Penetrance and expressivity The terms penetrance and expressivity quantify the modification of the influence on phenotype of a particular genotype by varying environment and genetic background they measure respectively the percentage of cases in which a particular phenotype is observed when the specific allele of a gene of interest is present and the extent of that phenotype MCB140 09 17 07 1 Cancer Free at 33 but Weighing a Mastectomy Deborah Lindner 33 did intensive research as she considered having a preventive mastectomy after a DNA test The New York Times Sunday Sep 16 2007 MCB140 09 17 07 2 The New York Times Sunday Sep 16 2007 MCB140 09 17 07 3 www nytimes com MCB140 09 17 07 4 Little gene cards what if it says BRCA1 mutant The Lindners share a defective copy of a gene known as BRCA1 for breast cancer gene 1 that raises their risk of developing breast cancer sometime in their lives to between 60 and 90 percent Only 30 000 of more than 250 000 American women estimated to carry a mutation in BRCA1 or a related gene BRCA2 have so far been tested The New York Times Sunday Sep 16 2007 MCB140 09 17 07 5 Never ask for whom the bell tolls MCB140 09 17 07 6 Hanahan and Weinberg 2000 Cell 100 57 70 MCB140 09 17 07 7 Knudsen two hit model MCB140 09 17 07 8 Malignancy of somatic cell hybrids B Ephrussi et al Nature 1969 224 1314 The studies of Ephrussi et al and Harris provided compelling evidence that the ability of cells to form a tumor is a recessive trait They observed that the growth of murine tumor cells in syngeneic animals could be suppressed when the malignant cells were fused to nonmalignant cells although reversion to tumorigenicity often occurred when the hybrids were propagated for extended periods in culture The reappearance of malignancy was found to be associated with chromosome losses Stanbridge and his colleagues studied hybrids made by fusing human tumor cell lines to normal diploid human fibroblasts Their analysis confirmed that hybrids retaining both sets of parental chromosomes were suppressed with tumorigenic variants arising only rarely after chromosome losses in the hybrids Moreover it was demonstrated that the loss of specific chromosomes and not simply chromosome loss in general correlated with the reversion to tumorigenicity The observation that the loss of specific chromosomes was associated with the reversion to malignancy suggested that a single chromosome and perhaps even a single gene might be sufficient to suppress tumorigenicity To directly test this hypothesis single chromosomes were transferred from normal cells to tumor cells using the technique of microcell mediated chromosome transfer It was found that the transfer of a single chromosome 11 into the HeLa cervical carcinoma cell line suppressed the tumorigenic phenotype of the cells Many studies have now demonstrated that transfer of even very small chromosome fragments will specifically suppress the tumorigenic properties of certain cancer cell lines MCB140 09 17 07 9 Ventura et al T Jacks Nature 445 661 MCB140 09 17 07 10 Beyond Mendel interactions of gene products in the formation of traits as revealed by highly modified progeny ratios in crosses EPISTASIS to stand on top of MCB140 09 17 07 11 Epistasis or an epistatic interaction between two loci 1 2 3 4 Pick a trait Find a mutant phenotype 1 Find a different mutant different phenotype 2 Cross the two mutants get not a mix of phenotypes but instead either phenotype 1 or 2 The term epistasis refers to a phenomenon in which an allele of one gene masks stops the effects on the phenotype of an allele of a different gene The discovery of epistatic interactions between gene products is one of the most powerful tools in genetics it allows the assembly of individual genes into pathways and understanding of pathways leads to an understanding of mechanism MCB140 09 17 07 12 For difference determined by one gene MCB140 09 17 07 13 Fig 3 7 MCB140 09 17 07 14 Fig 3 6 MCB140 09 17 07 15 How can one tell if two organisms under study that exhibit mutant phenotypes for a particular trait have a mutation in different genes or in the same gene MCB140 09 17 07 16 Complementation test Complementation is the production of a wild type phenotype when two haploid genomes bearing different recessive mutations are united in the same cell MCB140 09 17 07 17 The cis trans test aka complementation test Edward Lewis NP 1995 Are two different recessive mutations that appear to affect the same trait in the SAME gene or in DIFFERENT genes MCB140 09 17 07 18 Orgo cis 2 butene trans 2 butene MCB140 09 17 07 19 The cis trans test 1949 lozenge M Greene Two different recessive mutants both with the same phenotype small eyes and fused facets Are they mutations in the same gene Make two different fly lines and compare their phenotypes Cis Trans wt wt wt lz g lz BS lz g lz BS wt MCB140 09 17 07 20 Cis Trans wt wt wt lz g lz BS lz g lz BS wt This is a control experiment The flies will be wild type regardless of whether BS and g are in the same gene or not If flies are normal then mutations are in different genes If the phenotype is still mutant then BS and g must be in the same gene MCB140 09 17 07 21 MCB140 09 17 07 22 Rine schematic mate to a cells Jasper Rine and Ira Herskowitz 1987 Genetics 116 9 22 Fig 17 14 MCB140 09 17 07 23 The data Colonies screened 675 000 Colonies that mated to a 295 Major complementation groups 4 silent information regulators SIR1 SIR2 SIR3 SIR4 Jasper Rine and Ira Herskowitz 1987 Genetics 116 9 22 MCB140 09 17 07 24 Baur et al Nature 444 337 Lagouge et al Cell 127 1109 MCB140 09 17 07 25 Xeroderma pigmentosum MCB140 09 17 07 26 What was actually done 1 By linkage analysis it was discovered that the same disease XP can be caused by mutations in 7 distinct loci 2 The cDNA from each gene was cloned 3 An assay was developed to measure how sensitive to UV light cells are 4 Experiment take cells from patient type A and introduce each of the 7 cDNAs one after another 5 Whichever cDNA restores the wild type phenotype corresponds to the gene that is mutated in that cell Bin XP mutations into complementation groups MCB140 09 17 07 27 Wait a minute Ahem Fine You take a cell that s mutant stick in a gene the cell is now wild type and you tell us this means the gene you stuck in is the gene that is mutated in the cell What if the cell has a mutation in a completely different gene and the gene you stuck in is just epistatic to the first one Good question We ll get to
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