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Genetics, Cells, and Mendel's Law Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 21, 2011

Mendel's Laws of Inheritance

When Gregor Mendel published the results of his genetic studies on the garden pea in 1866 the foundation of modern genetics was laid. He proposed some basic genetic principles. One of these is known as the principle of segregation. In his model any one parent contains two copies of a unit of inheritance (now called a gene) for each trait; however, only one of these two genes (an allele) is transmitted through a gamete to the offspring. For example, a plant that contains two allelic forms of a gene for seed shape, one for round and one for wrinkled seeds, will transmit only one of these two alleles through a gamete to its offspring. Mendel knew nothing of DNA, chromosomes or meiosis, as they had not yet been discovered. We now know that the physical basis for segregation is in anaphase I, where homologous chromosomes (each containing a different allele of the gene for seed shape, in this case) separate from each other. If the gene for round seed is on one chromosome and its allelic form for wrinkled seed is on the homologous chromosome, then both alleles will not normally be found in the same gamete.

The second important principle that Mendel's work helped to establish is the principle of independent assortment. This law states that the segregation (or separation) of one gene pair occurs independently of any other gene pair. We now know that this is true only for unlinked genes on nonhomologous chromosomes. For example, on one homologous pair of chromosomes are the alleles for seed shape (round vs. wrinkled) and on another pair of homologues are alleles for green and yellow seed color. The segregation of the alleles for seed shape occurs independently of the segregation of the alleles for seed color because each pair of homologues behaves as an independent unit during meiosis. Furthermore, because the orientation of bivalents on the first meiotic metaphase plate is essentially random, four combinations of factors could be found in the meiotic products: (1) round-yellow, (2) wrinkled-green, (3) round-green, (4) wrinkled-yellow.

Genetic Model Systems

Genetic model systems are organisms that can be easily grown and manipulated in laboratory settings to explore various genetic and developmental effects. There are viral, bacterial (Escherichia coli), yeast (Saccharomyces cerevisiae, Schizosaccharomyces pombe), plant (Arabidopsis thaliana), and animal model systems. The soil dwelling nematode (Caenorhhabditis elegans), the freshwater zebrafish (Danio rerio), the fruit fly (Drosophila melanogaster), and the mouse (Mus musculus) are common animal systems used to study genetics.

Practice problems for these concepts can be found at:

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