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Mendel’s Rules People have been attempting to explain inheritance patterns for years. Failed propositions: Pangenesis o Gametes contain particles from the somatic cells Acquired traits are passed to future generations Blending o An offspring’s traits are an average of its parents’ traits Can’t explain why traits disappear in one generation and reappear in later ones. Gregor Mendel – 1860’s Used pea plants to determine the fundamental principles of genetics. Why were pea plants good specimens for Mendel to use? 1. Short life cycles 2. Reproduce sexually 3. Produce many offspring in a short amount of time 4. Easy to manipulate (control crosses, easy to care for) Male reproductive structures 5. Many contrasting characteristics (tall vs. short, purple vs. white flowers, green seeds vs. yellow, etc) 6. True-Breeding strains – when crossed with self or like plants, offspring look like parents. http://www2.edc.org/weblabs/mendel/mendel.html Questions 1. What conclusion can we draw from the flower color in the F1 generation? Purple is dominant Questions: 1. If the F1 plants are crossed with each other or self, what color flowers do you expect to see in the offspring? How can we explain the observed results? 2. How many alleles does each plant in the P generation have for flower color? 2 3. Assign alleles to each parent in the P generation F = purple allele f = white allele Purple (FF) White (ff) 4. What allele combination do the F1 plants have? Ff 5. What types of alleles could be in the gametes produced by these plants? F or f 6. What possible allele combinations could be found in the F2 generation? FF Ff ff Mendel’s observations lead to some important conclusions: He determined that parents pass on discrete “factors” to their offspring that control traits. What do we call those factors? Genes 1. There are alternate forms of a gene that account for the variation in inherited characteristics. a. What do we call different forms of a gene? Alleles 2. For each trait, an organism inherits two alleles, one from each parent. (Genotype) a. Homozygous – if the two alleles are the same b. Heterozygous – if the two alleles are different Genotype – an organism’s allele combination. Ex. GG Gg gg 3. If an organism inherits two different alleles (heterozygous), one allele will determine how the trait is expressed. (Phenotype) a. Dominant allele – the expressed allele from a heterozygous genotype Only need one to be expressed Not “normal” or more common in nature b. Recessive allele – the allele not expressed in a heterozygous genotype Must have two to be expressed Genotype – an organism’s allele combination. Ex. GG - Homozygous Dominant Gg - Heterozygous gg - Homozygous Recessive Phenotype – an organism’s expressed trait Ex. Tall or short 4. Law of Segregation – Sex cells (gametes/sperm and eggs) carry only one allele for a specific trait because homologous chromosome separate during meiosis. Law of Independent Assortment: Each pair of alleles assorts independently of the other pairs during meiosis (gamete formation). The sides of a Punnett Square only needs to be as big as the number of different gametes produced by each parent. Rules of Probability Test Cross How do you determine the genotype for an organism that expresses the dominant phenotype? What would be the best cross to determine the genotype? Cross with a recessive individual What data would you need to support your claim? Pedigrees – Family trees Genetic traits can be tracked through families using pedigrees. Allows scientists to determine how traits are inherited without controlling human matings. Carriers – People in a pedigree that have one copy of a recessive trait, but don’t express the symptoms of the disorder. Heterozygous Consanguineous marriage M Examples: 1. The pedigree below is for a genetic disease or abnormality. We do not yet know if it is dominant or recessive. Determine if the trait is autosomal dominant or recessive. Try the following designations: A = the trait (a genetic disease or abnormality, dominant) a = normal (recessive) a) Assign a genotype to each individual. If more than one genotype is possible, write both. Is this a dominant or recessive autosomal trait? Explain your answer. Recessive – two recessive parents can’t produce dominant offspring. b) Write the genotypes next to the symbol for each person in the pedigree below assuming that it is for a dominant trait. If more than one is possible, list both. c) Is it possible that this pedigree is for an autosomal dominant trait? YES 2. We will determine if the pedigree below can be for a trait that is autosomal dominant. Use "A" and "a" as you did for the pedigrees above. a) Write the genotype of each individual next to the symbol. If more than one is possible, list both. b) Is it possible that this pedigree is for an autosomal dominant trait? YES 3. We will determine if the pedigree below can be for a trait that is autosomal recessive. Use the following designations: A = normal a = the trait (a genetic disease or abnormality) a) Assuming that the trait is recessive, write the genotype of each individual next to the symbol. If more than one is possible, list both. b) Is it possible that the pedigree above is for an autosomal recessive trait? NO 5. We will determine if the pedigree below can be for a trait that is autosomal recessive. a) Write the genotype of each individual next to the symbol. If more than one is possible, list both. b) Is it possible that this pedigree is for an autosomal recessive trait? YES c) In this pedigree, two generations have been skipped. What can you conclude about recessive traits skipping generations (is it possible or not)? (Circle the correct answer below.) --Recessive traits cannot skip generations. --Recessive traits can skip generations. #2 Pedigree A Determine if the trait is dominant, recessive or if the pedigree is not possible. Assign a genotype to each person. If more than one genotype is possible, list both. Recessive Trait Pedigree B Determine if the trait is dominant, recessive or if the pedigree is not possible. Assign a genotype to each person. If more than one genotype is possible, list both. Dominant Trait Dihybrid Cross A cross involving 2 characteristics/traits, each controlled by their own gene on different homologous pairs. How many total chromosomes? 4 How many genes (types of letters)? 2 How many total letters in a genotype? 4 Mendel crossed two true-breeding plants: Plant 1: Yellow and Round seed Plant 2: Green and Wrinkled seed Parental Genotype: Gametes: F1 Offspring: Phenotype: YYRR X yyrr YR yr YyRr Yellow and Round Works with other traits as well: Develop a key for the above Dihybrid Cross: R = Green r = Yellow Y = Constricted y = Inflated Law of Independent Assortment: Each pair of alleles assorts independently of the other pairs during meiosis (gamete formation). The sides of a Punnett Square only needs to be as big as the number of different gametes produced by each parent. Rules of Probability Rule of Multiplication When using two coins (one egg and one sperm) the outcome for each coin is an independent event. o The probability of both coins landing heads up is the product of the separate probabilities. ½ X ½ =¼ When crossing two heterozygous Rr X Rr individuals, the probability of a homozygous genotype RR is ¼. Same for rr. What about the heterozygous genotype? ¼ +¼ =½ Rule of Addition – if there are 2+ outcomes, the probability is the sum of the separate probabilities. Most human genetic disorders are recessive Most genetic disorders are not evenly distributed across ethnic groups. Prolonged geographic and/or “class” isolation leads to inbreeding. o Increase the chance that both parents will have a harmful recessive allele. Ex. Cystic Fibrosis 1/2,500 Caucasians Albinism Sickle-cell disease* 1/400 African-Americans Tay-Sachs disease 1/3,500 Jewish people from C. Eu. Disorders can also be caused by dominant alleles. Dominant alleles that cause lethal diseases are much less common than lethal recessives. Why? Heterozygotes are affected Lethal dominant that don’t kill until later in life can be passed to future generations. Ex. Huntington’s disease 1/25,000 Achondroplasia (Dwarfism) 1/25,000 Alzheimer’s Disease (some cases)