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Wk 8_Mendelian Genetics - GENERAL BIOLOGY LAB 1(BSC1010L...

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GENERAL BIOLOGY LAB 1 (BSC1010L) Lab #8: Mendelian Genetics _____________________________________________________________________________ _ OBJECTIVES : Understand Mendel’s laws of segregation and independent assortment. Differentiate between an organism’s genotype and phenotype. Recognize different patterns of inheritance. Perform monohybrid and dihybrid crosses. Use pedigree analysis to identify inheritance patterns. _____________________________________________________________________________ _ INTRODUCTION: Through his studies of the inheritance patterns of the garden pea, Pisum sativum, Gregor Mendel changed our understanding of heredity. Mendel studied characters/traits that differed between plants and designed cross-fertilization experiments to understand how these characters transmit to the next generation. The results of Mendel’s work refuted the prevailing hypothesis of blending inheritance and provided a new framework for understanding genetics. Ultimately, Mendel postulated two laws to explain heredity: (1) the law of segregation and (2) the law of independent assortment . Monohybrid crosses and the law of segregation The law of segregation, also termed the “first law,” states that during gamete formation the alternate forms of a gene (i.e. alleles ) on a pair of chromosomes segregate randomly so that each allele in the pair is received by a different gamete. For example, if you were to examine the gene responsible for petal color, you may discover that the gene can be expressed as either yellow or white flowers. In this scenario, the gene is petal color, while the alleles are yellow and white. Depending on which allele is expressed, petal color will vary. Examine Figure 1 below making sure that you can follow the path of each allele from parent to offspring. 1
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Figure 1. Schematic of Mendel’s law of segregation In diploid organisms, all alleles exist in pairs; identical alleles within a pair are homozygou s, while different alleles are heterozygous . Allele forms are represented by a single letter that explains whether a particular trait is dominant or recessive . Dominant alleles are assigned an uppercase letter (E), while recessive alleles are lowercase (e).In general, a dominant trait is expressed when at least one of the alleles present in the resulting allelic pair is dominant (EE or Ee). In contrast, for a recessive trait to be expressed, both alleles within the pair must be recessive (ee). For example, when considering ear lobe shape, two forms (attached and unattached) are apparent (Fig. 2). This trait is regulated by a single gene where unattached ear lobes are dominant (E) while attached ear lobes (e) are recessive. Figure 2. (a) Unattached (EE or Ee) vs. (b) attached earlobes (ee) An organism’s genotype (EE, Ee, ee) is the combination of alleles present whereas the phenotyp e is the physical expression of the genotype. In the earlobe shape example above, an individual can have a genotype of EE, Ee or ee. People with EE or Ee genotypes have the unattached earlobe phenotype (Fig 2a), while those with an ee genotype express the attached earlobe form (Fig 2b). Note that dominant traits can be either homozygous (EE) or heterozygous (Ee) while recessive traits are always homozygous (ee).
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