Classical and Modern Genetics

 “Genetics”: study of how biological information is carried from one generation to the next –Classical Laws of inheritance developed from observations of Gregor Mendel (1800’s) –Modern Studies of how genes pass information from parent to offspring on the basis of molecular chemistry (1950 to present).

Classical and Modern Genetics  Classical (beginning in the 1800’s) –Mendel observed traits in pea plants being passed from parent to offspring via “unit of inheritance” –Observed traits could be dominant or recessive Dominant traits expressed even if only present in one parent Recessive traits expressed only if neither parent has the dominant trait –Tested crosses in pea plants Tested in both 1-trait and 2-trait crosses

Classical and Modern Genetics  When studying genetic crosses must distinguish between genotype and phenotype for a particular trait: –Example: tall (dominant) / short (recessive) genotypephenotype The actual pair of genes for a particular traitVisual expression of a genotype One gene from each parentReflects the dominant trait if it is present Either pure (both genes the same) or hybrid (one of each) Reflects the recessive trait only if both genes are recessive TT = pure for tall Tt = hybrid for tall tt = short Looks tall Looks short

Classical and Modern Genetics  Mendel’s test crosses - Cross-pollinated purebred and hybrid plants to observe offspring If parents are pure for a single trait: –Female: TT –Male: tt All four possible offspring in first generation look “tall” but are hybrid Tt If cross hybrids above: –Female: Tt –Male: Tt 3 of 4 possible offspring in second generation look tall and 1 of 4 looks short. One of tall is pure TT, Two of tall are hybrid Tt, The one short is pure tt. TT tTt t Tt TTTTt t tt

Classical and Modern Genetics  Modern (beginning in the 1950’s) –Mendel’s “units of inheritance” recognized as “genes” –Genes located on chromosomes –Chromosomes present in pairs in nucleus of eukaryotic cells –Chromosomes contain nucleic acids which code for all inheritance

Classical and Modern Genetics  Human Chromosomes –A total of 46 chromosomes (23 pairs) –All cells have 46 chromosomes except reproductive cells –Reproductive cells have just 23 – one member of each pair –22 pairs are “autosomes” and 1 pair is the “sex chromosome pair” –Males have the sex chromosome XY, Females have XX

Classical and Modern Genetics  Nucleic Acids –4 th category of organic molecules –Include DNA, RNA, (and ATP) –Large polymers made from chain of monomers –Monomer unit = nucleotide Sugar Phosphate group Base

Classical and Modern Genetics  DNA –Double strand in helix formation –Described by Watson and Crick (1953) –DNA nucleotides contain Sugar –deoxyribose Phosphate group Base –A, Adenine –T, Thymine –C, Cytosine –G, Guanine –Base pairs include A-T, T-A, C-G, G-C –Sequence of bases determines genetic code

Classical and Modern Genetics  RNA –Uses code from DNA to direct production of protein by the cell –Single strand nucleic acid –Nucleotide contains Sugar –Ribose Phosphate group Base –A, Adenine –U, Uracil (instead of thymine as found in DNA) –C, Cytosine –G, Guanine –Three different types: mRNA, tRNA, rRNA

Classical and Modern Genetics  Transcription of DNA message –Method by which coded DNA message is read by RNA mRNA (messenger RNA) copies a specific DNA sequence and carries it from the nucleus to the ribosomes

Classical and Modern Genetics  Translation of message into a protein –A “codon” (set of three bases) on the mRNA is read by a tRNA (transfer RNA) which picks up the proper amino acid for the code and brings it to the ribosome –Protein synthesis (joining the correct amino acids together) occurs at the ribosomes

Classical and Modern Genetics  Definitions –Genetic code Correspondence between base pair sequences and amino acids –Mutation Error in the coded sequence –Genome Complete description of an organisms genetic code –Mapping Position of every gene on every chromosome –Sequencing Exact order of base pairs on every gene Human genome has 3 billion base pair sequences!