Genetics
In 1865, a Moravian Augustinian friar named Gregor Mendel presented his paper Experiments on Plant Hybridization to the Naturforschender Verein in Brno. He had spent years observing pea plants in the monastery garden of St Thomas Abbey. Mendel traced how specific traits like flower color or seed shape passed from parent plants to offspring over multiple generations. His work showed that inheritance was not a smooth blend of parental features but relied on discrete units he called elements of inheritance. These units would later become known as genes. Before Mendel, most scientists believed in blending inheritance, where traits mixed like paint colors. Charles Darwin himself implied this view in his 1859 book On the Origin of Species. Mendel proved that traits remained distinct through hybridization. He found that crossing heterozygous organisms produced dominant traits three times more often than recessive ones. This mathematical ratio became the foundation for modern genetics.
Scientists struggled for decades to identify which molecule carried genetic information inside cells. Frederick Griffith discovered transformation in 1928 when dead bacteria transferred material to living ones. The Avery, MacLeod, McCarty experiment in 1944 confirmed DNA as the responsible molecule. The Hershey, Chase experiment in 1952 provided further evidence using viruses that infect bacteria. James Watson and Francis Crick determined the structure of DNA in 1953. They used X-ray crystallography data created by Rosalind Franklin and Maurice Wilkins. Their model revealed two strands forming a double helix with nucleotides pointing inward. Each strand contained all necessary information to reconstruct its partner. This semi-conservative nature allowed replication to occur when strands separated. Tomoko Ohta published her nearly neutral theory of molecular evolution in 1973. She emphasized how natural selection and environmental factors influenced evolutionary rates. Frederick Sanger developed chain-termination sequencing in 1977. Kary Banks Mullis invented the polymerase chain reaction in 1983. These tools enabled scientists to read and amplify specific sections of DNA quickly.
Genes reside on chromosomes, long chains of DNA wrapped around histone proteins. In humans, the largest chromosome spans approximately 247 million base pairs. Thomas Hunt Morgan argued in 1911 that genes sit linearly along these chromosomes based on white eye mutations in fruit flies. Alfred Sturtevant mapped gene positions in 1913 using genetic linkage. Harriet Creighton and Barbara McClintock provided cytological evidence for crossing over in corn plants during 1931. During meiosis, homologous chromosomes exchange stretches of DNA through chromosomal crossover. This process shuffles alleles between parents to create new combinations in offspring. Genes located close together on a chromosome demonstrate genetic linkage because they rarely separate. The probability of crossover increases with distance between points on the chromosome. Mary Frances Lyon discovered X-chromosome inactivation during reproduction to prevent passing twice as many genes to offspring. Her work led to understanding X-linked diseases. Bacteria utilize different methods like conjugation or transformation to acquire new genetic information without sexual reproduction.
Mutations alter an organism's genotype and occasionally produce different phenotypes. Studies in Drosophila melanogaster suggest about 70 percent of protein-changing mutations are harmful. Most others remain neutral or weakly beneficial. Population genetics tracks how allele frequencies change within populations over time. Natural selection favors alleles providing reproductive advantages while other factors like genetic drift influence distribution. Adaptation allows species to evolve forms better suited to their environment. Speciation often occurs when geographical separations prevent gene exchange between populations. Genetic comparisons calculate evolutionary distances between species more accurately than phenotypic traits alone. These comparisons form evolutionary trees representing common descent and divergence over millions of years. Horizontal gene transfer remains most common in bacteria, allowing unrelated species to share genetic material. Reactive oxygen species produced by cellular respiration create natural DNA damage leading to mutations. Error rates during replication stay low due to proofreading abilities of DNA polymerases.
Geneticists narrowed their focus from diverse organisms to specific model systems for convenience. The gut bacterium Escherichia coli serves as a primary research tool for studying gene regulation. Baker's yeast Saccharomyces cerevisiae offers short generation times ideal for experimentation. The nematode Caenorhabditis elegans provides insights into development and cancer mechanisms. Common fruit flies Drosophila melanogaster remain popular for tracking inheritance patterns. Zebrafish Danio rerio help researchers understand vertebrate genetics. House mice Mus musculus offer mammalian models closely resembling human biology. Arabidopsis thaliana plants allow study of plant-specific genetic processes. Researchers chose these organisms partly because significant existing data encouraged further investigation. Short generation times enabled rapid testing of hypotheses about gene function. Easy manipulation made them accessible tools for new scientists entering the field. These model organisms continue to drive discoveries in molecular genetics today.
Medical genetics examines how genetic variation relates to human health and disease. Genome wide association studies identify locations in the genome associated with multigenic traits. Pharmacogenetics explores how genotype affects individual drug responses. Cancer develops when cells accumulate mutations in three to seven genes allowing uncontrolled growth. Loss of p53 protein function represents one of the most frequent mutations found in tumors. Gain of function mutations in Ras proteins also promote tumor formation. Somatic mutations occur within body cells but do not transmit to progeny. On the 19th of March 2015, leading biologists urged a worldwide ban on clinical use of CRISPR editing methods. Chinese researchers reported editing DNA of non-viable human embryos using CRISPR technology in April 2015. The Human Genome Project completed sequencing the entire human genome in 2003. Costs have dropped dramatically since then with hopes reaching one thousand dollars per resequencing. Next-generation technologies produce millions of sequences concurrently while lowering expenses significantly.
Common questions
When did Gregor Mendel present his paper on plant hybridization?
Gregor Mendel presented his paper Experiments on Plant Hybridization to the Naturforschender Verein in Brno in 1865. He had spent years observing pea plants in the monastery garden of St Thomas Abbey before this presentation.
Who discovered that DNA is the molecule carrying genetic information?
The Avery, MacLeod, McCarty experiment in 1944 confirmed DNA as the responsible molecule for genetic information. Frederick Griffith discovered transformation in 1928 when dead bacteria transferred material to living ones prior to this confirmation.
What year did James Watson and Francis Crick determine the structure of DNA?
James Watson and Francis Crick determined the structure of DNA in 1953 using X-ray crystallography data created by Rosalind Franklin and Maurice Wilkins. Their model revealed two strands forming a double helix with nucleotides pointing inward.
How many base pairs does the largest human chromosome contain?
In humans, the largest chromosome spans approximately 247 million base pairs. Genes reside on chromosomes which are long chains of DNA wrapped around histone proteins.
When did Chinese researchers report editing DNA of non-viable human embryos using CRISPR technology?
Chinese researchers reported editing DNA of non-viable human embryos using CRISPR technology in April 2015. Leading biologists had urged a worldwide ban on clinical use of CRISPR editing methods on the 19th of March 2015.