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Unit 4: Genetics & Heredity Chapters 14 and 15 Biology, 9th Ed By Campbell & Reece Chapter 14 – Mendel & the Gene Idea Many suggested the “blending” hypothesis: genetic material from parents mixes Correct model is the “particulate” hypothesis: genes are passed to offspring in units called genes. Gregor Mendel Around 1857, Mendel began breeding garden peas to study inheritance. Used experimental method Used quantitative analysis – b/c he collected data & counted peas Excellent example of the scientific method Mendel’s Experiment Why peas? Mendel noticed many variations in peas. Control the mating of the pea plants & record the results! Traits were distinct! Started w/ true-breeding plants Most traits are controlled by a single gene & each gene has 2 alleles (one is completely dominant to the other) Mendel’s Work Bred pea plants Cross pollinated true-breeding parents (P) Raised seeds & then observed traits (F1) Allowed offspring (F1) to cross-pollinate & observed the next generation (F2) P = parents F – filial generation Mendel Collected Data for 7 Traits Overview of Mendelian Genetics Character/Gene: heritable feature; i.e., fur color Trait: variant of a character; i.e. brown fur or white fur Allele: form of a gene; represented by letters; i.e., B or b True-Bred: all offspring are the same variety Hybridization: crossing of 2 different traits P generation: parents F1 generation: first filial generation Closer Look at Mendel’s Work P purple flowers X white flowers F1 100% purple flowers 4 purple:0 white Self-pollinate F2 75% purple & 25% white 3 purple:1white Leading to the Law of Segregation Traits come in alternative versions Ex. Purple vs. white flower color Alleles different alleles vary in the sequence of nucleotides (nitrogen bases) at the specific locus of a gene Purple flower allele & white flower allele are 2 DNA variations at the flower color locus Law of Segregation Law of Segregation: The alleles for each character segregate (separate) during the formation of gametes (meiosis). When gametes are produced during meiosis, homologous chromosomes separate from each other. Each allele for a trait segregates (is packaged into a separate gamete) Law of Segregation Law of Segregation What meiotic event creates the law of segregation? Between anaphase I and telophase I when the homologous chromosomes separate & are packaged into different cells Remember, Mendel didn’t even know DNA or genes existed! Traits are inherited as discrete units For each gene/character, an organism inherits two alleles, 1 from each parent Diploid: organism inherits one set of chromosomes from each parent; 2 sets of chromosomes Law of Dominance If the two alleles differ, then the dominant allele is fully expressed in the organism’s appearance; the other, the recessive allele, has no effect on the organism’s appearance Purple X White = Light purple --- NO!!!! Purple masked white Genetic Vocabulary Punnett Square: predicts the results of a cross b/w individuals of a known genotype Homozygous: same alleles for a character – PP or pp Heterozygous: different alleles for a character – Pp or pP Phenotype: physical appearance (words) – purple or white flowers Genotype: genetic make-up (letters) Testcross: crossing a homozygous recessive to a dominant phenotype (unknown genotype) Genotype vs. Phenotype 2 organisms can have the same phenotype, but different genotypes PP = homozygous dominant; purple flowers Pp = heterozygous; also purple flowers Monohybrid Cross Practice Problems – Complete Dominance 1) A homozygous cream colored mouse (dd) is crossed with a heterozygous (Dd) dark mouse. a. What are the odds that this couple will have a cream colored baby? b. What are the odds of a dark mouse? 2) In sheep, white is due to a dominant gene (W), black is due to its recessive allele (w). A white ewe mated to a white ram produces a black lamb. How does this happen? What are the genotypic and phenotypic ratios of the offpspring? 