Biology, 9th ed,Sylvia Mader Chapter 11 Mendelian Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents TT tt Ee Ee T t eggs E e Tt E eggs T t EE spem Punnett square Ee T TT Tt sperm e t Tt tt Ee ee Offspring Offspring

Outline Blending Inheritance Monohybrid Cross Modern Genetics Law of Segregation Modern Genetics Genotype vs. Phenotype Punnett Square Dihybrid Cross Law of Independent Assortment Human Genetic Disorders

Gregor Mendel Austrian monk Studied science and mathematics at University of Vienna Conducted breeding experiments with the garden pea Pisum sativum Carefully gathered and documented mathematical data from his experiments Formulated fundamental laws of heredity in early 1860s Had no knowledge of cells or chromosomes Did not have a microscope

© Ned M. Seidler/Nationa1 Geographic Image Collection Gregor Mendel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ned M. Seidler/Nationa1 Geographic Image Collection

Fruit and Flower of the Garden Pea Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Flower Structure anther stamen filament stigma style carpel ovules in ovary a.

Garden Pea Traits Studied by Mendel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cutting away anthers Brushing on pollen from another plant All peas are yellow when one parent produces yellow seeds and the other parent produces green seeds.

Blending Inheritance Theories of inheritance in Mendel’s time: Based on blending Parents of contrasting appearance produce offspring of intermediate appearance Mendel’s findings were in contrast with this He formulated the particulate theory of inheritance Inheritance involves reshuffling of genes from generation to generation

One-Trait Inheritance Mendel performed cross-breeding experiments Used “true-breeding” (homozygous) plants Chose varieties that differed in only one trait (monohybrid cross) Performed reciprocal crosses Parental generation = P First filial generation offspring = F1 Second filial generation offspring = F2 Formulated the Law of Segregation

Mendel’s Monohybrid Crosses: An Example Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation TT tt P gametes T t F1 generation Tt F1 gametes T t T TT Tt F2 generation sperm t Tt tt Offspring Allele Key Phenotypic Ratio T = tall plant t = short plant 3 tall 1 short

Law of Segregation Each individual has a pair of factors (alleles) for each trait The factors (alleles) segregate (separate) during gamete (sperm & egg) formation Each gamete contains only one factor (allele) from each pair Fertilization gives the offspring two factors for each trait

Modern Genetics View Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) Dominant allele (capital letter) masks the expression of the recessive allele (lower-case) Alleles occur on a homologous pair of chromosomes at a particular gene locus Homozygous = identical alleles Heterozygous = different alleles

Homologous Chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids alleles at a gene locus G g G G g g Replication R r R R r r S s S S s s a. Homologous chromosomes have alleles for same genes at specific loci. b. Sister chromatids of duplicated chromosomes have same alleles for each gene. t T t t T T

Genotype versus Phenotype Refers to the two alleles an individual has for a specific trait If identical, genotype is homozygous If different, genotype is heterozygous Phenotype Refers to the physical appearance of the individual

Genotype versus Phenotype

Punnett Square Table listing all possible genotypes resulting from a cross All possible sperm genotypes are lined up on one side All possible egg genotypes are lined up on the other side Every possible zygote genotypes are placed within the squares

Punnett Square Allows us to easily calculate probability, of genotypes and phenotypes among the offspring Punnett square in next slide shows a 50% (or ½) chance The chance of E = ½ The chance of e = ½ An offspring will inherit: The chance of EE =½!½=¼ The chance of Ee =½!½=¼ The chance of eE =½!½=¼ The chance of ee =½!½=¼

Punnett Square Showing Earlobe Inheritance Patterns Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents Ee Ee eggs E e E EE Ee spem Punnett square e Ee e e Offspring Allele key Phenotypic Ratio E = unattached earlobes e = attached earlobes 3 unattached earlobes 1 attached earlobes

Monohybrid Test cross Individuals with recessive phenotype always have the homozygous recessive genotype However, individuals with dominant phenotype have indeterminate genotype May be homozygous dominant, or Heterozygous Test cross determines genotype of individual having dominant phenotype

One-Trait Test Cross Insert figure 11.7a here Tt tt eggs t Allele Key Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tt tt Insert figure 11.7a here eggs t Allele Key T = tall plant t = short plant sperm T t Tt Phenotypic Ratio 1 tall 1 short tt a. Offspring

One-Trait Test Cross TT tt Allele Key T t T = tall plant Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TT tt Allele Key T t T = tall plant t = short plant eggs sperm Phenotypic Ratio All tall plants Tt b. Offspring

Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traits Observed phenotypes among F2 plants Formulated Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traits All possible combinations of factors can occur in the gametes Mendel tracked each trait through two generations. P generation is the parental generation in a breeding experiment. F1 generation is the first-generation offspring in a breeding experiment. F2 generation is the second-generation offspring in a breeding experiment

Two-Trait (Dihybrid) Cross Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation TTGG ttgg P gametes TG tg F 1 generation TtGg eggs F 1 gametes TG Tg tG tg TG TTGG TTGg TtGG TtGg F 2 generation Tg TTGg TTgg TtGg Ttgg sperm tG TtGG TtGg ttGG ttGg tg TtGg Ttgg ttGg ttgg Offspring Allele Key Phenotypic Ratio T = tall plant 9 tall plant, green pod t = short plant 3 tall plant, yellow pod G = green pod 3 short plant, green pod g = yellow pod 1 short plant, yellow pod

Independent Assortment and Segregation during Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A B A A B B AB A B A A a a B B b b a a a b b b ab either a b A a B b A A A b or b b Ab A Parent cell has two A A a a b pairs of homologous chromosomes. b b B B a a a B B B aB a B All orientations of ho- At metaphase II, each All possible combina- mologous chromosomes daughter cell has only tions of chromosomes are possible at meta- one member of each and alleles occur in phase I in keeping with homologous pair in the gametes as the law of independent keeping with the law of suggested by Mendel's assortment. segregation. two laws.

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Human Genetic Disorders Genetic disorders are medical conditions caused by alleles inherited from parents Autosome - Any chromosome other than a sex chromosome (X or Y) Genetic disorders caused by genes on autosomes are called autosomal disorders Some genetic disorders are autosomal dominant An individual with AA has the disorder An individual with Aa has the disorder An individual with aa does NOT have disorder Other genetic disorders are autosomal recessive An individual with AA does NOT have disorder An individual with Aa does NOT have disorder, but is a carrier An individual with aa DOES have the disorder

Autosomal Recessive Pedigree Chart Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I aa A? II A? Aa Aa A? * III Aa Aa A? A? IV Key aa aa A? aa = affected Aa = carrier (unaffected) AA = unaffected A? = unaffected (one allele unknown) Autosomal recessive disorders Most affected children have unaffected parents. Heterozygotes (Aa) have an unaffected phenotype. Two affected parents will always have affected children. Close relatives who reproduce are more likely to have affected children. Both males and females are affected with equal frequency.

Autosomal Dominant Pedigree Chart Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I Aa Aa * II aa Aa A? aa aa aa III Aa Aa aa aa aa aa Key AA = affected Aa = affected Autosomal dominant disorders affected children will usually have an affected parent. • Heterozygotes (Aa) are affected. Two affected parents can produce an unaffected child. Two unaffected parents will not have affected children. Both males and females are affected with equal frequency. A? = affected (one allele unknown) aa = unaffected •

Autosomal Recessive Disorders Tay-Sachs Disease Progressive deterioration of psychomotor functions Cystic Fibrosis Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous Phenylketonuria (PKU) Lack enzyme for normal metabolism of phenylalanine

Cystic Fibrosis Cl- Cl- H2O Cl- H2O Cl- Cl- H2O nebulizer defective Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cl- Cl- H2O Cl- H2O Cl- Cl- H2O nebulizer defective channel percussion vest thick mucus © Pat Pendarvis

Courtesy Division of Medical Toxicology, University of Virginia Methemoglobinemia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Courtesy Division of Medical Toxicology, University of Virginia

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Autosomal Dominant Disorders Neurofibromatosis Tan or dark spots develop on skin and darken Small, benign tumors may arise from fibrous nerve coverings Huntington Disease Neurological disorder Progressive degeneration of brain cells Severe muscle spasms Personality disorders

A Victim of Huntington Disease Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. JK JJ JJ JK JK JK K L JK JK JK JK K L JK JK K L KK K L J L JJ J L JK J L J L J L J L JJ K L JJ K L JJ K L K L J L K L KL a. b. a: © Steve Uzzell

Incomplete Dominance Heterozygote has phenotype intermediate between that of either homozygote Homozygous red has red phenotype Homozygous white has white phenotype Heterozygote has pink (intermediate) phenotype Phenotype reveals genotype without test cross

Incomplete Dominance R R R R eggs R R R R R R R sperm Key 1 R R red R Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. R R R R 1 2 1 2 eggs R R 1 2 R 1 R R R R 1 1 1 2 sperm Key 1 R R red 1 1 R 2 R R pink 2 1 2 1 R R white 2 2 R R R R 1 2 2 2 Offspring

