Development of genetics: Mendel and later
Inheritance, variation and evolution • The development of understanding of genetics and evolution
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Mendel’s experiments and laws
Mendel studies seven discrete characteristics of Pisum sativum (pea) plants using true-breeding lines and controlled crosses. Mendel observes that crosses between contrasting traits produce uniform F1 offspring and predictable ratios in the F2 generation, revealing consistent patterns of inheritance. Mendel formulates rules now known as Mendelian inheritance: dominant and recessive phenotypes, segregation of alleles during gamete formation, and independent assortment of different gene pairs when on different chromosomes. Mendel publishes these findings in 1866, but contemporary acceptance is limited because blended inheritance remains the prevailing view.
Particulate inheritance versus blended inheritance
Blended inheritance proposes that parental characteristics mix and produce intermediate offspring. Mendel’s data contradict this model by showing that discrete units (alleles) preserve identity across generations, so traits reappear rather than blend away. The particulate model explains why heterozygotes show a dominant phenotype while recessive traits persist in hidden form and reappear in later generations according to predictable ratios. This explanation underpins the use of genotype and phenotype to describe inheritance.
Chromosome theory and the definition of a gene
Observation of chromosomes in the late 19th and early 20th centuries links Mendel’s factors to physical structures: genes occupy positions on chromosomes and segregate during meiosis. This chromosome theory of inheritance connects cytology to Mendel’s statistical rules. The modern definition of a gene becomes a section of DNA on a chromosome that codes for a protein, replacing the older term 'trait' with a molecular explanation for inherited characteristics.
DNA, the genetic code and protein synthesis
DNA is a double helix of four bases (A, T, C, G) that pair specifically (A with T, C with G) and form the template for protein synthesis. Groups of three bases (triplet code) specify amino acids, which join to form polypeptides and proteins that determine characteristics. Understanding of DNA explains mutation, variation and the molecular mechanism behind Mendel’s inheritance patterns. Later techniques in molecular biology and sequencing expand the ability to map genes and to link specific genes to inherited disorders.
Later developments and applications
Rediscovery of Mendel’s work around 1900 triggers rapid integration of genetics with cytology and evolution. Chromosome studies, gene mapping and molecular genetics produce applications such as genetic testing and experimental evolution studies that demonstrate rapid evolutionary change in microbes. Antibiotic resistance in bacteria provides observable evidence of natural selection acting on genetic variation. Large collaborative efforts, including the Human Genome Project (1984–2003), provide complete reference sequences and accelerate identification of genes linked to disease and to human ancestry. Ethical and social implications arise from access to genomic information.
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