3) In chickens, yellow legs (Y) are dominant over white legs (y). A yellow legged rooster was crossed with a white legged hen. Both kinds of offspring were produced. What are the genotypes of the parents and the offspring? Law of Independent Assortment Law of Segregation involves 1 character/gene (monohybrid) What about two different genes? (dihybrid) The two pairs of alleles segregate independently of each other (in Metaphase I of meiosis) Law of Independent Assortment Law of Independent Assortment Each pair of alleles – for each trait – segregates into gametes independently YyRr YR, Yr, yR, yr (four gametes formed) Law of Independent Assortment Law of Independent Assortment What meiotic event creates the law of independent assortment? When the homologous chromosomes line up independently of each other during metaphase I of meiosis Interesting Historical Facts While Mendel was acknowledged by his peers as an outstanding plant breeder, his revolutionary work was overlooked for 34 years. Mendel published “Experiments on Plant Hybrids” in 1865. In 1900, 16 years after his death, a number of scientists independently rediscovered his work. Interesting Historical Facts Charles Darwin proposed that evolution by natural selection was dependent on variation in the population 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! Probability and Genetics Mendel’s Laws: A) Segregation B) Independent Assortment Reflect same laws of probability that apply to tossing coins or rolling dice Probability & Genetics Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin Probability of tossing heads? Probability of making a P gamete……. P P Pp = 50% or PP =100% p P Probability & Genetics Outcome of one toss has no impact on the outcome of the next toss Probability of tossing heads each time? 50% Probability of making a P gamete each time… P Pp = 50% p Rule of Multiplication Chance that 2 or more independent events will occur together Probability that 2 coins tossed at the same time will land heads up ½X½=¼ Probability of Pp X Pp pp ½X½=¼ Rule of Addition Chance that an event can occur 2 or more different ways Sum of the separate probabilities Sperm Egg Offspring P ½ p ½ Pp ¼ p ½ P ½ pP ¼ 1/4 +1/4 -----1/2 Calculating Probability Pp X Pp = ????? Sperm Egg Offspring P ½ P ½ PP 1/4 P ½ p ½ p ½ P ½ Pp ¼ pP +¼ = ½ p ½ p ½ pp ¼ Calculating Dihybrid Probability Rule of Multiplication also applies to Dihybrid Crosses If you have heterozygous parents,YyRr, what is the probability of producing yyrr offpspring? A) Probability of producing y gamete = ½ B) Probability of producing r gamete = ½ C) Probability of producing yr gamete is … ½X½=¼ D) Probability of producing yyrr offspring is… ¼ X ¼ = 1/16 Dihybrid Cross Practice Problems 1) Cross a pea plant that is heterozygous for purple (P) flowers and homozygous dominant for yellow (Y) seeds with a plant that is heterozygous for purple flowers and homozygous recessive for green seeds. Test Cross Cross-breed the dominant unknown phenotype with a homozygous recessive to determine the identity of the unknown allele. If parent is PP offspring are all purple (Pp) If parent is Pp offspring are ½ purple (Pp) & ½ white (pp) Test Cross Test Cross Practice Problems 1) In Border Collies, black coat (B) is dominant to red coat (b). A breeder has a black male that has won numerous awards. The breeder would like to use the dog for breeding if he is purebred or BB. To learn this information, she testcrosses him with a red female (bb). Answer the following questions A, B, C, and D. A. If the black male is BB, what kind of gamete (sperm) can he produce? B. If the red female is bb, what kind of gamete (eggs) can she produce? C. If the black male is Bb, what kind(s) of gametes (sperm) can he produce? D. If any of the puppies are red, what is the father's genotype? Extending Mendelian Genetics Mendel worked with a simple system A) Peas are genetically simple B) Most traits are controlled by a single gene C) Each gene has only 2 alleles; 1 of which is completely dominant to the other The relationship b/w genotype and phenotype is rarely this simple!! Non-Single Gene Genetics – Incomplete Dominance Incomplete Dominance: appearance b/w phenotypes of the 2 parents; an intermediate/mixture; Ex. Snapdragons RR = red flowers RR’ = pink flowers R’R’ = white flowers Red flower X White flower = ? Non-Single Gene Genetics – Co-dominance Codominance: Two alleles are both dominant to each other, so they are both expressed in a heterozygote Ex. Black & White Checkered Chickens and M, N, and MN human blood groups B = Black chicken W = White chicken Black Chicken X White Chicken = ? BB X WW = All BW (Black & White Chickens) Incomplete Dominance & CoDominance Practice Problems 1) 2) 3) 4) The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants? Cross a red fruit with an orange fruit and give the phenotypic ratio. