Mendelian Genetics

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Mark Huyler

Mendelian Genetics In many species, individuals have two alleles of each gene. The principle of segregation states that prior to the formation of eggs and sperm, the allele of each gene separate so that each egg or sperm cell receives only one of them. The principle of independent assortment states that alleles of different genes are transmitted to egg cells and sperm cells independently of each other. Genes are located on chromosomes: o Principle of segregation separation of homologous chromosomes in anaphase of meiosis I o Principle of independent assortment genes found on different chromosomes and is explained by chromosomes lining up randomly in metaphase of meiosis I There are important exceptions and extensions to the basic patterns of inheritance 1865 Gregor Mendel, an Austrian monk, worked out the rules of inheritance through a series of brilliant experiments on garden peas Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance which proposes that meiosis causes the patterns of inheritance that Mendel observed Genetics is the branch of biology that focuses on inheritance Mendel was interested in heredity. Heredity is the transmission of traits from parents to offpsprings Addressing the basic question of why offspring resemble their parents and how transmission of traits occurs Two hypotheses: o Blending inheritance: parental traits blend such rhat heir offspring have intermediate traits o Inheritance of acquired characteristics: parental traits are modified and then passed on to their offspring Mendel chose the common garden pea: - easy to grow - short reproductive cycle - produces large number of seeds - matings are easy to control - traits are easily recognizable Peas normally pollinate themselves aka self-fertilization Mendel could prevent this by removing the male reproductive organs (stamen) containing pollen from each flower. He then used this pollen to fertilize the female reproductive organs (pistil) of flowers on different plants, thus performing cross-pollination Mendel worked with pea varieties that differed in seven easily recognizable traits: seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length. An individuals observable features comprise its phenotype. Mendels pea population had two distinct phenotypes for each of the seven traits. Mendel worked with pure lines that produced identical offspring when self-pollinated. He used these plants to create hybrids by mating two different pure lines that differed in one or more traits. Mendel's first experiments involved crossing pure lines that differed in just one trait. The adults in the cross were the Mendels first experimented with crossing plants that differed in only one trait. When Mendel crossed pure line plants with round seeds and pure line plants with wrinkled seeds, all of the F offspring had round seeds. 1 This contradicted the hypothesis of blending inheritance. The genetic determinant for wrinkled seeds seemed to have disappeared. Mendel allowed the F p1ogeny to self-pollinate. The wrinkled seed trait reappeared in the next F2g Mendel called the genetic determinant for wrinkled seeds recessive and the determinant for round seeds dominant. In modern genetics, the terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants. Mendel repeated these experiments with each of the other traits. In each case, the dominant trait was present in a 3:1 ratio over the recessive trait in th2 F generation. Mendel wanted to determine if gender influenced inheritance. He performed a reciprocal cross, in which the mother's phenotype in the first cross is the father's phenotype in the second cross, and the father's phenotype in the first cross is the mother's phenotype in the second cross. The results of the two crosses were identical. This established that it does not matter whether the genetic determinants for seed shape are located in the male or female parent. Hereditary determinants for a trait are now called genes. Mendel also proposed that each individual has two versions of each gene. Today these different versions of a gene are called alleles. Different alleles are responsible for the variation in the traits that Mendel studied. The alleles found in an individual are called its genotype. An individuals genotype has a profound effect on its phenotype. Mendel developed the principle of segregation: the two members of each gene pair must segregatethat is, separateinto different gamete cells during the formation of eggs and sperm in the parents. Mendel used a letter to indicate the gene for a particular trait. For example, R represented the gene for seed shape. He used uppercase (R) to show a dominant allele
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