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Genetics Mendel’s work • Bred pea plants • Cross-pollinated true breeding parents (P) then raised the seed & observed traits (F1) • Allowed offspring to cross-pollinate & observed next generation (F2) P = parents F = filial • • • In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation. What did Mendel’s findings mean? • Traits come in alternative versions o Purple vs. white flower color (Alleles) Different alleles vary in the sequence of nucleotides at the specific locus of a gene Traits are inherited as discrete units • For each character, an organism inherits 2 alleles, 1 from each parent • Diploid organism inherits 1 set of chromosomes from each parent o Homologous chromosomes o Diploid = 2 sets of chromosomes o Like having 2 different editions of an encyclopedia What did Mendel’s findings mean? • Some traits mask others o Purple & white-flower colors are separate traits that do not blend Purple x white ≠ light purple Purple masked white o Dominant allele Fully expressed o Recessive allele No noticeable effect Non-functional protein 1 Genotype vs. phenotype • Difference between how an organism “looks” & its genetics o Description of an organism’s trait = phenotype o Description of an organism’s genetic makeup = genotype Explain Mendel’s results using: Dominant & Recessive and Phenotype & Genotype. Phenotype vs. Genotype • 2 organisms can have the same phenotype but have different genotypes. 2 Mendel chose peas wisely • Pea plants are good for genetic research o Available in many varieties with distinct heritable features with different variations Flower color, seed color, seed shape, etc. o Mendel had strict control over which plants mated with which Each pea plant has male & female structures Pea plants can self-fertilize Mendel could also cross-pollinate plants: moving pollen from one plant to another Mendel chose peas luckily • Pea plants are good for genetic research o Relatively simple genetically Most characters are controlled by a single gene Each gene has only 2 alleles, one of which is completely dominant to the other Mendel’s Law of Heredity (#1) • Law of Segregation o When gametes (eggs & sperm) are produced during meiosis, homologous chromosomes separate from each other o Each allele for a trait segregates (is packaged) into a separate gamete o What meiotic event creates the law of segregation? _________________________ o And Mendel didn’t even know DNA or genes existed! Test Cross • It is possible to predict the genotype of an organism with a dominant phenotype? • Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele. 3 Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters o Flower color o Seed color Dihybrid cross • Some of Mendel’s experiments followed the inheritance of 2 different characters o Seed color & seed shape • Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry. • Both homozygous dominants and heterozygotes produce enough enzymes to convert all the monosaccharides into starch and form smooth seeds when they dry. Mendel’s Law of Heredity (#2) • Law of Independent Assortment o Each pair of alleles — for each trait — segregates into gametes independently = independent assortment. o 4 classes of gametes — YR, Yr, yR, yr — are produced in equal amounts. Review: Mendel’s Laws of Heredity • Law of Segregation o Monohybrid cross = single trait o Each allele for a trait segregates (is packaged) into separate gametes • Established by Meiosis 1 • Law of Independent Assortment o Dihybrid (or more) cross • 2 or more traits o Each pair of alleles for each trait segregates into gametes independently • Established by Meiosis 1 Some interesting historical facts • While Mendel was acknowledged by his contemporaries as an outstanding plant breeder, his revolutionary genetics work was overlooked for 34 years. • Mendel published “Experiments on Plant Hybrids” in 1865. In 1900, 16 years after Mendel’s death, a number of scientists independently rediscovered his work. • Charles Darwin, a contemporary of Mendel, proposed that evolution by natural selection was dependent on variation in the population, but Darwin was unable to propose a mechanism for how this variation was transmitted. • The key was Mendel’s work and nearly a century after Mendel published his findings historians found a copy of Mendel’s paper in Darwin’s study. He presumably never read it. 4 Chapter 14: Probability & Genetics Mendel’s laws: Reflect same laws of probability that apply to tossing coins or rolling dice. • Segregation • Independent assortment Probability & Genetics • Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin o Probability of tossing heads? 