Genetics
Genetics is the study of heredity—the passing of physical, biochemical, and physiologic traits, both healthy and pathogenic, from biological parents to their children. In this transmission, mistakes or mutations can cause susceptibility to disease, disability, or death.
Genetic information is carried in genes, which are strung together on the deoxyribonucleic acid (DNA) double helix to form chromosomes. Every normal human cell (except reproductive cells) has 46 chromosomes, 22 paired chromosomes called autosomes, and two sex chromosomes (a pair of X’s in females and an X and a Y in males). A person’s individual set of chromosomes is called his karyotype. The human genome has been under intense study for about 15 years to determine the structure of each gene in the genome and its location within each of the 23 chromosomes comprising the set of human chromosomes. In April 2003, scientists announced the completion of the entire genome sequence. The sequence consists of more than 3.1 billion pairs of DNA bases. Decoding the genome will enable people to know who’s likely to get a specific inherited disease and enable researchers to eradicate or improve the treatment of many diseases.
For a wide variety of reasons, not every gene is expressed in every cell. Genetic principles are based on studies of thousands of individuals. Those studies have led to generalities that are usually true, but exceptions occur. Genetics remains an inexact science.
Genetic components
Each of the two strands of DNA in a chromosome consists of thousands of combinations of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G), arranged in complementary triplet pairs (called codons), each of which represents an amino acid; a specific sequence of triplets represents a gene. The strands are held together loosely by chemical bonds between adenine and thymine or between cytosine and guanine. The looseness of the bonds allows the strands to separate easily during DNA replication. The genes carry a code for each trait a person inherits, from blood type to eye color to body shape and a myriad of other traits.
DNA ultimately controls the formation of essential substances throughout the life of every cell in the body. It does this through the genetic code, the precise sequence of AT and CG pairs on the DNA molecule. Genes control not only hereditary traits, transmitted from parents to offspring but also cell reproduction and the daily functions of all cells. Genes control cell function by controlling the structures and chemicals that are synthesized within the cell.
Transmitting traits
Germ cells, or gametes, are one of two classes of cells in the body; each germ cell (ovum or sperm) contains 23 chromosomes (called the haploid number) in its nucleus. All the other cells in the body are somatic cells, which are diploid; that is, they contain 23 pairs of chromosomes.
When human ovum and sperm unite, the corresponding chromosomes pair up, so that the fertilized cell as well as every somatic cell of the new individual has 23 pairs of chromosomes in its nucleus.
Germ cells
The body produces germ cells through a type of cell division called meiosis. Meiosis occurs only when the body is creating haploid germ cells from their diploid precursors. Each of the 23 pairs of chromosomes in the diploid precursor cell replicates and undergoes two cell divisions, so that, on completion, each new germ cell (ovum or sperm) contains one set of 23 chromosomes.
Most of the genes on one chromosome are identical or almost identical to those on its mate. The location (or locus) of a gene on a chromosome is specific and doesn’t vary from person to person. This allows each of the thousands of genes on a strand of DNA in an ovum to join the corresponding gene in a sperm when the chromosomes pair up at fertilization.
Sex chromosomes
Only one pair of the 23 pairs of chromosomes in each cell is primarily involved in determining a person’s sex. These are the sex chromosomes; the other 22 chromosome pairs are called autosomes. Females have two X chromosomes and males have one X and one Y chromosome.
Each germ cell produced by a male contains either an X or a Y chromosome. When a sperm with an X chromosome fertilizes an ovum, the offspring is female (two X chromosomes); when a sperm with a Y chromosome fertilizes an ovum, the offspring is male (one X and one Y chromosome). Extremely rare errors in cell division can result in a germ cell that has no sex chromosome or has two sex chromosomes. After fertilization, the zygote may have an XXX, XYY, XO, or XXY karyotype and still survive. Most other errors in sex chromosome division are incompatible with life.
Mitosis
The fertilized ovum—now called a zygote—undergoes a kind of cell division called mitosis. Before a cell divides, its chromosomes replicate. During this process, the double helix of DNA separates into two chains; each chain serves as a template for constructing a new chain. Individual DNA nucleotides are linked into new strands with bases complementary to those in the originals. In this way, two identical double helices are formed, each containing one of the original strands and a newly formed complementary strand. These double helices are duplicates of the original DNA chain.
Mitotic cell division occurs in five phases: interphase, prophase, metaphase, anaphase, and telophase. The result of every mitotic cell division is two new daughter cells, each genetically identical to the original and to each other. Then, each of the two resulting cells divides, and so on, eventually forming a many-celled human embryo. Thus, each cell in a person’s body (except ovum or sperm) contains an identical set of 46 chromosomes that are unique to that person.
Trait predominance
Each parent contributes one set of chromosomes (and therefore one set of genes) so that every offspring has two genes for every locus on the autosomes. Some characteristics, or traits, such as eye color, are determined by one gene that may have many variants (alleles). Others, called polygenic traits, require the interaction of two or more genes. In addition, environmental factors may affect how genes are expressed, although the environmental factors don’t affect the genetic structure.
Variations in a particular gene—such as brown, blue, or green eye color—are called alleles. A person who has identical genes on each member of the chromosome pair is homozygous for that gene; if the alleles are different, the person is said to be heterozygous.
Autosomal inheritance
On autosomes, one allele may be more influential than another in determining a specific trait. The more powerful, or dominant, gene product is more likely to be exhibited in the offspring than the less influential, or recessive, gene product. Offspring will exhibit a dominant allele when one or both chromosomes in a pair carry it. A recessive allele won’t be exhibited unless both chromosomes carry identical copies of the allele. For example, a child may receive a gene for brown eyes from one parent and a gene for blue eyes from the other parent. The gene for brown eyes is dominant, and the gene for blue eyes is recessive. Because the dominant allele is more likely to mask the recessive allele, the child is more likely to have brown eyes.
Sex-linked inheritance
The X and Y chromosomes aren’t literally a pair because the X chromosome is much larger than the Y, with more genetic material. The male has only one copy of the genes on the X chromosome. Inheritance of those genes is called X-linked. A man will transmit one copy of each X-linked gene to his daughters and none to his sons. A woman will transmit one copy to each child, whether male or female.
Inheritance of genes on the X chromosome is different in another way. Females have two X chromosomes in each of their cells; however, only one X chromosome is active in each cell because of a process called X inactivation. X inactivation occurs during early embryogenesis in the female, and the X that’s inactivated in each cell is random. In some cells, the X the female received from her mother is inactivated, and in other cells the X she received from her father is inactivated. For this reason, at the cellular level a heterozygous female will express the recessive gene in some cells and the dominant gene in others.
Multifactorial inheritance
Multifactorial inheritance is inheritance that’s determined by multiple factors, including genetic and possible nongenetic (environmental), each with only a partial effect. The genetic factor may consist of variations for multiple genes: some that provide susceptibility and some that provide protection. Examples of environmental factors that may contribute to such a trait are nutrition, exposure to teratogens or carcinogens
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