Mendel and the Gene Idea

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1 Chapter 14 BIOL 221 Mendel and the Gene Idea Historical Views of Heredity What gene.c principles account for the passing of traits From parents to offspring? Acquired traits Traits gained by parents in their lives are passed to offspring blending hypothesis Idea that gene.c material from the two parents blends together Like blue and yellow paint blend to make green Mendels Theory The par.culate hypothesis Idea that parents pass on discrete heritable units (genes) Mendel Documented a par.culate mechanism through his experiments with garden peas

2 Fig. Fig Mendel Mendel s Experimental, QuanEtaEve Approach Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned experiments Advantages of pea plants for gene.c study: Many varie.es with dis.nct heritable characters or traits Ma.ng of plants can be controlled Each pea plant Has sperm producing stamens and egg producing carpels Cross pollina.on Can be achieved by dus.ng one plant with pollen from another Fig. Fig TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial generation offspring (F1 ) 5

3 Mendel s Experimental, QuanEtaEve Approach Mendel chose to track only those characters that varied in an either or manner He also used varie.es that were true breeding Produce offspring of same variety when self pollinated Mendel s Experimental, QuanEtaEve Approach Mendel mated two contras.ng, true breeding varie.es Called hybridizaeon P generaeon Hybrid offspring of the P genera.on Called the F 1 generaeon When F 1 individuals self pollinate, the F 2 generaeon is produced The Law of SegregaEon Mendel Crossed contras.ng, true breeding white and purple flowered pea plants All of the F 1 hybrids were purple Crossed the F 1 hybrids Many of the F 2 plants had purple flowers But some had white Discovered a ra.o of about three to one Purple to white flowers In F 2 genera.on

4 Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

5 Mendel s Experimental, QuanEtaEve Approach Mendel Reasoned that only the purple flower factor was affec.ng flower color In the F 1 hybrids Called the purple flower color a dominant trait And the white flower color a recessive trait Observed the same pauern of inheritance In six other pea plant characters Each represented by two traits What Mendel called a heritable factor is what we now call a gene Table 14-1 Mendel s Model Mendel developed a hypothesis to explain the 3:1 inheritance pauern he observed in F 2 offspring Four concepts can be related to what we now know about genes and chromosomes 1st: Alterna.ve versions of genes account for varia.ons in inherited characters ie purple or white Alterna.ve versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome

6 Fig Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers Mendel s Model 2nd: For each character an organism inherits two alleles One from each parent Did not know the role of chromosomes Two alleles at a locus on a chromosome may be iden.cal True breeding plants Alterna.vely, the two alleles at a locus may differ Hybrids Mendel s Model 3rd: If the two alleles at a locus differ Then one determines the organism s appearance Dominant allele And the other has no no.ceable effect on appearance Recessive allele In the flower color example, the F 1 plants had purple flowers because the allele for that trait is dominant

7 Mendel s Model 4th: Now known as the law of segregaeon Two alleles for a heritable character separate (segregate) During gamete forma.on and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles That are present in the soma.c cells of an organism Corresponds to the distribu.on of homologous chromosomes To different gametes in meiosis CalculaEng possible combinaeons Mendel s segrega.on model Accounts for the 3:1 ra.o he observed in the F 2 genera.on of his numerous crosses Possible combina.ons of sperm and egg can be shown Using a PunneQ square A diagram for predic.ng the results of a gene.c cross between individuals of known gene.c makeup A capital leuer represents a dominant allele P = purple allele Lowercase leuer represents a recessive allele p = white allele Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p

8 Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F 1 Generation Appearance: Genetic makeup: Purple flowers Gametes: 1 / 2 P 1 / 2 p Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F 1 Generation Appearance: Genetic makeup: Purple flowers Gametes: 1 / 2 P 1 / 2 p F 2 Generation P Sperm p Eggs P p PP pp 3 1 GeneEc Vocabulary Homozygous An organism with two iden.cal alleles for the gene for a character Homozygous dominant Homozygous recessive Heterozygous An organism that has two different alleles for a gene for a character Unlike homozygotes heterozygotes are not true breeding