Multiple Allelic Traits Some traits controlled by multiple alleles The gene exists in several allelic forms (but each individual only has two) ABO blood types The alleles: IA = A antigen on red cells, anti-B antibody in plasma IB = B antigen on red cells, anti-AB antibody in plasma I = Neither A nor B antigens, both antibodies

Multiple Allelic Traits

Pleioptropic Effects Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits. Marfan syndrome have disproportionately long arms, legs, hands, and feet; a weakened aorta; poor eyesight

(Left): © AP/Wide World Photos; (Right): © Ed Reschke Marfan Syndrome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Connective tissue defects Skeleton Heart and blood vessels Eyes Lungs Skin Chest wall deformities Long, thin fingers, arms, legs Scoliosis (curvature of the spine) Flat feet Long, narrow face Loose joints Mitral valve prolapse Enlargement of aorta Lens dislocation Severe nearsightedness Stretch marks in skin Recurrent hernias Dural ectasia: stretching of the membrane that holds spinal fluid Collapsed lungs Aneurysm Aortic wall tear (Left): © AP/Wide World Photos; (Right): © Ed Reschke

Polygenic Inheritance Occurs when a trait is governed by two or more genes having different alleles Each dominant allele has a quantitative effect on the phenotype These effects are additive Result in continuous variation of phenotypes

Frequency Distributions in Polygenic Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation F1 generation F2 generation 20 — 64 15 — 64 Proportion of Population 6 — 64 1 — 64 aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC Genotype Examples

X – Linked Inheritance In mammals The X and Y chromosomes determine gender Females are XX Males are XY The term X-linked is used for genes that have nothing to do with gender Carried on the X chromosome. The Y chromosome does not carry these genes Discovered in the early 1900s by a group at Columbia University, headed by Thomas Hunt Morgan. Performed experiments with fruit flies They can be easily and inexpensively raised in simple laboratory glassware Fruit flies have the same sex chromosome pattern as humans

X – Linked Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation XrY XRXR P gametes Xr Y XR F1 generation XRY XRXr eggs F1 gametes XR Xr XR XRXR XRXr F2 generation sperm Y XRY XrY Offspring Allele Key Phenotypic Ratio XR = red eyes Xr = white eyes females: all red-eyed males : 1 red-eyed 1 white-eyed

Human X-Linked Disorders Several X-linked recessive disorders occur in humans: Color blindness The allele for the blue-sensitive protein is autosomal The alleles for the red- and green-sensitive pigments are on the X chromosome. Menkes syndrome Caused by a defective allele on the X chromosome Disrupts movement of the metal copper in and out of cells. Muscular dystrophy Wasting away of the muscle Adrenoleukodystrophy X-linked recessive disorder Failure of a carrier protein to move either an enzyme or very long chain fatty acid into peroxisomes. Hemophilia Absence or minimal presence of a clotting factor VIII, or clotting factor IX Affected person’s blood either does not clot or clots very slowly.

X-Linked Recessive Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. XBXB XbY grandfather XBY XBXb daughter XBY XbXb XbY XBY XBXB XBXb XbY grandson Key XBXB = Unaffected female XBXb = Carrier female XbXb = Color-blind female XbY = Unaffected male XbY = Color-blind male X-Linked Recessive Disorders • More males than females are affected. • An affected son can have parents who have the normal phenotype. • For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. • The characteristic often skips a generation from the grandfather to the grandson. • If a woman has the characteristic, all of her sons will have it.

Muscle Dystrophy fibrous tissue abnormal muscle normal tissue Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. fibrous tissue abnormal muscle normal tissue (Abnormal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center; (Boy): Courtesy Muscular Dystrophy Association; (Normal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center.

Terminology Pleiotropy Codominance Epistasis A gene that affects more than one characteristic of an individual Sickle-cell (incomplete dominance) Codominance More than one allele is fully expressed ABO blood type (multiple allelic traits) Epistasis A gene at one locus interferes with the expression of a gene at a different locus Human skin color (polygenic inheritance)

Review Blending Inheritance Monohybrid Cross Modern Genetics Law of Segregation Modern Genetics Genotype vs. Phenotype Punnett Square Dihybrid Cross Law of Independent Assortment Human Genetic Disorders

Biology, 9th ed,Sylvia Mader Chapter 11 Mendelian Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents TT tt Ee Ee T t eggs E e Tt E eggs T t EE spem Punnett square Ee T TT Tt sperm e t Tt tt Ee ee Offspring Offspring