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CHAPTER 11 GENETICS 1 Gregor Mendel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ned M. Seidler/Nationa1 Geographic Image Collection 2 Gregor Mendel •The garden pea: • Organism used in Mendel’s experiments • A good choice for several reasons: • Easy to cultivate • Short generation • Normally self-pollinating, but can be cross-pollinated by hand • True-breeding varieties were available • Simple, objective traits 3 Garden Pea Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Flower Structure stamen anther filament stigma style ovules in ovary carpel a. 4 Garden Pea Anatomy 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. 5 Garden Pea Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Trait Stem length Characteristics *Dominant *Recessive Tall Short Pod shape Inflated Constricted Seed shape Round Wrinkled Seed color Yellow Green Flower position Axial Terminal Flower color Purple White Pod color Green Yellow b. Figure 14.5 Genotype versus phenotype Figure 14.6 A testcross Figure 14.2 Mendel tracked heritable characters for three generations 11.2 Mendel’s Laws • 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 10 Mendel’s Laws • 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 of factors • Fertilization gives the offspring two factors for each trait 11 Mendel’s Laws • Classical Genetics and Mendel’s Cross: • 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 12 Figure 14.4 Mendel’s law of segregation (Layer 2) Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross Mendel’s Laws • Genotype • 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 15 Mendel’s Laws • A dihybrid cross uses true-breeding plants differing in two traits • Mendel tracked each trait through two generations. • Started with true-breeding plants differing in two traits • The F1 plants showed both dominant characteristics • F1 plants self-pollinated • Observed phenotypes among F2 plants • Mendel formulated the 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 • 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 16 Classical View of Homologous Chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids alleles at a gene locus a. Homologous chromosomes have alleles for same genes at specific loci. G g R r S s t T G Replication b. Sister chromatids of duplicated chromosomes have same alleles for each gene. R G g g R r r S S s s t t T T 17 Figure 15.1 The chomosomal basis of Mendel’s laws Mendel’s Laws • 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 19 Rules of Probability • Probability scale ranges from 0 to 1 • The possibilities of all outcomes must add up to 1 • In every fertilization involving gametes, the ovum has a ½ chance of carrying a dominant allele and ½ chance of carrying a recessive allele • Two basic laws of probability can help are the rule of multiplication and the rule of addition Rule of Multiplication • How can we determine the chance that two or more independent events will occur together in some combination? • Compute the probability for each independent event, then multiply these individual probabilities to obtain the overall probability of these events occurring together Rule of Addition • The probability of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways Mendel’s Laws • 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 the disorder • Other genetic disorders are autosomal recessive • An individual with AA does NOT have the disorder • An individual with Aa does NOT have the disorder, but is a carrier • An individual with aa DOES have the disorder 23 Autosomal Recessive Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generations I aa II III IV A? Aa A? Aa A? * Aa Aa aa aa A? A? A? Key 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. 24 Autosomal Dominant Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generations Aa Aa I * II III Aa aa Aa Aa aa A? aa aa aa aa aa aa Key AA = affected Aa = affected Autosomal dominant disorders A? = affected (one allele unknown) • Affected children will usually have an aa = unaffected 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. 25 Non-single gene genetics • Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons • Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease • Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types Figure 14.9 Incomplete dominance in snapdragon color Blood Types • Some traits are controlled by multiple alleles (multiple allelic traits) • The gene exists in several allelic forms (but each individual only has two alleles) • ABO blood types • The alleles: • IA = A antigen on red blood cells, anti-B antibody in plasma • IB = B antigen on red blood cells, anti-A antibody in plasma • i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma • The ABO blood type is also an example of codominance • More than one allele is fully expressed • Both IA and IB are expressed in the presence of the other 28 ABO Blood Type Phenotype A B AB O Genotype IAIA, IAi B B B I I ,I i IAIB ii 29 Non-single gene genetics • Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia • Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color • Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote Figure 14.11 An example of epistasis Figure 14.12 A simplified model for polygenic inheritance of skin color Extending the Range of Mendelian Genetics •X-Linked Inheritance •In mammals • The X and Y chromosomes determine gender • Females are XX • Males are XY 34 Extending the Range of Mendelian Genetics • X-Linked Inheritance • The term X-linked is used for genes that have nothing to do with gender • X-linked genes are 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 35 X – Linked Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation XrY Xr P gametes XRXR XR Y F1 generation XRY XRXr eggs F1 gametes XR Xr F2 generation sperm XR XRXR XRXr XRY XrY Y Offspring Allele Key R X = red eyes Xr = white eyes Phenotypic Ratio females: all red-eyed males: 1 red-eyed white-eyed 1 36 X – Linked Inheritance Extending the Range of Mendelian Genetics • 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. • Phenotypes include kinky hair, poor muscle tone, seizures, and low body temperature • Muscular dystrophy • Wasting away of the muscle • Caused by the absence of the muscle protein dystrophin • 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 clotting factor VIII or clotting factor IX 38 • 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. XbY XBXB XBY XBXb daughter grandfather XBY XbXb XbY XBY XBXB XBXb XbY grandson XBXB XBXb XbXb XbY XbY Key = Unaffected female = Carrier female = Color-blind female = Unaffected male = 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. 39 Expressivity refers to variations in a phenotype among individuals carrying a particular genotype. Penetrance is the proportion of individuals carrying a particular variant of a gene (allele or genotype) that also express an associated trait (phenotype). 40 Chromosomal errors, I • Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II • Aneuploidy: chromosome number is abnormal • Monosomy ~ missing chromosome • Trisomy ~ extra chromosome (Down syndrome) • Polyploidy~ extra sets of chromosomes Figure 15.11 Meiotic nondisjunction Chromosomal errors, II • Alterations of chromosomal structure: • Deletion: removal of a chromosomal segment • Duplication: repeats a chromosomal segment • Inversion: segment reversal in a chromosome • Translocation: movement of a chromosomal segment to another Figure 15.13 Alterations of chromosome structure Figure 15.14 Down syndrome Figure 15.x2 Klinefelter syndrome Figure 15.x3 XYY karyotype Chromosomal Disorders • Down syndrome • XXY Klinefelter syndrome • XYY • XXX • XO Turner syndrome Extranuclear Genes • Not all genes are located on the nuclear chromosomes. These genes do not exhibit Mendelian genetics. • Mitochondrial DNA which is given from the mother only, can in some rare cases cause some disorders. If the Mitochondrial DNA is defected this would reduce the amount of ATP made.