9 Traits do not always reveal gene.c composi.on Different effects of dominant and recessive alleles Phenotype physical appearance Genotype gene.c makeup PP and plants have the same phenotype (purple) but different genotypes GeneEc Vocabulary Fig Phenotype Genotype Purple PP (homozygous) 1 3 Purple (heterozygous) 2 Purple (heterozygous) 1 White pp (homozygous) 1 Ratio 3:1 Ratio 1:2:1 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Must have one dominant allele, But could be either homozygous dominant or heterozygous Testcross: Breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous

10 Fig TECHNIQUE Predictions Dominant phenotype, unknown genotype: PP or? Recessive phenotype, known genotype: pp If PP If or Sperm Sperm p p p p P Eggs P P Eggs p pp pp RESULTS All offspring purple or 2 offspring purple and 2 offspring white The Law of Independent Assortment Mendel derived the law of segrega.on by following a single character The F 1 offspring produced in this cross were monohybrids individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Mendel iden.fied his second law of inheritance by following two characters at the same.me Crossing two true breeding parents differing in two characters Produces dihybrids in the F 1 genera.on Heterozygous for both characters A dihybrid cross The Law of Independent Assortment Can determine whether two characters are transmiued to offspring as a package or independently

11 Fig. 14- EXPERIMENT P Generation YYRR yyrr Gametes YR yr F 1 Generation Predictions Hypothesis of dependent assortment YyRr Hypothesis of independent assortment Predicted or Sperm offspring of 1 /4 YR 1 /4 Yr 1 /4 yr 1 /4 yr Sperm F 2 generation 1 /2 YR 1 /2 yr 1 /4 YR YYRR YYRr YyRR YyRr 1 /2 YR YYRR YyRr 1 /4 Yr Eggs YYRr YYrr YyRr Yyrr 1 yr Eggs /2 YyRr yyrr 1 /4 yr YyRR YyRr yyrr yyrr 3 /4 1 /4 Phenotypic ratio 3:1 1 /4 yr YyRr Yyrr yyrr yyrr 9 /16 3 /16 3 /16 1 /16 RESULTS Phenotypic ratio 9:3:3:1 Phenotypic ratio approximately 9:3:3:1 The Law of Independent Assortment Law of independent assortment Each pair of alleles segregates independently of any other pair of alleles during gamete forma.on Applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together The laws of probability govern Mendelian Mendelian inheritanceinheritance Mendel s laws of segrega.on and independent assortment Reflected in the rules of probability When tossing a coin Outcome of one toss has no impact on the outcome of the next toss In the same way Alleles of one gene segregate into gametes independently of another gene s alleles

12 The MulEplicaEon and AddiEon Rules Rule of mul.plica.on states The probability that two or more independent events will occur together Is the product of their individual probabili.es Pone(XY) = P(X) * P(Y) Probability in an F 1 monohybrid cross can be determined Using the mul.plica.on rule Segrega.on in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele Fig Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm 1 / 2 R 1 / 2 r 1 / 2 R R R R r Eggs 1 / 4 1 / 4 1 / 2 r r R r r 1 / 4 1 / 4 Rule of addi.on The MulEplicaEon and AddiEon Rules The probability that any one of two or more exclusive events will occur Is calculated by adding together their individual probabili.es Pan(XY) = P(XY 1 ) + P(XY 2 ) + P(XY 3 ) + P(XY n ) The rule of addi.on can be used to figure out the probability that An F 2 plant from a monohybrid cross will be heterozygous rather than homozygous

13 Solving GeneEcs Problems with the Rules of Probability Predic.ng the outcome of crosses involving mul.ple characters Apply the mul.plica.on and addi.on rules Dihybrid (or greater) cross Equivalent to two or more independent monohybrid crosses Occurring simultaneously Calcula.ng the chances for various genotypes Considered each character separately Mul.ply individual probabili.es together Fig. 14 UN1 Complex Inheritance PaQerns Other pauerns of inheritance Rela.onship between genotype and phenotype Rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segrega.on and independent assortment apply Even to more complex pauerns of inheritance

14 Extending Mendelian Mendelian GeneEcs for a Single Gene Inheritance of characters by a single gene May deviate from simple Mendelian pauerns in the following situa.ons: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces mul.ple phenotypes Complete dominance Degrees of Dominance Phenotypes of the heterozygote and dominant homozygote are iden.cal Incomplete dominance Intermediate phenotype Somewhere between the phenotypes of the two parental varie.es Codominance Two dominant alleles affect the phenotype In separate, dis.nguishable ways Fig P Generation Red C R C R White C W C W Gametes C R C W

