Classical Genetics Lecture HU-BT III PDF

CLASSICAL GENETICS HU-BT III Kazim Ali Assistant Professor Biotechnology  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...

CLASSICAL GENETICS HU-BT III Kazim Ali Assistant Professor Biotechnology  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  Mendel’s peas Mendel looked at seven traits or characteristics of pea plants:  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  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. 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  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  Testcross Gametes from Gametes from homozygous homozygous recessive recessive parent parent yy yy y y y y 1/ 1/ 1/ 1/ 2 2 2 2 Gametes Gametes from Yy Y from YY Y individual 1/ Yy Yy individual 1/ Yy Yy 2 1/ 1/ 2 1/ 1/ Yy 4 4 YY 4 4 y Y 1/ yy yy 1/ Yy Yy 2 1/ 1/ 2 1/ 1/ 4 4 4 4 ALL Heterozygous & Heterozygous Homozygous genotypes recessive genotypes 1:1.