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Unformatted text preview: 1 Mendel and the Gene Idea Mendel’s Law of Segregation Mendel’s Law of Independent Assortment Readings: Chapter 14 Topics of Ch. 14 not covered: Extending Mendelian genetics for two or more genes (epistasis and quantitative genetics); multiple alleles. Table 14.1 Results of crosses for seven characters • Mendel developed a hypothesis to explain these results that consisted of four related ideas . • Alternative version of genes (different alleles ) account for variations in inherited characters. • The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus . Alleles of the same locus (= the location of a gene) are located on different chromosomes of the homologous chromosome pair Recessive Dominant Fig. 14.4 2. For each character, an organism inherits two alleles, one from each parent. • A diploid organism inherits one set of chromosomes from each parent. • Each diploid organism has a pair of homologous chromosomes and therefore two copies of each locus. These homologous loci may be identical, as in the true-breeding plants of the P generation. Alternatively, the two alleles may differ; purple-flower allele from one parent and a white-flower allele from the other. One allele may be dominant to the other. The two alleles for each character segregate (separate) during gamete production. • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis. • The separation of alleles into separate gametes is summarized as Mendel’s law of segregation . Mendel’s law of segregation: the two alleles for a character are packaged into separate gametes Fig. 14.5 Punnett square 2 • Some vocabulary of genetics : • An organism with two identical alleles for a character is homozygous for that character. • Organisms with two different alleles for a character is heterozygous for that character. • A description of an organism’s traits is its phenotype . • A description of its genetic makeup is its genotype . • Two organisms can have the same phenotype but have different genotypes if one is homozygous dominant and the other is heterozygous. • It is not possible to predict the genotype of an organism with a dominant phenotype. The organism must have one dominant allele, but it could be homozygous dominant or heterozygous. • A test cross : breeding a homozygous recessive with dominant phenotype. The unknown genotype, can determine the identity of the unknown allele. Fig. 14.7 • Mendel’s monohybrid crosses – led to formulation of law of segregation ....
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