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur? Dominant Alleles NOTE: Dominant alleles are NOT always more common than recessive alleles!! Polydactyly – dominant alleles Only 1 in 400 people are polydactyl Most people are homozygous recessive for polydactyly Non-Single Gene Genetics – Multiple Allele Problems Multiple Alleles: more than 2 possible alleles for a gene; Ex. Human blood types (ABO) 3 alleles = IA, IB, and I IA & IB are dominant to the i allele IA & IB are co-dominant to each other Phenotype Genotype A IAIA or IAi B IBIB or IBi AB IAIB O ii Human Blood Types Genotype Phenotype Phenotype Status IAIA or IAi Type A Type A Oligosaccharides on the surface of RBC -------- IBIB or IBi Type B Type B oligosaccharides on surface of RBC -------- I AI B Type AB Both Type A & Type B oligosaccharides on surface of RBC Universal Recipient ii Type O No oligosaccharides on surface of RBC Universal Donor Blood Compatibility Matching compatible blood groups is critical for blood transfusions A person produces antibodies against foreign blood factors oligosaccharides If a 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 Binding causes the donated blood cells to clump together & can kill the recipient Multiple Alleles Practice Problems 1) A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid? 2) What are the chances of a woman with Type AB and a man with Type A having a child with Type O? 3) A test was done to determine the biological father of a child. The child's blood Type is A and the mother's is B. Man #1 has a blood type of O & man #2 has blood type AB. Which man is the biological father? Non-Single Gene Genetics – Polygenic Inheritance Polygenic Inheritance: an additive effect of 2 or more genes on a single phenotypic character Ex. human skin color and height Phenotypes on a continuum Skin Color – 3 genes A, B, & C – dark skin a, b, & c – light skin Alleles have a cumulative effect; therefore……. AaBbCc intermediate/medium skin color Nature vs. Nurture Phenotype is controlled by both environment and genes Color of hydrangea flowers is influenced by the acidity of the soil Chi-Square Test Test to see if your data supports your hypothesis Compare “observed” vs. “expected” data A) Is variance from “expected” due to random chance? B) Is there another factor influencing data? Chromosomal Theory of Inheritance Genes have specific locations on chromosomes and chromosomes undergo segregation and independent assortment Chromosomal Linkage Thomas Hunt Morgan Drosophilia melanogaster Sex Linkage – genes located on sex chromosomes (pair #23 in humans) Linked Genes – genes located on the same chromosome tend to be inherited together! Morgan’s Research – First Mutant 1st to associate a specific gene with a specific chromosome Fruit flies have 4 pairs of chromosomes Wild type (Normal Phenotype) Fly = red eyes Discovered mutant white-eyed male Morgan’s Experiment P = White eyed male X Red Eyed Female F1 = All Red Eyed Males & Females F2 = 3 red: 1 white; only males had white eyes Q: How was the possible? A: The trait was sex-linked!!! Sex-Linked Traits Humans & other mammals have 2 sex chromosomes X & Y 2 X chromosomes = female X & Y = male Human Female Karyotype Human Male Karyotype Genes on Sex Chromosomes Y Chromosome: SRY: sex-determining region Master regulator for maleness Turns on genes for production of male hormones X Chromosome: Other traits, rather than sex determination Hemophilia Colorblindness Duchenne Muscular Dystrophy Sex-Linked Traits Summary X-Linked: Follow the X chromosome Males get their X from their mother Trait is never passed from father to son Y-Linked: Very few traits Only 26 genes Trait is only passed from father to son Females cannot inherit the trait X-Inactivation Female mammals inherit two X chromosomes One X becomes inactivated during embryonic development Condenses into a compact object called a Barr Body X Inactivation & Barr Bodies: Tortoise Shell Cat Sex-Influenced Traits Male Pattern Baldness autosomal trait influenced by sex hormones age effect as well = onset after 30 years old dominant in males & recessive in females B_ = bald in males; bb = bald in females Linked Genes Genes on the same chromosomes tend to be inherited together Close Together more likely to be inherited together Far Apart More likely to inherited separately (behave as if they are on separate chromosomes); more likely to “cross over” in meiosis What is Recombination? Occurs when offspring have different combinations of traits than the parents Parental Types: Same genotype as parents Recombitants: different genotype from parents Chromosomal Basis of Recombination