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Reebop Ratios Part 1: Introduction Part 2: Mendelian Genetics and Beyond Part 1: Introduction Remember your baby Reebops? • Let’s think about the tail trait. • What were the possible phenotypes? curly tail straight tail • What were the possible genotypes? TT = curly tail Tt = curly tail tt = straight tail Important Words • Homozygous: Having two alleles (forms of a gene) that are identical (TT or tt). • Heterozygous: Having two alleles (forms of the gene) that are different (Tt). • Dominant: The dominant allele is expressed when homozygous (TT) and when heterozygous (Tt). • Recessive: The recessive allele is only expressed when homozygous (tt). • Which of the genotypes below are heterozygous? Homozygous? Tt TT tt • Based on these genotypes and phenotypes, which allele is dominant, T or t? How do you know? TT = curly tail Tt = curly tail tt = straight tail Quick-Think Time • If your original Reebop parents had 100 offspring, how many of them would you expect to have a curly tail? • How many of them would you expect to have a straight tail? • Why do you think so? • Why would someone want to predict the numbers of offspring to have a particular trait? Punnett’s Square • We can find out the expected number of curly-tailed and straight-tailed babies with a tool called Punnett’s Square. • We will call it Punnett’s “Fertilization Probability” Square. Who was Punnett and what is his square? • REGINALD CRUNDALL PUNNETT (1875-1967) was among the first English geneticists. He created the “Punnett Square” to work out what the possible offspring of two parents will be. Curly-tailed Dad Tt possible gametes Tt Curly-tailed Mom T t T TT Tt t Tt tt Quick-Think Time TT T t T TT Tt t Tt tt Tt t t How many out of 100? Expected Genotype TT Tt 50% t t 25% 25% T t T TT Tt t Tt tt Expected Phenotype TT Tt 50% + 25% = 75% curly-tailed t t 25% straight-tailed T t T TT Tt t Tt tt From Percentages to Ratios 25% TT to 50% Tt to 25% tt = Genotypic ratio of 1 to 2 to 1, or 1:2:1 75% curly-tailed to 25% straight-tailed = Phenotypic ratio of 3 to 1 or 3:1 How can we test this prediction? Expected genotypic ratio: 1:2:1 Expected phenotypic ratio: 3:1 STOP to complete Reebop Ratios activity Part 2: Mendilian Genetics and Beyond You and Gregor Mendel… • In the first Reebop activity, you looked at the variety of offspring produced by two parents. • You looked at 7 traits, the gene for each residing on a different chromosome. • You learned about how meiosis and fertilization contribute to variation in organisms. …You and Gregor Mendel • In the second Reebop activity, you looked at the expected and experimental ratios of offspring with curly and straight tails. • You, and a nineteenth-century monk named Gregor Mendel have a lot in common. • Let’s find out why. Mendel & the Garden Pea • Gregor Mendel, Austrian Monk • In 1866, Mendel performed breeding experiments with garden pea plants and observed consistent, predictable patterns of inheritance. • From his observations, Mendel developed a number of principles, today referred to as Mendel's Laws of Inheritance. Before Mendel… • In the 19th century, when both Gregor Mendel and Charles Darwin were alive, “blending inheritance” was a popular idea. • According to blending inheritance, traits from parents “blended” in offspring producing intermediate traits. • Example: The offspring of one short parent and one tall parent would be of medium height. This offspring would pass the new “medium sized” trait to its own offspring. Quick-Think Time • Imagine that blending inheritance occurs in Reebops. • What type of tail would the offspring of curly-tailed and straight-tailed parents have? • What would happen to variation in this population over time? Darwin and Mendel… 1809–1882 1822–1884 Darwin and Mendel… • Darwin observed that variation, in the form of individual differences, exists in every species, and is not reduced from one generation to the next. • He proposed that natural selection acts on this variation. • Darwin knew that in order for his idea to be valid, a mechanism for preserving and inheriting variation had to exist. Darwin and