Genetics BT 1011 Lectures PDF

Genetics BT 1011 15 Lectures Genetics Welcome to the study of Genetics! Genetics is one of the most interesting branches of Biology! Genetic information determines an organism’s – cellular function – external appearance – behaviour. It serves as the link between generations.  Sci...

Genetics BT 1011 15 Lectures Genetics Welcome to the study of Genetics! Genetics is one of the most interesting branches of Biology! Genetic information determines an organism’s – cellular function – external appearance – behaviour. It serves as the link between generations.  Science of Genetics Genetics as a science began with Gregor Mendel = Father of Genetics He carried out experiments on garden Gregor Mendel pea plant 1822 - 1884 Published his classic paper in 1866 (His contemporary - Charles Darwin published his paper on Theory of Evolution in 1859) Charles Darwin 1809 - 1882  Genetics is a late discipline! But Mendel’s findings were not appreciated and his research was ignored Three other scientists “re-discovered” Mendel’s work in 1900 – Carl Correns (German Botanist) – Hugo de Vries (Dutch Biologist) – Eric Von Tschermak (Austrian Botanist) So Genetics is barely 150 years old!!!  Era of Classical Genetics (1900 – 1940s) Important findings – Chromosomal basis of inheritance – Nuclear division (division & segregation of chromosomes) Based on improved equipment and techniques – Microscope (Hand lens binocular microscope compound microscope) – Staining techniques to observe chromosomes  Era of Modern Genetics (1940s – 1970s) Rapid progress made in the middle of the 20th century Important findings – DNA as the hereditary molecule (Avery, 1944) – Structure of DNA (Watson and Crick, 1953) – Cracking the genetic code (Nirenberg, 1966)  Era of Molecular Genetics (1970s – 1990) Important findings – Restriction enzymes (Arber, Smith, Nathans & co- workers, 1968 – 1973) » Cutting and cloning DNA – Sequencing DNA (Sanger, 1977) » Read nucleotide sequence – Polymerase Chain Reaction (PCR) (Mullis,1986) » Amplify DNA Recombinant DNA Technology Genetic Engineering (Genetically Modified Organisms - GMOs), DNA fingerprinting  Era of Genomics and Bioinformatics (1990s to Present) Genomics & Bioinformatics – Making sense with the vast quantities of sequence data using computational methods Genome projects (complete sequencing of genomes) Eg. Human genome project 2002, Rice genome project Functional genomics Drug discovery Personalized therapy? Applications of Genetics in Society Medicine Genetic diseases Diagnosis/ Counseling Gene therapy / Genome editing Agriculture Plant & animal breeding Genetically Modified Organisms (GMO) Law DNA fingerprinting Criminal investigations & paternity testing  Different branches of Genetics Transmission Genetics / Classical Genetics / Mendelian Genetics Cytogenetics Population & Evolutionary Genetics Behavioral Genetics Quantitative Genetics Molecular Genetics Genomics & Bioinformatics etc. Transmission Genetics / Classical Genetics / Mendelian Genetics Foundation or basis on which all other branches of Genetics are built on.  Transmission Genetics Studies the inheritance of characters from generation to generation The offspring of two parents usually resemble the parents = Heredity Many traits in the offspring are not exactly like those of either parent = Variation Genetics is the scientific study of heredity and hereditary variation Gregor Mendel – documented patterns of inheritance through his experiments with garden peas – discovered basic principles of heredity from carefully planned experiments with the pea plant – used the scientific approach to identify two laws of inheritance Mendel’s experimental approach Designed experiments – Select parents – Carry out crosses (hybridization) – Examine progeny – Analyze ratios/ proportions Mendel chose to work with pea plants because – they were available in many varieties – he could strictly control mating Self fertilization (natural) Cross fertilization (artificial) – generations could be produced rapidly (annual plants)  Some genetic terms – Character (trait): a heritable feature, such as flower colour – True breeding (Pure bred) : an organism that always passes down the trait to its offspring – Monohybrid cross: A genetic cross between two individuals involving one character  Mendel’s monohybrid crosses Experimental Design - Stage 1 Obtain true-breeding pea plants for a given character. Eg: Character: flower colour Forms of the character: purple, white When plants breed true; – Purple flower plants produce only purple flower plants – White flower plants produce only white flower plants Experimental Design - Stage 2 Perform monohybrid crosses between true- breeding plants exhibiting alternative forms of the character – purple x white Designated as the parental (P) generation The offspring of this cross is the first filial (F1) generation (hybrids) Experimental Design - Stage 3 Allow the F1 hybrid offspring to self-fertilize – F1 x F1 Designate progeny as the second filial (F2) generation  Observations When Mendel crossed contrasting, true- breeding white and purple flowered pea plants – all of the offspring were purple flowered Mendel reasoned that – in the F1 plants, only the purple factor was affecting flower color in these hybrids  Observations When Mendel selfed the F1 plants – many of the F2 plants had purple flowers (705), but some had white flowers (224) – demonstrated that hereditary factors (white) had not been lost in F1 but were hidden – Mendel discovered a ratio of about 3: 1, purple : white flowers in F2 P Generation (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation  With other monohybrid crosses involving 6 more pairs of contrasting characters similar observations were made. – F1 plants look like one parent – In F2 both forms of the trait appear in a ratio of 3:1 Mendel observed the same pattern in six other pea plant characters Character Dominant trait x F2 Ratio Recessive trait generation Flower 651 : 207 3.14 : 1 position Seed colour 6022 : 2001 3.01 : 1 Seed shape 5474 : 1850 2.96 : 1 Pod shape 882 : 299 2.95 : 1 Pod colour 428 : 152 2.82 : 1 Stem length 787 : 277 2.84 : 1  Mendel’s Conclusions (Based on monohybrid crosses) Law of segregation (Mendel’s First Law) Characters are determined by units / factors In individuals the factors appear in pairs Paired factors segregate (separate) when germ cells (gametes) are formed Only one member of a pair is transmitted through any one gamete.  Terminology We now use the term genes rather than factors, and different forms of a gene are called alleles. We use a single letter in the alphabet to designate each allele; in this case P for purple and p for white. Also note that the same letter (P or p) , and not P vs w is used for the different alleles of one gene. Thus P and p are alleles of the ‘flower colour gene’ Mendel’s law of segregation & the Punnett square P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: P p F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 :1  More genetic words An organism is homozygous for a particular gene if it has a pair of identical alleles for that gene – exhibits true-breeding An organism is heterozygous if the two alleles of a gene are different Phenotype is the appearance Genotype is the genetic composition Dominance When two plants expressing different forms of the character are crossed, only one form of the character is expressed in the F1. – The allele that is masked (hidden) is recessive – The allele that is expressed in dominant  Phenotype (appearance) versus genotype (genetic makeup) Phenotype Genotype Purple PP 1 (homozygous) Pp 3 Purple (heterozygous) 2 Pp (heterozygous) Purple pp 1 White 1 (homozygous) Ratio 3:1 Ratio 1:2:1