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Chapter 14 Mendel and the Gene Idea PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Mendel’s Experimental, Quantitative Approach • Characters - varieties with distinct heritable features (such as flower color) • Traits - character variants (purple or white flowers) • Cross-pollination -fertilization between different plants • True-breeding -plants that produce offspring of the same variety when they self-pollinate • Mated two contrasting, true-breeding varieties – hybridization • P generation - true-breeding parents • F1 generation - hybrid offspring of the P generation • F2 generation - F1 individuals self-pollinate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings •The first concept is that alternative versions of genes account for variations in inherited characters •These alternative versions of a gene are now called alleles •Each gene resides at a specific locus on a specific chromosome Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers • The second concept is that for each character an organism inherits two alleles, one from each parent • The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-5-3 •Punnett square diagram for predicting the results of a genetic cross between individuals of known genetic makeup •Capital letter dominant allele P Generation Purple flowers White flowers Appearance: Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1/ 2 1/ 2 P Sperm F2 Generation P p PP Pp Pp pp P •Lowercase letter recessive allele Eggs p 3 1 p Useful Genetic Vocabulary • Homozygous - two identical alleles for a character • Heterozygous - two different alleles for a gene • Phenotype- physical appearance • Genotype- genetic makeup Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-6 3 Phenotype Genotype Purple PP (homozygous) Purple Pp (heterozygous) 1 2 1 Purple Pp (heterozygous) White pp (homozygous) Ratio 3:1 Ratio 1:2:1 1 The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • The individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • Testcross - breeding the mystery individual with a homozygous recessive individual Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-7 TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If PP Sperm p p P Pp Eggs If Pp Sperm p p or P Pp Eggs P Pp Pp pp pp p Pp Pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white • Law of independent assortment - each pair of alleles segregates independently of each other pair of alleles during gamete formation • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Multiplication and Addition Rules Applied to Monohybrid Crosses • Multiplication rule - probability that two or more independent events will occur together is the product of their individual probabilities • Rule of addition - probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities Fig. 14-9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm 1/ R 2 R 1/ 2 r R R Eggs 4 r 2 r 2 R 1/ 1/ 1/ 1/ 4 r r R r 1/ 4 1/ 4 Fig. 14-UN1 Degrees of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • Many heritable characters are not determined by only one gene with two alleles • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – Incomplete dominance - phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties – When a gene has more than two alleles (multiple alleles) – Codominance - two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-10-3 P Generation Red CRCR White CWCW CR Gametes CW Pink CRCW F1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-11 Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes Pleiotropy and Epistasis • Pleiotropy - most genes have multiple phenotypic effects – Ex. responsible for the multiple symptoms of cystic fibrosis and sickle-cell disease • Epistasis - a gene at one locus alters the phenotypic expression of a gene at a second locus • Ex. Some mammals, coat color depends on two genes [Pigment color (B for black and b for brown); whether the pigment will be deposited (C for color and c for no color)] Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings BbCc BbCc Sperm 1/ 4 BC 1/ 4 bC 1/ 4 Bc 1/ 4 bc Eggs 1/ 1/ 1/ 1/ 4 BC BBCC BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCc BbCc BBcc Bbcc BbCc bbCc Bbcc bbcc 4 bC 4 Bc 4 bc 9 : 3 : 4 Polygenic Inheritance • Quantitative characters - vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Ex. Skin color in humans Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-13 AaBbCc AaBbCc Sperm 1/ Eggs 1/ 8 1/ 8 1/ 8 1/ 8 1/ 1/ 8 1/ 1/ 8 8 1/ 8 1/ 64 15/ 8 1/ 1/ 8 8 8 1/ 8 1/ 8 1/ 8 8 1/ Phenotypes: 64 Number of dark-skin alleles: 0 6/ 64 1 15/ 64 2 20/ 3 64 4 6/ 64 5 1/ 64 6 Nature and Nurture: The Environmental Impact on Phenotype • Norm of reaction - phenotypic range of a genotype influenced by the environment – Ex. hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity • Norms of reaction are generally broadest for polygenic characters • Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-14 Pedigree Analysis • Pedigree -family tree that describes the interrelationships across generations • Pedigrees can also be used to make predictions about future offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) Ww ww ww Ww ww ww Ww Ww Ww ww 3rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Recessively Inherited Disorders • Carriers - heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented (albinism) • If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low • Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-16 Parents Normal Aa Normal Aa Sperm A a A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino Eggs Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-17 Parents Dwarf Dd Normal dd Sperm D d d Dd Dwarf dd d Dd Dwarf Eggs Normal dd Normal Multifactorial Disorders • Many diseases, such as heart disease and cancer, have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Testing and Counseling • Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders • Amniocentesis - liquid that bathes the fetus is removed and tested • Chorionic villus sampling (CVS) - sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero • Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-18 Amniotic fluid withdrawn Centrifugation Fetus Fetus Placenta Uterus Placenta Cervix Fluid Fetal cells BioSeveral chemical hours tests Several weeks Several weeks (a) Amniocentesis Karyotyping Chorionic villi Several hours Suction tube inserted through cervix Fetal cells Several hours (b) Chorionic villus sampling (CVS)