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Chapter 13 Mendel and the Gene

Principle of independent assortment is the concept

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Principle of independent assortment is the concept that each pair of hereditary elements behaves independently of other genes during meiosis. Using a Testcross to Confirm Predictions A testcross uses a parent that contributes only recessive alleles to its offspring, to help determine the unknown genotype of the second parent. Mendel provided a powerful conceptual framework for thinking about transmission genetics- the patterns that occur as alleles pass from one generation to the next. This framework was based on 1. The segregation of discrete, paired genes into separate gametes 2. The independent assortment of genes that affect different traits. The Chromosome Theory of Inheritance Walter Sutton and Theodor Boveri realized that meiosis could be responsible for Mendel's rules. Meiosis Explains Mendel's Principles Chromosomes are composed of Mendel's hereditary determinants or genes. The gene for seed shape is at a particular position along a certain chromosome. This location is called a locus. A genetic locus is the physical location of a gene.
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The physical separation of alleles during anaphase of meiosis I is responsible for Mendel's principle of segregation. If the alleles for different genes are located on different nonhomologous chromosomes, they assort independently of one another at meiosis I. This is the physical basis of Mendel's principle of independent assortment. Sutton and Boveri formalized these observations in the chromosome theory of inheritance. Testing the Chromosome Theory Thomas Morgan and his students adopted the Drosophila as a model organism for testing the chromosome theory. The most common phenotype for each trait was referred to as wild type. Morgan discovered a male fly that had white eyes rather than the wild-type red eyes. Morgan inferred that the white-eyed phenotype resulted from a mutation- a change in a gene. Individuals with white eyes are referred to as mutants. Morgan mated a red-eyed female fly with the mutant white-eyed male fly. All of the F 1 progeny had red eyes, but when Morgan did the reciprocal cross by mating white-eyed females to red-eyed males, all F 1 females had red eyes, but all F 1 males had white eyes. Suggested a definite relationship between the sex of the progeny and the inheritance of eye color. Nettie Stevens discovered sex chromosomes. XX= female; XY= male Morgan proposed that the gene for white eye color in fruit flies is located on the X chromosome and that the Y chromosome does not carry an allele of this gene. Called X-linked inheritance, or X-linkage. A gene residing on the Y-chromosome is said to have Y-linked linkage, or Y-linkage. General term is sex- linked inheritance, or sex-linkage. According to the hypothesis of X-linkage, a female fruit fly has two copies of the gene that specifies eye color because she has two X chromosomes. Genes on non-sex chromosomes are said to show autosomal inheritance.
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