50% o Probability making a P gamete… • Outcome of 1 toss has no impact on the outcome of the next toss o Probability of tossing heads each time? 50% o Probability making a P gamete each time? 50% Calculating Probability Rule of multiplication • Chance that 2 or more independent events will occur together o probability that 2 coins tossed at the same time will land heads up ½x½=¼ o probability of Pp x Pp → pp ½x½=¼ Calculating dihybrid probability • Rule of multiplication also applies to dihybrid crosses o Heterozygous parent — YyRr o Probability of producing yyrr? probability of producing y gamete = 1/2 probability of producing r gamete = 1/2 probability of producing yr gamete = ½x½=¼ probability of producing a yyrr offspring =¼ x ¼ = 1/16 1 Rule of Addition • Chance that an event can occur 2 or more different ways o Sum of the separate probabilities o Probability of Pp x Pp → Pp Extending Mendelian genetics • Mendel worked with a simple system. o Peas are genetically simple. o Most traits are controlled by a single gene. o Each gene has only 2 alleles, 1 of which is completely dominant to the other. • The relationship between genotype & phenotype is rarely simple. Incomplete dominance • Heterozygotes show an intermediate phenotype o RR = red flowers o rr = white flowers o Rr = pink flowers make 50% less color 2 Co-dominance • 2 alleles affect the phenotype in separate, distinguishable ways. o ABO blood groups o 3 alleles = IA, IB, and i. Both the IA & IB alleles are dominant to the i allele IA & IB alleles are codominant to each other. o Determines presences of oligosaccharides on the surface of red blood cells. Blood compatibility • Matching compatible blood groups is critical for blood transfusions. • A person produces antibodies against foreign blood factors = oligosaccharides. o If donor’s blood has an A or B oligosaccharide that is foreign to the recipient, antibodies in the recipient’s blood will bind to the foreign molecules. o Cause the donated blood cells to clump together & can kill the recipient. Pleiotropy • Most genes are pleiotropic o One gene affects more than one phenotypic character Wide-ranging effects due to a single gene: Dwarfism (achondroplasia) Gigantism (acromegaly) Epistasis • One gene masks another o Coat color in mice = 2 genes Pigment (C) or no pigment (c) More pigment (black=B) or less (brown=b) cc = albino, no matter B allele 3 Polygenic inheritance • Some phenotypes determined by additive effects of 2 or more genes on a single character o Phenotypes on a continuum o Human traits skin color height weight eye color intelligence behaviors Nature vs. Nurture • Phenotype is controlled by both environment & genes • A single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun • For humans, nutrition influences height, exercise alters build, suntanning darkens the skin, and experience improves performance on intelligence tests • Even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences Thomas Hunt Morgan • Morgan was an embryologist at Columbia University 1st to associate a specific gene with a specific chromosome Drosophila breeding • prolific • 2 week generations • 4 pairs of chromosomes • XX=female, XY=male Morgan’s first mutant… • Wild type fly = red eyes • Morgan discovered a mutant white-eyed male o Trace a gene for eye color to a specific chromosome • Discovery of Sex Linkage 4 Genes on sex chromosomes • Y chromosome o SRY: sex-determining region Master regulator for maleness Turns on genes for production of male hormones • Pleiotropy! • X chromosome o Other traits beyond sex determination Hemophilia Duchenne muscular dystrophy Color-blind Sex-linked traits summary • X-linked o Follow the X chromosomes o Males get their X from their mother o Trait is never passed from father to son • Y-linked o Very few traits o Only 26 genes o Trait is only passed from father to son o Females cannot inherit trait X-inactivation • Female mammals inherit two X chromosomes o One X becomes inactivated during embryonic development • Condenses into compact object = Barr body • X-inactivation & Tortoise shell cat o 2 different cell lines in cat Male pattern baldness • Sex influenced trait o Autosomal trait influenced by sex hormones Age effect as well: onset after 30 years old o Dominant in males & recessive in females B– = bald in males; bb = bald in females 5 Chapter 14: Studying Inheritance in Humans Pedigree Analysis • Pedigree analysis reveals Mendelian patterns in human inheritance. • Data mapped on a family tree. Recessive diseases • The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at all. o Heterozygotes (Aa) Carriers Have a normal phenotype because one “normal” allele produces enough of the