Lab Manual - Heredity - Foundations of Biological Sciences...

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Foundations of Biological Sciences I - Lab Manual Principles of Heredity -1 Principles of Heredity There are several terms that need to be clarified so that you can more easily follow the exercise. A gene is a piece of DNA that directs the expression of a particular characteristic ( trait ). Genes are located on chromosomes, and the location where a particular gene is found is referred to as the locus (plural: loci) of that gene. An allele is a gene for which there is an alternative expression. For example, a diploid organism carries the allele “ A ” on one homologous chromosome, and the allele “ A ” on the other. The genotype of this organism is then AA and it is said to be homozygous . An organism may also carry two different alleles. For example on one chromosome it could carry the allele “ A ” and on the other it could carry the allele “ a ”. The genotype of such an organisms is then Aa , and it is described as heterozygous for this chromosomal locus. The genotype of an organism is the listing of the two alleles for each trait that it possesses. The phenotype of an organism is a description of the way a trait is displayed in the structure, behavior, or physiology of the organism. Some alleles are dominant to others and mask the presence of other alleles. The dominant condition is indicated by uppercase letters (e.g., “ A ”). The alleles that are masked are called recessive alleles. The recessive condition is indicated by lowercase letters (e.g., “ a ”). When both dominants are present in the genotype ( AA ), the organism is said to be homozygous dominant for the trait, and the organisms will show the dominant phenotype (trait expression A). When both recessives are present in the genotype ( aa ), the organism is said to be homozygous recessive for the trait, and the organisms will show the recessive phenotype (trait expression a). In the case of complete dominance, the dominant allele completely masks the recessive allele, and an organism with a heterozygous genotype ( Aa ) will show the dominant phenotype (trait expression A). The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. In today’s lab you will review the principles of heredity on a series of exercises. MONOHYBRID INHERITANCE (CLASS EXERCISE + LAB TABLES 1-3) This introductory exercise is done by the whole class at the same time. Your TA will explain the concepts of genotype and phenotype, how to do a simple 2 x 2 Punnett Square. Monohybrid inheritance is the inheritance of a single characteristic. The different forms of the characteristic are usually controlled by different alleles of the same gene. For example, a monohybrid cross between two pure- breeding plants (homozygous for their respective traits), one with red flowers (the dominant trait, R ) and one with white flowers (the recessive trait, r ), would be expected to produce an F 1 (first) generation with only red flowers because the allele for red flower is dominant to that of white. Using a Punnett square is a convenient method to keep track of the kinds of gametes that can be produced and the
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Lab Manual - Heredity - Foundations of Biological Sciences...

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