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BIOL 1020 14

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Auburn University
BIOL 1020
Anne- Marie Singh

BIOL 1020 – CHAPTER 14 LECTURE NOTES Chapter 14: Mendel and the gene idea I. the basic rules of inheritance were first demonstrated by Mendel a. at the time of Mendel’s work, most thought that parental traits were fluids that “blend” in offspring b. Mendel recognized that this model did not explain what he observed c. Mendel chose a model system and carefully established testing conditions i. he used pea plants that he could outcross or allow to self- fertilize ii. he chose traits that had two clear possible outcomes (yellow or green seeds, etc.) iii.he established true-breeding or “pure” lines to use for genetic crosses d. terminology for genetic crosses i. Pgeneration (or P1) = parental generation ii. F1 generation = first generation offspring (from filial) iii.F2 generation = second generation offspring iv. phenotype – appearance or characteristic of an organism v. genotype – genetic makeup of an organism, determines phenotype vi. gene – unit of heredity; controls a trait that determines a phenotype vii. locus – the location of a particular gene on a chromosome viii. alleles – alternative versions of a gene ix. dominant – allele that dominates over others in determining phenotype x. recessive – allele whose phenotypic expression is “hidden” when a dominant allele is present xi. hybrid – offspring from a cross between two “pure” lines of different, competing phenotypes II. rules and terminology for examination of genetic inheritance a. Mendel’s law of segregation i. when Mendel crossed pure lines of different, competing phenotypes, he found that the F1 generation was uniform and matched one of the parents’phenotypes 1. example: P1 yellow seed X green seedàall F1 yellow seed ii. when F1 plants were crossed or selfed, the F2 plants had both P1 phenotypes in a ratio of roughly 3:1 1. using offspring from above F1 X F1 F2 3 yellow seed: 1 green seed iii. thus, contrary to the popular belief of the time, recessive traits are not lost in a mixing of parental phenotypes – they are merely hidden in some “carrier” individuals iv. 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 cells v. later investigations of cell division revealed the mechanism for segregation: the pairing and subsequent separation of homologous chromosomes during meiosis b. genotype vs. phenotype i. 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) ii. genotype is the listing of the actual alleles present; if you know the genotype, you should be able to predict the phenotype 1. genotypes are either homozygous or heterozygous a. homozygous – the homologous chromosomes have the same allele at the locus in question b. 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 2. the same letter is used to indicate all alleles (superscripts or subscripts are sometimes needed, if there are more than 2 alleles known) 3. DOMINANTALLELES ARE CAPITALIZED; recessive alleles are lowercase c. rules of probability govern genetic inheritance i. 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) ii. the combination of sex cells to form a zygote is generally ruled by probability as well iii.thus, the rules of probability govern genetics iv. 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) v. 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 results 1. the sum of all possibilities is one (no more, no less) d. Punnett square – way of diagramming genetic crosses that uses the laws of probability e. more terminology i. 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 genotype ii. 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 ratio f. practice applying the law of segregation: following one gene in a cross i. Apea 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? ii. Two of the F1 plants from above are crossed. What are the expected ratios of phenotypes and genotypes in the F2 generation? 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 test III. expanding the rules and terminology to follow two (or more) genes in a cross a. law of independent assortment i. dihybrid cross – cross between individuals that are both heterozygous for two different genes that you are following ii. when Mendel performed dihybrid crosses he found phenotype ratios of 9:3:3:1, which is explained by the product rule iii.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 1. we now know that this is also a consequence of events in meiosis 2. this doesn’t hold perfectly true for all genes (see genetic linkage below) b. using the law of independent assortment in genetic problems i. with independent assortment a dihybrid cross is simply two separate monohybrid crosses multiplied ii. avoid making tedious and difficult Punnett squares like in Fig. 14.8; pay attention in class for an easier method 1. make sure to try some on your own IV. Beyond simple genetics: Mendel picked easy fights a. 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 used b. incomplete dominance – the heterozygote has a phenotype that is intermediate between the two homozygous states 1. really, the term dominance has no true meaning here 2. example: red, pink, and white snapdragon flowers c. codominance – the heterozygote expresses characteristics of both alleles; very much like incomplete dominance i. not an intermediate form, instead you see each allele distinctly expressed ii. roan cattle, expressing both red and white hairs, are a good example (the difference between incomplete dominance and codominance is essentially a case of splitting hairs) iii. one of the best examples is theABO human blood type, which will be covered below iv. how to spot codominance or incomplete dominance: monohybrid crosses with a 1:2:1 phenotype ratio d. multiple alleles – it is very common for there to be more than two allele types for a give locus; any time there are three or more alleles types involved, we say that there are multiple alleles i. dominance relationships can vary between multiple alleles ii. example: rabbit coat color is influenced by a gene that has four known alleles iii. example: humanABO blood types 1. the main blood type is determined by a single locus with three known alleles (IA, IB, iO) 2. IAand IB alleles are codominant with respect to each other a. the IAallele leads to the expression of type Aantigen on the surface of red blood cells b. the IB allele leads to the expression of type B antigen on the surface of red blood cells 3. iO is a recessive allele; the iO allele does not lead to expression of a cell surface antigen 4. resulting blood types: a. IAIAor IAiO genotype produce only the A antigen; blood type A b. IBIB or IBiO genotype produce only the B antigen; blood type B c. IAIB genotype produces both theAantigen and B antigen; blood typeAB d. iOiO genotype produces noAor B antigens; blood type O 5. blood transfusions (or any transplants) must be of the appropriate type, because the blood of individuals contains antibodies against the antigens not contained on its red blood cells a. thus, type O can only accept type O blood or organs b. type AB can accept any type blood or organs (A, B, AB or O); etc. c. there are other blood type factors, such as Rh factor, that must be taken into account 6. blood type is used in paternity or maternity cases only as a means to rule out possible parents iv. other key component tested for human blood typing is the Rh factor 1. while there are actually several Rh factors, one (antigen D) is most commonly tested and referred to as the Rh factor; mostAmericans are Rh+ 2. expression of antigen D on red blood cell surfaces is controlled by a single gene; the dominant phenotype leads to expression of the antigen (recessive = no expression) 3. inheritance of the Rh factor is thus described by classical Mendelian inheritance; if you express the dominant phenotype, you are Rh+; if you are Rh-, then you are homozygous recessive for the gene controlling the factor 4. someone who is Rh- should not be given Rh+ blood or organs, because they will develop antibodies to antigen D and reject the blood or organs 5. the Rh factor can cause complications during pregnancy (something not seen with theABO bloodgroup) 6. there are other blood typings and tissue matchings that are done, but the ABO/Rh blood typing is the one most commonly used (for example, ABO/Rh is usually all that matters for blood donation or reception e. pleiotrophy: one gene, many phenotypes i. one gene affects more than one characteristic ii. usually only one gene product is directly involved, and its status affects many things iii. many disease genes are pleiotrophic (examples, cystic fibrosis, sickle cell anemia) f. one phen
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