BIOL 1020 – CHAPTER 14 LECTURE NOTESChapter 14: Mendel and the gene ideaI. the basic rules of inheritance were first demonstrated by MendelA. at the time of Mendel’s work, most thought that parental traits were fluids that “blend” in offspringB. Mendel recognized that this model did not explain what he observedC. Mendel chose a model system and carefully established testing conditions1. he used pea plants that he could outcross or allow to self-fertilize2. he chose traits that had two clear possible outcomes (yellow or green seeds, etc.)3. he established true-breeding or “pure” lines to use for genetic crossesD. terminology for genetic crosses1. P generation (or P1) = parental generation2. F1 generation = first generation offspring (from filial)3. F2 generation = second generation offspring4. phenotype – appearance or characteristic of an organism5. genotype – genetic makeup of an organism, determines phenotype6. gene – unit of heredity; controls a trait that determines a phenotype7. locus – the location of a particular gene on a chromosome8. alleles – alternative versions of a gene9. dominant – allele that dominates over others in determining phenotype10. recessive – allele whose phenotypic expression is “hidden” when a dominant allele is present11. hybrid – offspring from a cross between two “pure” lines of different, competing phenotypesII. rules and terminology for examination of genetic inheritanceA. Mendel’s law of segregation1. when Mendel crossed pure lines of different, competing phenotypes, he found that the F1 generation was uniform and matched one of the parents’ phenotypes - example: P1 yellow seed X green seed à all F1 yellow seed2. when F1 plants were crossed or selfed, the F2 plants had both P1 phenotypes in a ratio of roughly 3:1- using offspring from above F1 X F1 à F2 3 yellow seed: 1 green seed3. thus, contrary to the popular belief of the time, recessive traits are not lost in a mixing of parental phenotypes – they aremerely hidden in some “carrier” individuals4. Mendel explained these ratios with what we now call his law of segregation; stated in modern terms: individuals normally carry two alleles for each gene, these alleles must segregate in production of sex cells5. later investigations of cell division revealed the mechanism for segregation: the pairing and subsequent separation of homologous chromosomes during meiosisB. genotype vs. phenotype1. phenotype is the actual appearance or characteristic, and is determined by genotype; knowing the phenotype will not always directly reveal the genotype (recessive traits can be masked)2. genotype is the listing of the actual alleles present; if you know the genotype, you should be able to predict the phenotype- genotypes are either homozygous or heterozygous homozygous – the homologous chromosomes have the same allele at the locus in question heterozygous – the homologous chromosomes have different alleles at the locus; if there is a dominant allele the trait of the dominant allele will be expressed- the same letter is used to indicate all alleles (superscripts or subscripts are sometimes needed, if there are more than 2 alleles known)- DOMINANT ALLELES ARE CAPITALIZED; recessive alleles are lowercaseC. rules of probability govern genetic inheritance1. the likelihood of a sex cell carrying a particular allele is determined by probability, its expected frequency of occurrence(expressed in fractions, decimal fractions, percentages, or ratios)2. the combination of sex cells to form a zygote is generally ruled by probability as well3. thus, the rules of probability govern genetics4. product rule – when independent but not mutually exclusive events are combined, you multiply their individual probabilities to get the overall probability of the result (genetic crosses, X, are multiplications of probabilities)5. sum rule– if there is more than one way to obtain a result (mutually exclusive events), you add their individual probabilities to get the overall probability of the result- the sum of all possibilities is one (no more, no less)D. Punnett square – way of diagramming genetic crosses that uses the laws of probabilityE. more terminology1 of 5BIOL 1020 – CHAPTER 14 LECTURE NOTES1. test cross – mating an individual that has the dominant phenotype for a trait with an individual with the recessive phenotype; this often will reveal the genotype of the dominant parent, or at least give some idea of the probably genotype2. monohybrid cross – cross between individuals that are both heterozygous for the gene that you are following; note that these give a 3:1 phenotype ratio and a 1:2:1 genotype ratioF. practice applying the law of segregation: following one gene in a cross1. A pea plant with yellow seeds is crossed with a pea plant with green seeds (P1 generation). All 131 offspring (F1 generation) have yellow seeds. What are the likely genotypes of the P1 plants?2. Two of the F1 plants from above are crossed. What are the expected ratios of phenotypes and genotypes in the F2 generation?3. be sure to work some examples on your own; the textbook and website have plenty of genetics problems – note how they are typically presented as word problems and expect that format on your testIII. expanding the rules and terminology to follow two (or more) genes in a crossA. law of independent assortment1. dihybrid cross – cross between individuals that are both heterozygous for two different genes that you are following2. when Mendel performed dihybrid crosses he found phenotype ratios of 9:3:3:1, which is explained by the product rule3. this led to Mendel’s law of independent assortment: segregation of any one pair of alleles is independent of the segregation of other pairs of alleles- we now know that this is also a consequence of events in meiosis- this doesn’t hold perfectly true for all genes (see genetic linkage below)B. using the law of independent assortment in genetic problems1. with independent assortment a dihybrid cross is simply two separate monohybrid crosses multiplied2. avoid making tedious and difficult Punnett squares like in Fig. 14.8; pay attention in class for an easier method- make sure to try some on your ownIV. Beyond simple genetics: Mendel picked easy fightsA. We have already seen that modifications must be made to Mendel’s laws for linked genes; there are other situations that do not fit the “simple” cases that Mendel