Mendelian Genetics (1) PDF

LECTURE 2: GENETICS Introduction to Genetics and heredity Gregor Johann Mendel – a brief bio Genetic terminology Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Gregor Johann...

LECTURE 2: GENETICS Introduction to Genetics and heredity Gregor Johann Mendel – a brief bio Genetic terminology Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Gregor Johann Mendel Austrian Monk, born in what is now Czech Republic in 1822 Son of peasant farmer, studied Theology and was ordained priest Order St. Augustine. Went to the university of Vienna, where he studied botany and learned the Scientific Method Worked with pure lines of peas for eight years Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.  Mendel s pea plants Mendel looked at seven traits or characteristics of pea plants:  In 1866 he published Experiments in Plant Hybridization, (Versuche über Pflanzen- Hybriden) in which he established his three Principles of Inheritance He tried to repeat his work in another plant, but didn t work because the plant reproduced asexually! If… Work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered Mendel s work.  Mendel was the first biologist to use Mathematics – to explain his results quantitatively. Mendel predicted The concept of genes That genes occur in pairs That one gene of each pair is present in the gametes  Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes)  Chromosomes (and genes) occur in pairs in diploid organisms --- Homologous Chromosomes  Genetics terms you need to know: Gene – a unit of heredity; a section of DNA sequence encoding a single protein Genome – the entire set of genes in an organism Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait; or, alternative forms of the same gene. Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.  Homozygous – having identical genes (one from each parent) for a particular characteristic. Heterozygous – having two different genes for a particular characteristic. Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.  Genotype – the genetic makeup of an organisms Phenotype – the physical appearance of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. P = Parental generation F1 = First filial generation; offspring from a genetic cross. F2 = Second filial generation of a genetic cross  Monohybrid cross for stem length: P = parentals TT × tt true breeding, (tall) (dwarf) homozygous plants: F1 generation Tt is heterozygous: (all tall plants)  Punnett square T T TT × tt Genotypes: t Tt Tt 100% T t Phenotypes: t Tt Tt 100% Tall plants  Monohybrid cross: F2 generation If you let the F1 generation self-fertilize, the next monohybrid cross would be: Tt × Tt (tall) (tall) Genotypes: 1 TT= Tall T t 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1:2:1 T TT Tt Phenotype: 3 Tall t Tt tt 1 dwarf Phenotypic ratio= 3:1  Key to the Punnett Square: Determine the gametes of each parent… How? By splitting the genotypes of each parent: T T × t t If this is your cross The gametes are: T T t t  Shortcut for Punnett Square… If either parent is HOMOZYGOUS T T × t t t Genotypes: 100% T t T Tt Phenotypes: 100% Tall plants  If you have another cross… A heterozygous with a homozygous T t × t t You can still use the shortcut! t Genotypes: 50% T t T Tt 50 % t t t t Phenotypes: t 50% Tall plants 50% Dwarf plants Cross the F1 generation: Pp × Pp Genotypes: P p 1 PP 2 Pp 1 pp P PP Pp Phenotypes: p Pp pp 3 Purple 1 White  Mendel s Laws 1. Law of Dominance: In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. 2. Law of Segregation: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.  Dihybrid crosses Matings that involve parents that differ in two genes (two independent traits) For example, flower color: P = purple (dominant) p = white (recessive) and stem length: T = tall t = short  Dihybrid cross: flower color and stem length TT PP × tt pp (tall, purple) (short, white) Possible Gametes for parents tp tp tp tp T P and t p TP TtPp TtPp TtPp TtPp TP TtPp TtPp TtPp TtPp TP TtPp TtPp TtPp TtPp TP TtPp TtPp TtPp TtPp F1 Generation: All tall, purple flowers (Tt Pp)  Dihybrid cross: flower color and stem length (shortcut) TT PP × tt pp (tall, purple) (short, white) Possible Gametes for parents T P TP tp t p Tt Pp F1 Generation: All tall, purple flowers (Tt Pp)  Dihybrid cross: 9 genotypes Genotype ratios (9): Four Phenotypes: 1 TTPP 2 TTPp Tall, purple (9) 2 TtPP 4 TtPp 1 TTpp Tall, white (3) 2 Ttpp 1 ttPP Short, purple (3) 2 ttPp 1 ttpp Short, white (1)  Dihybrid cross F2 If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Tt Pp × Tt Pp (tall, purple) (tall, purple) TP Tp tP tp Possible gametes: TP Tp tP tp TP TTPP TTPp TtPP TtPp Tp TTPp TTpp TtPp Ttpp tP TtPP TtPp ttPP ttPp tp TtPp Ttpp ttPp ttpp Four phenotypes observed Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1)  Dihybrid cross 9 Tall purple TP Tp tP tp TP TTPP TTPp TtPP TtPp 3 Tall white Tp TTPp TTpp TtPp Ttpp tP TtPP TtPp ttPP ttPp tp TtPp Ttpp ttPp ttpp 3 Short purple Phenotype Ratio = 9:3:3:1 1 Short white Dihybrid Cross  Principle of Independent Assortment Based on these results, Mendel postulated the 3. Principle of Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation Genes get shuffled – these many combinations are one of the advantages of sexual reproduction  Genetic Variation arising from chromosomal shuffling and independent assortment during meiosis The diagram shows how different gametes arising from the same individual goes to form different