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Chapter 4 Mendelian Inheritance Gregor Johann Mendel was born on July 22, 1822, in Heizendorf, Austria. He was the only son of a peasant farmer. In 1843 he began studying at the St. Thomas Monastery of the Augustinian Order in Brunn. He was ordained into the priesthood in August of 1847. After his ordination, Mendel was assigned to pastoral duties, but it soon became apparent that he was more suited to teaching. In 1849, he was assigned to a secondary school in the city of Znaim. It was there that he took the qualifying examination for teacher certification and failed. In 1851 he entered the University of Vienna to train to be a teacher of Mathematics and Biology. It was at the University of Vienna that he developed his skills as a researcher which he utilized later in his life. Mendel returned to teaching in Brunn in 1854. Two years later he again attempted the state certification examination. He became quite ill, perhaps as a result of severe debilitating test anxiety, and he withdrew. He did attempt to take the examination again, but returned to Brunn in 1856 where he continued to teach parttime. Toward the end of his life, in 1868, Mendel was promoted in the monastery to Abbot. He died on January 6, 1884. During the middle of Mendel's life, Mendel did groundbreaking work into the theories of heredity. Using simple pea pod plants, Mendel studied seven basic characteristics of the pea pod plants. By tracing these characteristics, Mendel discovered three basic laws which governed the passage of a trait from one member of a species to another member of the same species. The first law states that the sex cells of a plant may contain two different traits, but not both of those traits. The second law stated that characteristics are inherited independently from another (the basis for recessive and dominant gene composition). The third theory states that each inherited characteristic is determined by two hereditary factors (known more recently as genes), one from each parents, which decides whether a gene is dominant or recessive. In other words, if a seed gene is recessive, it will not show up within the plant, however, the dominant trait will. Mendel's work and theories, later became the basis for the study of modern genetics, and are still recognized and used today. His work led to the discovery of particulate inheritance, dominant and recessive traits, genotype and phenotype, and the concept of heterozygous and homozygous. Unfortunately, Gregor Mendel was not recognized for his work by his scientific peers. He found actual proof of the existence of genes, and is considered to be the father of genetics, though his work was relatively unappreciated until the early 1900's. References: Olby, Robert C., The Origins of Mendelism. 2d ed. 1985. Hugo Iltis. Vitezslav,Orel. Life of Mendel. 1984. 4.1 Following the Inheritance of One Gene Segregation Mendel’s Laws Apply to Humans, Too – Ex. Cystic fibrosis - recessive disorder in humans that can be unexpected in the offspring of normal parents – Defective allele of the CFTR gene on chromosome 7. Mendel’s Experiments – Mendel deduced his first law (segregation) from observations of crosses involving tall pea plants that could produce short offspring. Chromosome Behavior in Meiosis Explains Mendel’s Law of Segregation – The law of segregation states that inherited "characters" (alleles) separate during meiosis, so that each offspring receives one copy of each allele from each parent. Representing Mendel’s Law of Segregation – The genotypic ratio for a monohybrid cross is 1:2:1, and the phenotypic ratio is 3:1. – A test cross reveals the presence recessive genes in an individual with an unknown genotype by crossing them with an individual homozygous recessive for the genes in question. – Punnett squares are used to calculate expected genotypic and phenotypic ratios among progeny. http://i.ehow.com/images/GlobalPhoto/Articles/4599080/PunnettSquare2_Full.jpg 4.2Mendelian Inheritance in Humans A Mendelian trait is caused by a single gene. Traits can be dominant or recessive and recur in a predictable pattern in subsequent generations. Autosomal dominant traits do not generally skip generations and can affect both sexes. Autosomal recessive traits can skip generations and can affect both sexes. Blood relatives that have children together have a much higher risk of having a child with a rare recessive disorder. Punnett squares apply Mendel's first law to predict recurrence risks for inherited disorders or traits. A Mendelian trait applies anew to each child. “On the Meaning of Dominance and Recessiveness” – Biochemical level, recessive disorders often result from alleles that cause the loss of function or production of a normal protein. – Dominant disorders can result from production of an abnormal protein that interferes with the function of a normal protein. 4.3 Following the Inheritance of Two GenesIndependent Assortment Mendel's law of independent assortment considers genes transmitted on different chromosomes. The phenotypic ratio of 9:3:3:1 of a dihybrid cross indicates that a gene on one chromosome does not influence transmission of a gene on a different chromosome. In meiosis, random assortment of maternally and paternally derived chromosomes results in gametes that have different combinations of genes. Punnett squares and probability are used to predict recurrence of more than one trait. http://www.biologycorner.com/resources/dihybrid_cross.gif 4.4 Pedigree Analysis Pedigrees Then and Now – Pedigree charts depict family relationships and transmission of inherited traits. – Squares represent males and circles represent females. – Horizontal lines indicate parents – vertical lines show generations – elevated horizontal lines depict siblings. – Symbols for heterozygotes are half-shaded – individuals with a particular phenotype, completely shaded. Pedigrees Display Mendel’s Laws – Pedigrees can reveal mode of inheritance, and can include molecular information, carrier status, and input from other genes and the environment. Simple Pedigree http://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/Autosomal_Recessive_Pedigree_Chart_.svg/ 600px-Autosomal_Recessive_Pedigree_Chart_.svg.png Family Mortality Pedigree Additional Resources Variety of Articles – http://www.esp.org/foundations/genetics/class ical/ Article - The Evolution of a Classic Genetic Tool – http://auth.mhhe.com/biosci/genbio/life/article s/article16.mhtml Mendel’s Data – http://www.stat.ucla.edu/cases/mendel/