required protein. Heterozygote Crosses • Heterozygotes as carriers of recessive alleles. Cystic Fibrosis • Primarily whites of European descent • Strikes 1 in 2500 births o 1 in 25 whites is a carrier (Aa) • Normal allele codes for a membrane protein that transports Cl- across cell membrane o Defective or absent channels cause high extracellular levels of Clo Thicker & stickier mucus coats around cells o Mucus build-up in the pancreas, lungs, digestive tract & causes bacterial infections • Without treatment children die before 5; • With treatment can live past their late 20s 1 Tay-Sachs • Primarily Jews of eastern European (Ashkenazi) descent & Cajuns o Strikes 1 in 3600 births 100 times greater than incidence among non-Jews or Mediterranean (Sephardic) Jews o Non-functional enzyme fails to breakdown lipids in brain cells. Symptoms begin few months after birth. Seizures, blindness & degeneration of motor & mental performance. Child dies before 5yo. Sickle cell anemia • Primarily Africans o Strikes 1 out of 400 African Americans. o Caused by substitution of a single amino acid in hemoglobin. o When oxygen levels are low, sickle-cell hemoglobin crystallizes into long rods. Deforms red blood cells into sickle shape. Sickling creates pleiotropic effects = cascade of other symptoms. o Substitution of one amino acid in polypeptide chain. Sickle Cell Phenotype • 2 alleles are codominant o Both normal & abnormal hemoglobins are synthesized in heterozygote (Aa) o Carriers usually healthy, although some suffer some symptoms of sickle-cell disease under blood oxygen stress • Heterozygote advantage o Sickle cell frequency High frequency of heterozygotes is unusual for allele with severe detrimental effects in homozygotes. • 1 out of 400 African Americans Suggests some selective advantage of being heterozygous. o Malaria Single-celled eukaryote parasite spends part of its life cycle in red blood cells. o In tropical Africa, where malaria is common: Homozygous normal individuals die of malaria. Homozygous recessive individuals die of sickle cell anemia. Heterozygote carriers are relatively free of both. o High frequency of sickle cell allele in African Americans is vestige of African roots. 2 Genetics & culture • Why do cultures have a taboo against incest? o Laws or taboos forbidding marriages between close relatives are fairly universal. • Fairly unlikely that 2 carriers of same rare harmful recessive allele will meet & mate. o But mating between close relatives increase risk. Consanguineous o Individuals who share a recent common ancestor are more likely to carry same recessive alleles. Chapter 15: Chromosomal Abnormalities Chromosomal Abnormalities • Incorrect number of chromosomes o Nondisjunction Chromosomes don’t separate properly during meiosis. o Breakage of chromosomes Deletion Duplication Inversion Translocation Nondisjunction • Problems with the meiotic spindle cause errors in daughter cells. o Tetrad chromosomes do not separate properly during Meiosis 1. o Sister chromatids fail to separate during Meiosis 2. • Baby will have wrong chromosome number o Trisomy Cells have 3 copies of a chromosome. o Monosomy Cells have only 1 copy of a chromosome. • Alteration of Chromosome Number 3 Human Chromosome Disorders • High frequency in humans o Most embryos are spontaneously aborted o Alterations are too disastrous o Developmental problems result from imbalance • Certain conditions are tolerated o Upset the balance less = survive o Characteristic set of symptoms = syndrome • Down Syndrome o Trisomy 21 3 copies of chromosome 21 1 in 700 children born in U.S. o Chromosome 21 is the smallest human chromosome. But still severe effects. o Frequency of Down syndrome correlates with the age of the mother. Sex chromosomes • Human development more tolerant of wrong numbers in sex chromosome • But produces a variety of distinct conditions in humans o XXY = Klinefelter’s syndrome male o XXX = Trisomy X female o XYY = Jacob’s syndrome male o XO = Turner syndrome female • Klinefelter’s syndrome o XXY male o 1 in every 2000 live births o Have male sex organs, but are sterile. o Feminine characteristics, tall, normal intelligence. • Jacob’s syndrome male o XYY Males o 1 in 1000 live male births o Extra Y chromosome. o Somewhat taller than average, more active. o Slight learning disabilities, delayed emotional immaturity. o Normal intelligence, normal sexual development. • Trisomy X o XXX o 1 in every 2000 live births. o Produces healthy females. Why? __________________ • Turner syndrome o Monosomy X or X0 o 1 in every 5000 births o Varied degree of effects Webbed neck Short stature, immature sterile females. 4 Changes in Chromosome Structure 5