Lab-Genetics of Organisms - APBiology Lab 7: Genetics of...

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AP Biology Lab 7: Genetics of Organisms Introduction: Gregor Mendel was the father of the study of genetics. Through studying genetic inheritance in pea plants, Gregor Mendel established two basic laws that serve as the cornerstones of modern genetics: Law of Segregation and Law of Independent Assortment. The Law of Segregation refers to the segregation (separation) of alleles and their chromosomes to individual gametes. In other words, one member of each chromosome pair migrates to an opposite pole so that each gamete contains only one copy of each chromosome and each allele. These alleles are a source of genetic variability among offspring. Mendel’s Law of Independent Assortment refers to the independent assortment of alleles and their chromosomes. The process is independent because the migration of homologues within one pair of homologous chromosomes to opposite poles does not influence the migration of homologues of other homologous pairs. Both of these laws contribute to genetic variability among offspring. Mendel’s laws have their limitations. For example, if two genes are on the same chromosome, the assortment of their alleles will not be independent. Also, for genes found on the X chromosome, expression of the trait can be linked to the sex of the offspring. Our knowledge of genetics and the tools we use in its study have advanced a great deal since Mendel’s time, but his basic concepts still stand true. When traits are expressed as if one allele is dominant to a second allele, the inheritance pattern is called complete (or full) dominance. There are three types of complete dominance crosses that are used in this lab: monohybrid, dihybrid, and sex-linked. In monohybrid crosses the mode of inheritance is determined when a single contrasting pair of characteristics is involved. In a dihybrid cross the mode of inheritance is determined when the two pairs of contrasting of characteristics are considered simultaneously. In a sex-linked cross the mode of inheritance is determined when the mutant characteristic is associated with the X chromosome. At around 1908, Morgan, a professor in experimental zoology at Columbia University, started cross-breeding fruit fly Drosophila melanogaster to find heritable mutations. Finally in 1909, a series of heritable mutants appeared, some of which displayed Mendelian inheritance patterns. In a paper published in Science in 1911, he concluded that some traits were sex-linked, some genes for a certain trait are probably carried on one of the sex chromosomes, and other genes are probably carried on specific chromosomes as well. Morgan referred to wild type as the phenotype of the typical form of a species as it occurs in nature. Morgan also coined the term mutant, which is an individual, organism, or new genetic character arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not found in the wild type. Genetics and inheritance are never exactly the same, so we use statistics to determine if
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This note was uploaded on 03/03/2010 for the course AP BIO AP BIO taught by Professor Apbioteacher during the Spring '09 term at École Normale Supérieure.

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Lab-Genetics of Organisms - APBiology Lab 7: Genetics of...

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