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Genetics and Mendel’s Rules People have been attempting to explain inheritance patterns for years. Failed propositions: “Homunculus” – Little man in sperm Pangenesis o Gametes contain particles that migrated from the somatic cells Acquired traits are passed to future generations Blending o An offspring’s traits are an average of its parents’ traits o 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. 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 d) What can you conclude from these two examples about the parents of a person that has a dominant characteristic? (Circle the correct answer below.) --If a person has a dominant trait, the parents will not have the trait. --If a person has a dominant trait, the parents might have the trait or they might not have it. --If a person has a dominant trait, at least one of the parents will have the trait. --If a person has a dominant trait, both of the parents will have the trait. #3 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 c) In conclusion, can two individuals that have an autosomal dominant trait have unaffected children? (Circle the correct answer below.) --If two individuals have a dominant trait, none of their offspring will have the trait. --If two individuals have a dominant trait, their offspring might or might not have the trait. --If two individuals have a dominant trait, their offspring will have the trait. #2 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 c) Assuming that the pedigree below is for a recessive trait, write the genotype next to the symbol for each person. If more than one is possible, list both. d) Is it possible that this pedigree is for an autosomal recessive trait? YES e) If a trait is autosomal recessive, what can you conclude about the children if both parents are affected? (Circle the correct answer below.) --If both parents are affected, none of the children will be affected. --If both parents are affected, the children might or might not be affected. --If both parents are affected, all of the children will be affected. #3 4. We will determine if the pedigree below can be for a trait that is autosomal recessive. Use "A" and "a" as you did for the previous example. 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) If a trait is autosomal recessive, what can you conclude about the children of two parents that are not affected? (Circle the correct answer below.) --If two parents have a dominant trait, the children will not have the trait. --If two parents have a dominant trait, the children might or might not have the trait. --If two parents have a dominant trait, the children will have the trait. #2 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: 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 pair 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. Trihybrid Cross o Maximum number of different gametes? 23 = 8 o Maximum size for Punnett Square? 8 X 8 (64 squares) o Easier to do three separate monohybrid crosses and multiply probabilities Given AaBbCc X AaBbCc Chances of aabbcc? ¼ X ¼ X ¼ = 1/64 Given AaBbCc X AaBbCc Chances of AabbCC? 2/4 X ¼ X ¼ = 2/64 (or 1/32) 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) Genetic Legacy and Technology With increases in our understanding of genetics and technology, people are now able to learn more about their children’s genetic legacy before conception, during pregnancy and after birth. Before Conception: Parents are screened for recessive alleles to determine if they are carriers. During Pregnancy: Often requires the collection of fetal cells. Chorionic Villus Sampling (CVS) – collects sample of chorionic villus tissue from the placenta. (After 8-10 wks) Amniocentesis – collection of fetal cells from the amniotic fluid using a needle (After 14-16 wks) Blood tests on the mother (After 15-20 wks) o Check protein levels Down Syndrome Ultrasound Imaging – uses sound waves to produce images. Fetoscopy – use of viewing scope and fiber optics Post-Birth: samples taken in the hospital after birth to screen for common genetic disorders. PKU – can’t break down an amino acid - adjust diet Greater technology also leads to great ethical questions. For people with a family history but no symptoms of a genetic disorder, a predictive test can help determine a person’s risk for developing the disorder in the future. People can seek early medical screening or care. Colon cancer Breast cancer Information needs to be used/handled responsibly Counseling Insurance Job security