Gregor Mendel's work on pea plants laid the foundation for genetics, but it was largely ignored until 1900. Three botanists independently rediscovered his research, sparking rapid development in the field. Scientists began applying Mendel's principles to other organisms, investigating inheritance mechanisms.

Mendel's laws of segregation and independent assortment explained observed ratios of phenotypes in his experiments. Early geneticists like William Bateson and Thomas Hunt Morgan made significant contributions, establishing genetics as a field. The chromosome theory of inheritance provided a physical basis for Mendel's abstract concepts.

Rediscovery of Mendel's Work

Mendel's Pioneering Research

• Gregor Mendel's work on pea plants, published in 1866, laid the foundation for the field of genetics
• Mendel's work was largely ignored by the scientific community during his lifetime
• Mendel's experiments involved carefully controlled crosses between pea plants with distinct characteristics (height, seed color, pod shape)
• Mendel's meticulous record-keeping and statistical analysis of the results allowed him to discern patterns of inheritance

Independent Rediscovery and Recognition

• In 1900, three botanists independently rediscovered Mendel's work: Hugo de Vries, Carl Correns, and Erich von Tschermak
• The rediscovery brought attention to Mendel's research and recognized the significance of his findings
• The rediscovery led to a rapid development in the field of genetics, as scientists began to apply Mendel's principles to other organisms
• Scientists investigated the mechanisms of inheritance, building upon Mendel's groundbreaking work
• The rediscovery sparked debates about the nature of heredity, leading to the development of new theories and experimental approaches in genetics
• Mendel's work provided a framework for understanding the inheritance of traits and the concept of genes, which became central to the field of genetics in the early 20th century

Principles of Mendelian Inheritance

Dominance and Recessiveness

• The principle of dominance states that when two different alleles for a gene are present, one allele (the dominant allele) masks the expression of the other allele (the recessive allele)
• In a heterozygous individual, the dominant allele determines the phenotype, while the recessive allele remains hidden
• The recessive phenotype is only expressed when an individual is homozygous for the recessive allele
• Mendel's experiments with pea plants demonstrated dominance in traits such as seed shape (round dominant over wrinkled) and flower color (purple dominant over white)

Segregation and Independent Assortment

• The principle of segregation, also known as the law of segregation, states that during gamete formation, the two alleles for a gene separate (segregate) from each other, so that each gamete carries only one allele for each gene
• Segregation ensures that an individual's genetic material is equally distributed among their gametes
• The principle of independent assortment, also known as the law of independent assortment, states that the segregation of alleles for one gene occurs independently of the segregation of alleles for other genes during gamete formation
  • This principle applies to genes located on different chromosomes or far apart on the same chromosome
  • Independent assortment results in the formation of all possible combinations of alleles in the gametes
• Mendel's principles of inheritance provided a mathematical foundation for predicting the outcomes of genetic crosses and understanding the transmission of traits from parents to offspring
• Mendel's laws of segregation and independent assortment explained the observed ratios of phenotypes in his pea plant experiments (3:1 ratio for a monohybrid cross, 9:3:3:1 ratio for a dihybrid cross)

Early Geneticists' Contributions

Establishing the Field of Genetics

• William Bateson coined the term "genetics" in 1905 and was instrumental in promoting the study of heredity and variation
• Bateson discovered genetic linkage, the tendency of certain genes to be inherited together due to their proximity on the same chromosome
• Bateson introduced the terms "homozygous" (having two identical alleles for a gene) and "heterozygous" (having two different alleles for a gene)

Drosophila and the Morgan School

• Thomas Hunt Morgan and his students, known as the "Drosophila group," used the fruit fly Drosophila melanogaster to study inheritance
• The Drosophila group discovered sex-linked inheritance, the inheritance of traits determined by genes located on the sex chromosomes (X and Y)
• Alfred Sturtevant, one of Morgan's students, constructed the first genetic map of a chromosome in 1913, demonstrating that genes are arranged linearly on chromosomes
• Calvin Bridges, another member of the Drosophila group, provided cytological evidence for the chromosome theory of inheritance by correlating the behavior of chromosomes with the inheritance of traits

Mutations and Population Genetics

• Hermann Muller demonstrated that X-rays could induce mutations in Drosophila, establishing the link between radiation and genetic mutations
• Muller's discovery had implications for understanding the nature of genes and the potential causes of genetic variation
• Ronald Fisher, J.B.S. Haldane, and Sewall Wright developed the mathematical foundations of population genetics
• Population genetics combined Mendelian genetics with Darwin's theory of natural selection to explain the genetic basis of evolution
• Fisher, Haldane, and Wright's work laid the groundwork for the modern synthesis of evolution and genetics

Chromosome Theory of Inheritance

Chromosomes as Carriers of Genetic Information

• The chromosome theory of inheritance, proposed by Walter Sutton and Theodor Boveri in 1902, stated that chromosomes are the carriers of genetic information
• The theory proposed that the behavior of chromosomes during meiosis is responsible for the Mendelian patterns of inheritance
• The chromosome theory provided a physical basis for the abstract concepts of genes and alleles, suggesting that they are located on chromosomes

Evidence for the Chromosome Theory

• The discovery of the sex chromosomes (X and Y) in 1905 by Nettie Stevens and Edmund B. Wilson provided further support for the chromosome theory
• The sex chromosomes explained the inheritance of sex-linked traits, such as eye color in Drosophila
• The physical mapping of genes onto chromosomes, as demonstrated by Alfred Sturtevant, provided evidence for the linear arrangement of genes on chromosomes
• Sturtevant's genetic maps supported the chromosome theory and helped establish the concept of genetic linkage

Unifying Genetics and Cell Biology

• The chromosome theory unified the fields of genetics and cell biology, demonstrating that the behavior of chromosomes during cell division and gamete formation is directly related to the transmission of genetic information
• The theory explained how the segregation and independent assortment of chromosomes during meiosis lead to the observed patterns of inheritance
• The chromosome theory laid the foundation for the development of cytogenetics, the study of the structure and behavior of chromosomes
• Cytogenetics has been applied in understanding genetic disorders (Down syndrome, Turner syndrome) and evolutionary relationships between species (karyotype comparisons)