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Chapter 14: Mendel and the Gene Idea Mendel used peas because Short generation time Can cross or self pollinate Clear cut/discrete traits (not continuous) Easy to control mating Can breed a large number of plants at one time The Process Start with a true breeding plant o Self pollinate until all offspring are of same variety o Eliminate all but one characteristic. Hybridization o cross two true breeding plants. o Hybrid: offspring of genetically dissimilar parents (not a clone) P Generation: Parental F1 Generation: First Filial (hybrid offspring) F2 Generation -2nd Filial (offspring of F1) Vocabulary to Know Gene: region of DNA that controls a given trait Each gene has variants called alleles Allele: variant of a gene. Locus: site on chromosome where specific gene located. Mendel’s terminology Character (heritable feature) Trait: variant of character (white/purple) Law of Segregation Allele pairs separate during gamete formation and re-pair during fertilization. 1. Alternate versions of genes (alleles) account for variations in inherited characteristics • Determined by nucleotide sequence on DNA • Found at specific locus (spot) on chromosome. 2. For each character, an organism inherits two alleles, one from each parent. • Each genetic locus represented twice in diploid cell, on homologous chromosomes. • We know now--not always true 3. If two alleles differ, then one, the dominant allele, is fully expressed in an organism’s appearance. The other, the recessive allele, has no noticeable effect on the organisms appearance. • Not always the case, applies only to situation with complete dominance 4. The two alleles for each character separate during gamete production Law of Independent Assortment Not only do alleles separate during gamete formation, but alleles for each trait separate independently of others located on different pairs of homologous chromosomes. It is a matter of chance as to which member of a homologous pair of chromosomes goes to which daughter cell during Meiosis I. Punnett Squares Use to predict results of a cross between individuals of known genotype Capital = dominant, lower case = recessive “monohybrid cross”: looking at one trait. Mate two heterozygotes in a monohybrid cross 3:1 phenotypic ratio. Homozygous: identical alleles Homozygous dominant: two dominant alleles Homozygous recessive: two recessive alleles Heterozygous: two different alleles Genotype: genetic makeup Phenotype: how the genes are expressed Appearance, physiology Dihybrid cross: looking at two traits. Law of Independent Assortment o Each allele segregates independently during gamete formation. o Exception is linked genes, will look at this in chapter 15. A dihybrid cross between two organisms that are heterozygous for two traits, each of which has two forms -dominant or recessive - always produces a phenotypic ratio of o 9 dominant for traits A and B: 3 dominant for trait A but recessive for trait B: 3 recessive for trait A but dominant for trait B: 3 recessive for traits A and B o 9:3:3:1 Practice Problems Genetic Crosses • What would be the genotypic and phenotypic ratios of the offspring of two individuals, one being homozygous dominant and the other being heterozygous for a trait? Genetic Crosses • John and his wife are both heterozygous for the traits of tongue rolling and freckles. – If John and his wife have 16 children, what combinations of phenotypes would you observe, and in what ratios would they occur. – A little more realistically, if John and his wife have two children, what is the probability that one will be a tongue roller (dominant) but not have freckles (recessive)? – What is the probability that both of their children will be unable to roll their tongues (recessive) and not have freckles (recessive)? Probability to Solve Genetics Problems • YyRr x Yyrr • What is the chance that the offspring will have each of the following Genotypes YYRr YYrr YyRr Yyrr yyRr yyrr Probability Practice • P: PpyyRr x PPYyrr • What is the probability that F1 will be PPYyrr? • What is the probability that F1 will be either PPYyrr or Ppyyrr? • What is the probability of having an offspring with a dominant phenotype for each trait? • What is the probability of having an offspring with a recessive phenotype for each trait? Other Forms of Inheritance Incomplete Dominance Codominance Multiple Alleles Sex Linked Inheritance Polygenic Inheritance