Plant Genetics PDF Lecture Notes

LECTURE 7 : GENETICS Introduction to Genetics and heredity Gregor Mendel – a brief bio Genetic terminology (glossary) Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Introduction to G...

LECTURE 7 : GENETICS Introduction to Genetics and heredity Gregor Mendel – a brief bio Genetic terminology (glossary) Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) Arrangement of nucleotides in DNA DNA → RNA → Proteins  Chromosomes (and genes) occur in pairs Homologous Chromosomes New combinations of genes occur in sexual reproduction – Fertilization from two parents  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 peas 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  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 (like ‘flavors’ of a trait). 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 Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant TT  tt  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 A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. Looks like a window pane… We use the Punnett square to predict the genotypes and phenotypes of the offspring.  Using a Punnett Square STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t Cross TT  tt  Punnett square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) T T TT  tt t Genotypes: 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  Secret of the Punnett Square Key to the Punnett Square: Determine the gametes of each parent… How? By “splitting” the genotypes of each parent: If this is your cross T T  t t The gametes are: T T t t Once you have the gametes… T T  t t t t T Tt Tt T Tt Tt  Shortcut for Punnett Square… If either parent is HOMOZYGOUS T T  t t t Genotypes: 100% T t T Tt Phenotypes: 100% Tall plants You only need one box!  Understanding the shortcut… t t t T Tt Tt = T Tt T Genotypes: Tt Tt 100% T t 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  Another example: Flower color For example, flower color: P = purple (dominant) p = white (recessive) If you cross a homozygous Purple (PP) with a homozygous white (pp): PP  pp Pp ALL PURPLE (Pp) 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 Principles 1. Principle of Dominance: One allele masked another, one allele was dominant over the other in the F1 generation. 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.  Human case: CF Mendel’s Principles of Heredity apply universally to all organisms. Cystic Fibrosis: a lethal genetic disease affecting Caucasians. Caused by mutant recessive gene carried by 1 in 20 people of European descent (12M) One in 400 Caucasian couples will be both carriers of CF – 1 in 4 children will have it. CF disease affects transport in tissues – mucus is accumulated in lungs, causing infections.  Inheritance pattern of CF IF two parents carry the recessive gene of Cystic Fibrosis (c), that is, they are heterozygous (C c), one in four of their children is expected to be homozygous for cf and have the disease: C c C C = normal C CC Cc C c = carrier, no symptoms c c = has cystic fibrosis c Cc cc  Probabilities… Of course, the 1 in 4 probability of getting the disease is just an expectation, and in reality, any two carriers may have normal children. However, the greatest probability is for 1 in 4 children to be affected. Important factor when prospective parents are concerned about their chances of having affected children. Now, 1 in 29 Americans is a symptom-less carrier (Cf cf) of the gene.  Gaucher Disease Gaucher Disease is a rare, genetic disease. It causes lipid-storage disorder (lipids accumulate in spleen, liver, bone marrow) It is the most common genetic disease affecting Jewish people of Eastern European ancestry (1 in 500 incidence; rest of pop. 1 in 100,000)  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 TP TtPp TtPp TtPp TtPp T P and t p 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 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: 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)  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 Relation of gene segregation to meiosis… There’s a correlation between the movement of chromosomes in meiosis and the segregation of alleles that occurs in meiosis  Test cross When you have an individual with an unknown genotype, you do a test cross. Test cross: Cross with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ?  pp  Test cross If you get all 100% purple flowers, then the unknown parent was PP… P P p Pp Pp p Pp Pp If you get 50% white, 50% purple flowers, P p then the unknown p Pp pp parent was Pp… p Pp pp  Dihybrid test cross?? If you had a tall, purple plant, how would you know what genotype it is? ?? ?? tt pp  1. TTPP 2. TTPp 3. TtPP 4. TtPp  Beyond Mendelian Genetics: Incomplete Dominance Mendel was lucky! Traits he chose in the pea plant showed up very clearly… One allele was dominant over another, so phenotypes were easy to recognize. But sometimes phenotypes are not very obvious…  Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) RR  rr You get PINK flowers (Rr)! Genes show incomplete dominance when the heterozygous phenotype Rr is intermediate.  Incomplete dominance When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white Incomplete Dominance R r R R R Rr r Rr rr  Incomplete dominance What happens if you cross a pink with a white?  A pink with a red?   Summary of Genetics Chromosomes carry hereditary info (genes) Chromosomes (and genes) occur in pairs New combinations of genes occur in sexual reproduction Monohybrid vs. Dihybrid crosses Mendel’s Principles: – Dominance: one allele masks another – Segregation: genes become separated in gamete formation – Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation  Thanks! Remember: Quiz due on Thursday, February 19th. Review Session: Friday, February 20 TBA. Exam on Tuesday, February 24th