blood vessels of the retina that show through, cannot tolerate light. Albinos tend to squint even in normal indoor lighting and frequently have vision problems.

DNA Molecule

A DNA molecule consists of a ladder, formed of sugars and phosphates, and four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The genetic code is specified by the order of the nucleotide bases, and each gene possesses a unique sequence of base pairs. Scientists use these base sequences to locate the position of genes on chromosomes and to construct a map of the entire human genome.

Genetic Code

Messenger RNA (mRNA) is the template for protein synthesis. It consists of a series of nucleotides, each containing one of four nitrogen bases: uracil (U), cytosine (C), adenine (A), and guanine (G). The order of nucleotides in a strand of mRNA specifies the order in which amino acids are added as a protein is built. Each series of three nucleotides specifies one amino acid. This chart identifies each amino acid by its three-letter codon(s). For example, G under the “first letter” column, C under the “second letter” column, and A under the “third letter” column intersect at alanine, the amino acid specified by the sequence GCA. Most amino acids are identified by more than one codon (for instance, GCU, GCC, GCA, and GCG all encode alanine).

Restriction Enzymes

Produced by some kinds of bacteria, restriction enzymes recognize specific sequences of DNA and cut the double strand where the sequence occurs. Treating the DNA of two different organisms with the same restriction enzyme produces complementary fragments, or fragments with ends that fit together. These can be combined in a hybrid DNA molecule that, if part of a living cell, expresses traits of both parents.

Genetic Engineering

1. In genetic engineering, scientists use restriction enzymes to isolate a segment of deoxyribonucleic acid (DNA) that contains a gene—for example, the gene regulating insulin production. 2. A plasmid removed from a bacterium and treated with the same restriction enzyme binds with the DNA fragment to form a hybrid plasmid. 3. The hybrid plasmid is re-inserted back into the bacterium, where it replicates as part of the cell’s DNA. 4. A large number of identical daughter cells (clones) can be cultured and their gene products extracted for human use.

Polymerase Chain Reaction

Polymerase chain reaction (PCR) uses an enzyme known as polymerase to rapidly multiply a small fragment of deoxyribonucleic acid (DNA)—a double-stranded, ladderlike molecule that carries the hereditary material in all living things. Each cycle of PCR consists of three phases. In the first phase, denaturation, the DNA is heated to cause its two linked strands to separate. In the second phase, annealing, the temperature of the mixture is lowered to allow primers—starter pieces of DNA—to bind to the separated DNA. In the third phase, polymerization, the temperature is raised to allow the polymerase enzyme to rapidly copy the DNA. Each PCR cycle duplicates the existing DNA, so over 1 billion copies of a single DNA fragment can be made in just a few hours.

Cromosomal Variations

A karyotype is a photographic image that depicts all of the chromosomes in an individual cell. Laboratory workers use computers to rearrange the images so that the chromosomes are lined up in pairs, typically beginning with the autosomes—chromosomes 1 through 22—and ending with the sex chromosomes—normally XX or XY. A complete karyotype helps doctors determine if a person has extra chromosomes, missing chromosomes, or chromosomes that have attached to one another in unusual ways.

Human Male Karyotype

 

This karyotype of a human male shows the 23 pairs of chromosomes that are typically present in human cells. The chromosome pairs labeled 1 through 22 are called autosomes, and have a similar appearance in males and females. The 23rd pair, shown on the bottom right, represents the sex chromosomes. Females have two identical-looking sex chromosomes that are both labeled X, whereas males have a single X chromosome and a smaller chromosome labeled Y.

Down Syndrome

  

Down syndrome is often called Trisomy 21 because most people with this condition have three copies of the number 21 chromosome—one of the smallest of the human autosomes. In this karyotype, the sex chromosomes—marked with letters instead of numbers—are XX rather than XY, showing that these chromosomes belong to a female. Down syndrome almost always results in mental retardation, though the severity of the retardation varies.

Klinefelter’s Syndrome

 

This karyotype is indicative of Klinefelter’s syndrome because it has three sex chromosomes—a single Y chromosome and two X chromosomes—instead of the usual two. People with Klinefelter’s syndrome are always male. They are typically tall, and they may have slight breast development and small testes.

Abnormal Cells and Cancer

Cancerous cells usually become much different from the tissue from which they arise. The ovarian tumor pictured here bears no resemblance to the normal tissue of the ovary.

Family Pedigree

Family pedigrees trace specific genetic characteristics through three or more generations. Pedigrees such as this one, which depicts the inheritance of a gene associated with cystic fibrosis, help genetic counselors to identify which individuals in a family are at risk of either inheriting a genetic disorder or being a carrier for a disorder.

Connecting Genetic Diseases

Gene therapy may someday be able to cure hereditary diseases, such as hemophilia and cystic fibrosis, which are caused by missing or defective genes. In one type of gene therapy, genetically engineered viruses are used to insert new, functioning genes into the cells of people who are unable to produce certain hormones or proteins necessary for the body to function normally.

Genetically Altered Aspen

Biologists found in 1995 that by turning on a gene called Leafy they could induce flowering in an aspen seedling, right, when it was only six months old and several centimeters high. An aspen tree usually does not flower until it is between 8 and 20 years old and 9 m (about 30 ft) tall. The aspen seedling on the left is also six months old, but was not altered.