Gregor Mendel
Known as the father of Modern Genetics, Gregor Mendel developed the principles of heredity while studying seven pairs of inherited characteristics in pea plants. Although the significance of his work was not recognized during his lifetime, it has become the basis for the present-day field of genetics.
Genetics, scientific study of how physical, biochemical, and behavioural traits are transmitted from parents to their offspring. The word itself was coined in 1906 by the British biologist William Bateson. Geneticists determine the mechanisms of inheritance whereby the offspring of sexually reproducing organisms do not exactly resemble their parents, and the differences and similarities between parents and offspring recur from generation to generation in repeated patterns. The investigation of these patterns has led to some of the most exciting discoveries in modern biology.
EMERGENCE OF GENETICS
The science of genetics began in 1900, when several plant breeders independently discovered the work of the Austrian monk Gregor Mendel, which, although published in 1866, had been virtually ignored. Working with garden peas, Mendel described the patterns of inheritance in terms of seven pairs of contrasting traits that appeared in different pea-plant varieties. He observed that the traits were inherited as separate units, each of which was inherited independently of the others (see Mendel's Laws). He suggested that each parent has pairs of units but contributes only one unit from each pair to its offspring. The units that Mendel described were later given the name genes.
PHYSICAL BASIS OF HEREDITY
Human Male Karyotype
Different groups of organisms have different numbers of chromosomes; for example, humans have 23 pairs (46 in total) of chromosomes. One chromosome in each pair comes from the mother, the other from the father. This photo of the human male karyotype shows the chromosome pairs labelled 1 to 22, called autosomes, which 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 labelled X, whereas males have a single X chromosome and a smaller chromosome labelled Y. Chromosomes contain the genetic blueprints for a specific organism. The variation present in individuals is a reflection of the genetic recombination of these sets of chromosomes from generation to generation.
Soon after Mendel's work was rediscovered, scientists realized that the patterns of inheritance he had described paralleled the action of chromosomes in dividing cells, and they proposed that the Mendelian units of inheritance, the genes, are carried by the chromosomes. This led to intensive studies of cell division.
Mitosis
This interactivity outlines the stages involved in mitosis, the division of a cell to produce two identical cells.
Every cell comes from the division of a pre-existing cell. All the cells that make up a human being, for example, are derived from the successive divisions of a single cell, the zygote (see Fertilization), which is formed by the union of an egg and a sperm. The great majority of the cells produced by the division of the zygote are, in the composition of their hereditary material, identical to one another and to the zygote itself (assuming that no mutations occur; see below). Each cell of a higher organism is composed of a jellylike layer of material, the cytoplasm, which contains many small structures. This cytoplasmic material surrounds a prominent body called the nucleus. Every nucleus contains a number of minute, threadlike chromosomes. Some relatively simple organisms, such as cyanobacteria and bacteria, have no distinct nucleus but do have cytoplasm, which contains one or more chromosomes.
Fruit Fly Chromosomes
The chromosomes of the fruit fly, Drosophila melanogaster, lend themselves well to genetic experiments. There are only 4 pairs—one of which, marked here X and Y, determines the fly’s sex—compared with the human complement of 23 pairs. In addition, the fly’s chromosomes are very large. Thomas Hunt Morgan and his associates based their theory of heredity on studies using Drosophila. They found that chromosomes were passed from parent to offspring in a way that Gregor Mendel ascribed to inherited characteristics. They proposed, correctly, that genes in fact occupy specific physical locations on chromosomes.
Chromosomes vary in size and shape and usually occur in pairs. The members of each pair, called homologues, closely resemble each other. Most cells in the human body contain 23 pairs of chromosomes, whereas most cells of the fruit fly Drosophila contain four pairs, and the bacterium Escherichia coli has a single chromosome in the form of a ring. Every chromosome in a cell is now known to contain many genes, and each gene is located at a particular site, or locus, on the chromosome.
Meiosis
The process of cell division by which a new cell comes to have an identical number of chromosomes as the parent cell is called mitosis (see Reproduction). In mitotic division each chromosome divides into two equal parts, and the two parts travel to opposite ends of the cell. After the cell divides, each of the two resulting cells has the same number of chromosomes and genes as the original cell (see Cell: Division, Reproduction, and Differentiation). Every cell formed in this process thus has the same genetic material. Simple one-celled organisms and some multicellular forms reproduce by mitosis; it is also the process by which complex organisms achieve growth and replace worn-out tissue.
Higher organisms that reproduce sexually are formed from the union of two special sex cells known as gametes. Gametes are produced by meiosis, the process by which germ cells divide. It differs from mitosis in one important way: in meiosis a single chromosome from each pair of chromosomes is transmitted from the original cell to each of the new cells. Thus, each gamete contains half the number of chromosomes that are found in the other body cells. When two gametes unite in fertilization, the resulting cell, called the zygote, contains the full, double set of chromosomes. Half of these chromosomes normally come from one parent and half from the other.
IV THE TRANSMISSION OF GENES
Albinism
Albinism, the lack of normal pigmentation, occurs in all groups of people. 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 ageing and skin cancer. The eyes, too, colourless 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. Tinted glasses or contact lenses can help.
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