C H A P T E R
Gregor Johann Mendel, who in 1866 put forward the major postulates of transmission genetics as a result of experiments with the garden pea.
■ Inheritance is governed by information ■ During gamete formation, chromosomes
stored in discrete factors called genes.
■ Genes are transmitted from generation
to generation on vehicles called chromosomes.
■ Chromosomes, which exist in pairs, provide
are distributed according to postulates first described by Gregor Mendel, based on his nineteenth-century research with the garden pea. ■ Mendelian postulates prescribe that
other segregating homologs during gamete formation.
■ Genetic ratios, expressed as probabilities,
are subject to chance deviation and may be evaluated statistically. ■ The analysis of pedigrees allows predictions
the basis of biparental inheritance.
homologous chromosomes segregate from one another and assort independently with
involving the genetic nature of human traits.
The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation 39 science for the next 16 years. Mendel received support from the monastery for his studies and research throughout his life. In 1856, Mendel performed his first set of hybridization experiments with the garden pea. The research phase of his career lasted until 1868, when he was elected abbot of the monastery. Although he retained his interest in genetics, his new responsibilities demanded most of his time. In 1884, Mendel died of a kidney disorder. The local newspaper paid him the following tribute: “His death deprives the poor of a benefactor, and mankind at large of a man of the noblest character, one who was a warm friend, a promoter of the natural sciences, and an exemplary priest.” Mendel first reported the results of some simple genetic crosses between certain strains of the garden pea in 1865. Although his was not the first attempt to provide experimental evidence pertaining to inheritance, Mendel’s success where others failed can be attributed, at least in part, to his elegant model of experimental design and analysis. Mendel showed remarkable insight into the methodology necessary for good experimental biology. He chose an organism that is easy to grow and hybridize artificially. The pea plant is self-fertilizing in nature but is easy to crossbreed experimentally. It reproduces well and grows to maturity in a single season. Mendel followed seven visible features (unit characters), each represented by two contrasting forms, or traits (Figure 3–1). For the character stem height, for example, he experimented with the traits tall and dwarf. He selected six other visibly contrasting pairs of traits involving seed shape and color, pod shape and color, and pod and flower arrangement. From local seed merchants, Mendel obtained true-breeding strains—those in which each trait appeared unchanged generation after generation in self-fertilizing plants. There were several reasons for Mendel’s success. In addition to his choice of a suitable organism, he restricted his examination to one or very few pairs of contrasting traits in each experiment. He also kept accurate quantitative records, a necessity in genetic experiments. From the analysis of his data, Mendel derived certain postulates that became principles of transmission genetics. The results of Mendel’s experiments were unappreciated until the turn of the century, well after his death. However, once Mendel’s publications were rediscovered by geneticists investigating the function and behavior of chromosomes, the implications of his postulates were immediately apparent. He had discovered the basis for the transmission of hereditary traits!
lthough inheritance of biological traits has been recognized for thousands of years, the first...
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