Unlinked Genes (genes on different chromosomes) have a 50% frequency of recombination Linked genes do NOT assort independently b/c they are on the same chromosome & tend to move together through meiosis & fertilization Why are there recombitants w/ linked genes if they do not assort independently? B/c crossing over exchanges genes b/w non-sister chromatids Crossing over b/w homologous chromosomes breaks linkages in parent chromosomes to form new, recombitant chromosomes. Chromosomal Basis of Recombination Notice that crossing over b/w non-sister chromatids makes recombitant chromosomes These recombitant chromosomes are packaged into gametes Production of Recombitant Offspring Genetic Maps Crossing Over – Genes that DO NOT assort independently of each other Genetic Maps – the further apart two genes are, the higher the probability that a crossover will occur b/w them; therefore, the higher the recombination frequency I Map Unit – 1% recombination frequency Genetic Maps Continued Linkage Maps – Genetic maps based on recombination frequencies Not a true picture of a chromosome and the relative distances b/w genes Shows the sequence of genes on a chromosome, not an exact location Genomic Imprinting Definition: parental effect on gene expression Identical alleles may have different effects on offspring; depending on whether they arrive in the zygote via the egg or sperm Genomic Imprinting Both disorders below are caused by a partial deletion of chromosome #15…………………………………… Prader-Willi Syndrome Mental retardation Obesity Short stature Inherits abnormal chromosome from father Angelman Syndrome Jerky movements Spontaneous laughter Motor and/or mental symptoms Inherits abnormal chromosome from mother Extranuclear Genes Small amounts of DNA are found in mitochondria & chloroplasts This extranuclear DNA is randomly assorted to gametes and does not follow simple Mendelian rules of inheritance. Maternal inheritance is the rule for mitochondrial DNA b/c it comes from the cytoplasm of the egg/ovum Genetic Diseases/Disorders Carried by Genes Autosomal – Chromosomes #1-#22 Sex-Linked – Chromosome #23 Dominantly Inherited Recessively Inherited Co-dominance Chromosomal Error Monosomy Trisomy Recessively Inherited Diseases – Cystic Fibrosis Primarily whites of European descent 1 in 2500 births 1 in 25 whites is a carrier Defective/absent Cl- channels cause high levels of Cl- in the body Thick & sticky mucus coats cells Build-up of mucus causes infections & affects pancreas, lungs & digestive tract Live until 20’s with treatment ( 5 yrs. w/o treatment) Cystic Fibrosis Recessively Inherited Diseases – Tay Sachs Primarily Jews of eastern European (Ashkenazi) descent & Cajuns 1 in 3600 births Non-functional enzyme fails to breakdown lipids in brain cells Symptoms begin a few months after birth Seizures, blindness, degeneration of motor and mental skills Death before 5 years of age Sickle-Cell Anemia – Co-dominance Inheritance Primarily Africans or of African descent 1 of 400 African Americans Caused by substitution of a single amino acid in hemoglobin When oxygen levels are too low, sickle-cell hemoglobin crystallizes into long rods 2 alleles are co-dominant Both normal & abnormal hemoglobins are made in the heterozygote (Ss) Carriers are usually healthy, although some suffer some symptoms of sickle-cell disease under oxygen stress Sickle-Cell Anemia Inheritance Heterozygote Advantage & Sickle-Cell Anemia High frequency of heterozygotes is unusual for an allele with severe detrimental effects May be a selective advantage for being heterozygote In Africa, where malaria is common…. Homozygous normal: die of malaria Homozygous sickle-cell: die of sickle cell Heterozygote carriers: relatively free from both malaria & sickle cell Dominantly Inherited Diseases Only need one copy of the dominant allele to have a dominantly inherited disease Huntington’s Disease: Degenerative disease of the nervous system Occurs later in life (35-45 years of age) Fatal Children of a person with Huntington’s Disease—what is their chance of getting it? Carried on chromosome #4 Huntington’s Disease Genetic Counseling & Testing Amniocentesis – uses needle Chorionic Villi Sampling – suction w/ a tube Ultrasound Fetoscopy Newborn Screening – blood tests Phenylketonuria - PKU Pedigrees – traces family genes Amniocentesis and Chorion Villi Sampling Pedigree Analysis Reveals patterns of inheritance Square = male Circle = female Filled in square/circle = person with trait Royal Hemophilia Pedigree