15 Fig P Generation Red C R C R White C W C W Gametes C R C W F 1 Generation Pink C R C W Gametes 1 / C R 1 2 / 2 C W Fig P Generation Red C R C R White C W C W Gametes C R C W F 1 Generation Pink C R C W Gametes 1 / C R 1 2 / 2 C W Sperm F 2 Generation 1 / C R 2 Eggs 1 / C R 2 1 / C W 2 C R C R C R C W 1 / 2 C W C R C W C W C W The RelaEon Between Dominance and Phenotype A dominant allele does not subdue a recessive allele Alleles don t interact Alleles Are simply varia.ons in a gene s nucleo.de sequence For any character Dominance/recessiveness rela.onships of alleles Depend on the level at which we examine the phenotype

16 The RelaEon Between Dominance and Phenotype Tay Sachs disease Fatal A dysfunc.onal enzyme causes an accumula.on of lipids in the brain At the organismal level The allele is recessive At the biochemical level The phenotype (i.e., the enzyme ac.vity level) is incompletely dominant At the molecular level The alleles are codominant Dominant alleles Not necessarily more common in popula.ons than recessive alleles For example Frequency of Dominant Alleles One baby out of 400 in the United States is born with extra fingers or toes Dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent Than the popula.on s dominant allele MulEple Alleles Most genes Exist in popula.ons in more than two allelic forms For example, ABO blood group In humans are determined by three alleles for the enzyme (I) that auaches A or B carbohydrates to red blood cells I A, I B, and I Enzyme encoded by the I A allele adds the A carbohydrate, Enzyme encoded by the I B allele adds the B carbohydrate Enzyme encoded by the i allele adds neither

17 Fig Allele I A Carbohydrate I B B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates A Genotype I A I A or I A i Red blood cell appearance Phenotype (blood group) A I B I B or I B i B I A I B AB ii (b) Blood group genotypes and phenotypes O Pleiotropy Pleiotropy One gene with mul.ple phenotypic effects For example Pleiotropic alleles are responsible for the mul.ple symptoms of certain hereditary diseases Such as cys.c fibrosis and sickle cell disease Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure

18 Epistasis Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus For example In mice and many other mammals, coat color depends on two genes One gene determines the pigment color With alleles B for black and b for brown The other gene determines whether the pigment will be deposited in the hair With alleles C for color and c for no color Fig BbCc BbCc Eggs 1 / 4 BC 1 / 4 bc Sperm 1 / 4 BC 1 / 4 bc 1 / 4 Bc 1 / 4 bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc 1 / 4 Bc BBCc BbCc BBcc Bbcc 1 / 4 bc BbCc bbcc Bbcc bbcc 9 : 3 : 4 Polygenic Inheritance QuanEtaEve characters Those that vary in the popula.on along a con.nuum Quan.ta.ve varia.on Usually indicates polygenic inheritance An addi.ve effect of two or more genes on a single phenotype Skin/eye color, height in humans

19 Fig. Fig AaBbCc Sperm Eggs Phenotypes: 64 Number of dark-skin alleles: 0 6/ / 64 2 AaBbCc 20/ 64 15/ / The Environmental Impact on Phenotype Phenotype for a character may depend on environment as well as genotype Norm of reaceon Phenotypic range of a genotype influenced by the environment For example Hydrangea flowers of the same genotype Range from blue violet to pink Depending on soil acidity Fig. Fig

20 The Environmental Impact on Phenotype Norms of reac.on Generally broadest for polygenic characters Such characters are called mulefactorial Because gene.c and environmental factors collec.vely influence phenotype Pedigree Pedigree Analysis A family tree that describes the interrela.onships of parents and children across genera.ons Inheritance pauerns of par.cular traits Can be traced and described using pedigrees Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis You should now be able to: 1. Define the following terms: true breeding, hybridiza.on, monohybrid cross, P genera.on, F 1 genera.on, F 2 genera.on 2. Dis.nguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype 3. Use a PunneU square to predict the results of a cross and to state the phenotypic and genotypic ra.os of the F 2 genera.on

21 You should now be able to: 4. Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance 5. Define and give examples of pleiotropy and epistasis 6. Explain why lethal dominant genes are much rarer than lethal recessive genes 7. Explain how carrier recogni.on, fetal tes.ng, and newborn screening can be used in gene.c screening and counseling