Mendelian Genetics

Gregor Mendel, an Austrian monk, is the undisputed father of the science of genetics. Working with garden peas, Mendel analyzed thousands of breeding experiments to describe laws that governed the inheritance of traits. Though Mendel studied a plant, his laws for the inheritance of traits apply to all sexually reproducing organisms, including humans.

Mendel chose seven distinct traits to study in his garden peas: flower color, plant height, seed shape, seed color, pod shape, pod color, and flower position. He concluded that each of these

Gregor Mendel Experiments

Gregor Mendel's experiments with garden peas laid the foundation for the study of genetics. (National Library of Medicine)

traits was determined by a single, discrete factor called a gene. For instance, there was a gene for flower color and a gene for seed shape. Each gene had several variations, or alleles. The gene for flower color had a white allele that produced white flowers and a purple allele that produced purple flowers.

Mendel's experiments revealed that organisms have two copies of any gene for a trait. Those two copies can be identical, two purple alleles of the flower color gene, for instance; or those two alleles can be different. A pea plant could have one purple allele of the flower color gene and one white allele of the flower color gene. When an organism has two identical alleles of a gene, it is homozy-gous for that gene. When an organism has two different alleles of a gene, it is heterozygous for that gene.

An organism inherits one allele, or copy of a gene, from one parent and one allele from the other parent. An organism, or cell, that has two copies of all of its genetic information is called diploid. In most sexually reproducing animals, the offspring are formed when a sperm cell from the male parent fertilizes an egg from the female parent. The sperm and the egg only contain half of all the genetic information. They are said to be haploid. However, the new organism they create is diploid because it gets one copy of the genetic information from the sperm and a second copy from the egg.

Mendel's first law, or the law of segregation, states that the two copies of each gene separate during the formation of gametes (eggs and sperm), and that fertilization of the egg by the sperm is a random event. Any sperm containing any allele of a gene can fertilize any egg of the same species, regardless of the allele carried by that egg.

Mendel noted that certain alleles seemed to dominate over others. For instance, when a plant had a purple allele for flower color and a white al-lele for flower color, the plant always had purple flowers. Mendel called the allele that was seen in the heterozygote, in this case the purple allele, the dominant allele. The allele that was hidden or masked, he called the recessive allele. In order to show a recessive allele, an organism has to have two identical copies of a gene, both containing the same recessive allele. This is known as the homo-zygous recessive condition. Garden peas that have white flowers are homozygous recessive for the white allele of the flower color gene.

Homozygous recessive describes the organism's genotype, or its genetic makeup. It has two copies of the recessive allele of the gene. The observable characteristic of the organism, having white flowers, is called its phenotype.

Mendel also demonstrated that the segregation of alleles of any one gene is not dependent on the segregation of alleles of any other gene. For instance, a gamete could receive a dominant al-lele for an eye color gene and a recessive allele for height, or that gamete could receive the recessive alleles for both genes or the dominant alleles of both genes. This is Mendel's second law, the law of independent assortment, and it applies to any genes that are located on separate chromosomes.

Mendel's work was far ahead of its time. Although Mendel published his research in the 1800's, it was not until after his death that his work gained recognition in the scientific community. In 1900, three other scientists, each working separately on inheritance, came across Mendel's work in the course of their research. They gave him credit for his insights, and Mendel's research provided the foundation for the new discipline of genetics.

Genes and Chromosomes

Although Mendel described the gene as the factor that was responsible for a particular trait, nothing was known about the physical makeup of a gene. One of the first questions scientists needed to answer was where genes are found in cells. Early studies in frogs and sea urchins indicated that the nucleus of the sperm and the nucleus of the egg combined with each other during fertilization. This observation suggested that the genetic material that determined how the fertilized egg would develop might reside in the nucleus.

As microscopes improved, scientists were able to distinguish structures within the nuclei of cells. These long, threadlike structures stained blue and were called chromosomes (Greek chroma, "color"). Several scientists observed that when animal and plant cells divided, the chromosomes duplicated, then separated, and each daughter cell inherited a complete set of chromosomes. The one exception to this was the cell division that produced the gametes (eggs and sperm). When an egg or a sperm cell was produced, it only contained half the number of chromosomes as the cell that produced it. If genetic information was carried on chromosomes, scientists reasoned that a sperm and an egg could each contribute half of the genetic information to the new organism at fertilization.

Some of the first evidence that chromosomes were linked to observable traits came from the studies of American graduate student Walter S.

Sutton. Sutton studied grasshoppers, and his observations indicated that male grasshoppers always had an X and a Y chromosome, whereas female grasshoppers contained two X chromosomes. Several other scientists observed similar things in other organisms, such as fruit flies, and concluded that the physical characteristic of sex was determined by the kind of chromosomes an organism possessed.

Since chromosomes determined the trait of sex, it was possible that chromosomes contained the genes that Mendel had shown to determine physical characteristics. The first scientist to demonstrate that genes were located on chromosomes was Thomas Hunt Morgan, who showed that an eye-color gene in the fruit fly, Drosophila melano-gaster, was located on the X chromosome.

Next, scientists wanted to know what kind of chemical molecule actually carried the genetic information. Chromosomes contain two kinds of molecules, protein and a weak acid called deoxy-ribonucleic acid (DNA). Experiments in the early 1930's first demonstrated that DNA is the genetic material. Oswald Avery, Colin MacLeod, and Maclyn McCarty showed that adding DNA to these bacterial cells could change their physical traits. In their experiments, they mixed a harmless strain of bacteria with DNA from bacteria that caused disease in mice. When they did this, the previously harmless bacteria changed (or transformed) into disease-causing bacteria. Two other scientists, Alfred Hershey and Martha Chase, later obtained similar results by studying a virus that infects E. coli.

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