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Transcript
Ch. 14 Mendelian Genetics • Gregor Mendel (1822-1884) – Determined particulate nature of inheritance – parents transmit discrete inheritable factors (genes) that remain as separate factors from one generation to the next. • Mendel liked peas. Peas are good. • Peas have several characters with two distinct forms (traits) – flower color is purple or white – Seed shape is round or wrinkled • Mendel did separate experiments looking at seven individual pea characters. He crossed two plants, each true breeding for alternate traits of a character (example: purple flowers and white flowers). This was the parental generation (p1). • Each time he found the offspring all had the same trait. This was the first filial (f1) generation. • He then crossed the f1 generation with itself to get the f2 generation. • He found that the other parental trait came back! And…… the two traits were always in a 3:1 ratio. • This happened for all 7 characters he studied. • Soooooo…… getting back to flower color… • Soooooo…… getting back to flower color… – Since the white color came back, the plants in the f1 generation must have still had a discrete factor for white color, but it was hidden. He called that factor recessive and the other dominant. • Soooooo…… getting back to flower color… – Since the white color came back, the plants in the f1 generation must have still had a discrete factor for white color, but it was hidden. He called that factor recessive and the other dominant. – Each plant must have 2 factors for each character, one from male parent, one from female parent. • Soooooo…… getting back to flower color… – Since the white color came back, the plants in the f1 generation must have still had a discrete factor for white color, but it was hidden. He called that factor recessive and the other dominant. – Each plant must have 2 factors for each character, one from male parent, one from female parent. – These factors must segregate during gamete formation, leaving the possibility for factors to randomly combine during fertilization • Soooooo…… getting back to flower color… – Since the white color came back, the plants in the f1 generation must have still had a discrete factor for white color, but it was hidden. He called that factor recessive and the other dominant. – Each plant must have 2 factors for each character, one from male parent, one from female parent. – These factors must segregate during gamete formation, leaving the possibility for factors to randomly combine during fertilization – Today we know these factors to be genes and the alternate forms alleles. • Mendel’s Law of Segregation • Mendel’s Law of Segregation – Allele pairs segregate during gamete formation and the paired condition is restored by the random fusion of gametes at fertilization. • Outcomes of crosses, in genotypic and phenotypic ratios, may be predicted with Punnett squares. • Test crosses can determine unknown genotypes • Mendel’s second group of experiments were like the first except he looked at the inheritance of two traits together. • Mendel’s second group of experiments were like the first except he looked at the inheritance of two traits together. – Were characters for different traits inherited together or independently?? If so, the f2 generation would have the same 3:1 phenotype ratio. • Mendel’s Law of Independent Assortment resulted from his dihybrid crosses: • Mendel’s Law of Independent Assortment resulted from his dihybrid crosses: – Each allele pair segregates independently of other gene pairs during gamete formation. • Using the laws of probability to predict mating outcomes. • Using the laws of probability to predict mating outcomes. – Probability scale: • Using the laws of probability to predict mating outcomes. – Probability scale: – Probabilities of all possible outcomes = • Using the laws of probability to predict mating outcomes. – Probability scale: – Probabilities of all possible outcomes = – Rule of multiplication • Using the laws of probability to predict mating outcomes. – Probability scale: – Probabilities of all possible outcomes = – Rule of multiplication • Used to determine the chance that two or more independent events will occur together. • Using the laws of probability to predict mating outcomes. – Probability scale: – Probabilities of all possible outcomes = – Rule of multiplication • Used to determine the chance that two or more independent events will occur together. • Determined by multiplying the probability of each independent event – Rule of addition – Rule of addition • The probability of an event that can occur in two or more different ways is the sum of the separate probabilities • Other forms of Inheritance • Other forms of Inheritance – Incomplete dominance • Other forms of Inheritance – Incomplete dominance • Neither allele is fully dominant • Other forms of Inheritance – Incomplete dominance • Neither allele is fully dominant • Heterozygote phenotype is intermediate • Other forms of Inheritance – Incomplete dominance • Neither allele is fully dominant • Heterozygote phenotype is intermediate • Mendel’s particulate nature of inheritance still applies. – Codominance – Codominance • Both alleles are fully expressed in the phenotype – Codominance • Both alleles are fully expressed in the phenotype – Example: MN blood groups, roan coat color in horses – Multiple alleles – Multiple alleles • When there are more than two alternate forms of a gene. – Multiple alleles • When there are more than two alternate forms of a gene. • Each individual carries only two – Multiple alleles • When there are more than two alternate forms of a gene. • Each individual carries only two – Example ABO blood group – Epistasis – Epistasis • A gene at one locus alters the phenotypic expression of a gene at another locus – Epistasis • A gene at one locus alters the phenotypic expression of a gene at another locus – Example coat color and melanin deposition in mice – Epistasis • A gene at one locus alters the phenotypic expression of a gene at another locus – Example coat color and melanin deposition in mice – Example labrador retriever coat color – Polygenic Inheritance – Polygenic Inheritance • Quantitative characters with a continuum of expression – Polygenic Inheritance • Quantitative characters with a continuum of expression • Characters controlled by more than one gene – Polygenic Inheritance • Quantitative characters with a continuum of expression • Characters controlled by more than one gene • Often are “multifactorial” and have a broad “Norm of reaction” – Polygenic Inheritance • Quantitative characters with a continuum of expression • Characters controlled by more than one gene • Often are “multifactorial” and have a broad “Norm of reaction” – Example : skin color