Mendel… THE BIG IDEA: • Mendel’s work answered Darwin’s difficult question. – Mendel showed that traits from parents do not blend. The traits remain distinct and can be passed on to later generations. Genetic variation is preserved. – Mendel also proposed a mechanism for how this variation is inherited and preserved. Mendel summarized his findings in three theories… Mendel’s first law, or the principle of segregation: During the formation of eggs and sperm, “paired factors” (now known as alleles, which reside on chromosomes) segregate, or separate. Quick-Think Time Explain how Mendel’s Principle of Segregation was demonstrated in the Reebop activities. Mendel summarized his findings in three theories… Mendel’s second law, or the principle of independent assortment: Each pair of “factors” (now known as alleles) separate and recombine into gametes independently of each other. Quick-Think Time Explain how Mendel’s Principle of Independent Assortment was demonstrated in the Reebop activities. Mendel summarized his findings in three theories. Mendel’s third law, or the law of dominance: Each characteristic is determined by the interaction of two hereditary factors (now called alleles), one from each parent. Mendel found that one allele was always dominant over the other. Quick-Think Time Explain how Mendel’s Law of Dominance was demonstrated in the Reebop activities. Mendel & Probability • coin (heads/tails) - probability of heads or tails = ½ - probability of getting heads 3 times in a row - multiplication rule: (½ x ½ x ½ = 1/8) Extensions of Mendelian Genetics • Many researchers have encountered exceptions to Mendelian Principles – was he wrong? • Majority of heritable characters not as simple as peas – Phenotypes affected by: many genes, sequential, jointly, environment, etc. Extensions of Mendel… • Incomplete Dominance – appearance between dominant and recessive CR F2 generation 1 : 2 : 1 CRCR:CRCW:CWCW F1 Generation All CRCW Eggs CRCR CR CW CRCR CRCW CRCW CWCW Sperm CR Cw CWCW CW 1:2:1 phenotypic ratio 1:2:1 genotypic ratio Another example of Incomplete Dominance Chestnut, CC Palomino, CCcr Cremello, CcrCcr Extensions of Mendel… • Codominance – both alleles affect phenotype in separate ways – Only observed at the cellular or molecular level – e.g., M N bloodgroups (RBC surface marker) • MM • NN • MN (both equally expressed) Extensions of Mendel… • Multiple alleles of a single gene– ABO blood groups – A, B, AB, O – Combinations of 3 different alleles – IA, IB, I – A and B dominant to O, but not to each other – A and B are codominant Extensions of Mendel… •Multiple alleles of a single gene– ABO blood groups • I – enzyme that adds sugar molecules to lipids on surface of RBC recognized by our immune system • IB –adds galactose (IBIB or IBi) “B” • IA – adds galactosamine (IAIA or IAi) “A” • i– does not add a sugar (ii) “O” • IA IB adds both sugars “AB” Multiple Alleles—ABO Blood Groups Possible alleles from male Possible alleles from female IA IB I IA IAIA IAIA IAi IB IAIB IBIB IBi i IAi IBi ii Blood Types A AB B O Extensions of Mendel… • Epistasis – one gene influences a second gene (gene interaction) For example in dogs: – B gene: determines whether pigment (melanin) is brown or black. – E gene:enables or prevents melanin from being deposited in the coat. Epistatic Ineractions on Coat Color ee No dark pigment in fur E_ Dark pigment in fur Yellow Lab eebb Yellow fur, brown nose, lips, eye rims eeB_ Yellow fur, black nose, lips, eye rims E_bb Chocolate Lab E_B_ Black Lab Brown fur, nose, lips, eye rims Black fur, nose, lips, eye rims Pedigree Analysis • Display of family relationships to help keep track of relationships and traits – phenotypes – genotypes – e.g., X-linked clotting disorder XX (normal) XXh (carrier) XhXh (hemophilia) XY (normal) XhY (hemophilia) Hemophilia in European Royalty XXh XXh XhY Click and choose the button Genetic Testing • fetal testing • amniocentesis – needle inserted in uterus to remove ~10 mL of fluid from amniotic sac (14-16th week) – chromosomal analysis • chorionic villus sampling (CVS) – tube inserted through cervix into uterus removing sample of fetal tissue from placenta