BIOL 3333 1st EditionExam # 2 Study Guide Lectures: 10-20*Be sure to review the terms in the last Study Guide for Exam 1 What causes modifications of Dihybrid Mendelian Ratios?Gene InteractionsWhat is a gene interaction?- they can lead to differences in phenotypic expression from that which is expected in the absenceof such interactionsWhat is epistasis?- the literal meaning is to “stand upon” - this is where one gene can modify the phenotypic expression of a non allelic gene when both genes are present in the genome What are the 4 separate comb phenotypes that are observed in chickens?- walnut- rose- pea- single How do they occur?*Remember that both rose and pea are dominant to single What happens when true-breeding rose and pea strains are crossed? Rose x Pea = all walnutSo what happens when F1 walnuts are crossed with one another?That would be Walnut x Walnut and would yield:9 Walnut: 3 Rose: 3 Pea: 1 Single *this suggests that a dihybrid cross with 2 separate genes are interacting in these phenotypesWhat is the model for Comb Phenotype in Chickens? Complementary Gene Action: - this is where the dihybrid has a different phenotype and different genes are involvedWhat is the model for Complementary Gene Action?-there is a dominant allele at both the A and B loci - this is necessary to get to phenotype Z -this yields 9/16 A_B_ = PURPLEand 3/16 aaB_ + 3/16 A_bb + 1/16 aabb = WHITEWhat is an example of Duplicate Gene Action?Flower Shape in Shepherd’s Purse:Heart x Narrow -> Heart x Heart -> 15 heart: 1 NarrowWhat is the model for Duplicate Gene Action? - Having is a dominant allele at either the A or B locus is sufficient to get to phenotype Z - Any dominant allele will give Z phenotype- Only the aabb genotypewill be insufficient - This is a 15:1 ratio- There is unequal crossing over here: What is the Model for Dominant Epistasis? - a dominant allele at the B locus will block phenotype Y - a dominant allele at the A locus will lead to phenotype ZWhat is an example of Dominant Epistasis? - Fruit color in squash What is the Model for Recessive Epistasis? = Black Pigment= Brown Pigment This Yields: 9/16 C_B_ = black3/16 C_bb = brown3/16 ccB_ = white1/16 ccbb = white 9 black: 3 brown: 4 white What are some examples of modified phenotypic ratios that are produced by Gene Interaction: - comb shape in chickens- flower color in sweet peas- albanism in mammals - fruit color in squash- fruit shape in shepherd’s purseWhat is the phenotypic ratio that represents a gene interaction of 4 distinct phenotypes (no gene interaction)?- 9:3:3:1 What is the phenotypic ratio that represents a gene interaction of Complementary Gene action? - 9:7What is the phenotypic ratio that represents a gene interaction of Recessive suppression by aa acting on bb? 13:7 What is the phenotypic ratio that represents a gene interaction of recessive epistasis of aa acting on B and b alleles? 9:3:4What is the phenotypic ratio that represents a gene interaction of Dominant epistasis of A acting on B and b alleles?12:3:1What is the phenotypic ratio that represents a gene interaction of Duplicate genes? 15:1What is continuous variation in populations? - a variation that occurs by adding more genes/alleles to the phenotype - the more interaction, the more possible phenotypic classesWhat shoes multifactorial inheritance? - Continuous traits What is the r+ gene needed for?- to stimulate mRNA synthesis of a+ gene* binding to DNA turns on gene a+Chart for suppression at the gene product level: First mutation blocks interactionSecond mutation now restores ability for interactionBoth mutations may be required for activity! In a test cross situation, what should the independent segregation of dominant alleles in the dihybrid should produce: AaBb x aabb = a 1:1:1:1 ratio of all 4 phenotypesWhat if the A/a B/b gene loci are close to one another on the same chromosome? * gametes ab and AB will become gametes aB and AbWhen will gene loci not show independent assortment?- when they are linked closely to ether on the same chromosomeWhat are the 2 possible gene configurations that can result when genes are close together on the same chromosome? What did A.H Sturtevant assume? - that crossing over occurs at random during meiosis- and that genes T and U are farther apart than V and WWhat is the outcome of Sturtevants assumption?- that chromatids cross over between T and U in larger proportion of meioses than between V and W. What can serve as a measure of gene distances? - the number of crossovers in a given interval When is there evidence of linkage?- when a ratio is not 1:1:1:1Know how to Determine Recombination and Map unit distancesWhat did T.H Morgans Laboratory suggest?- that linkage experiments in Drosophila support the chromosomal theory of inheritance- genes are linked thru all generations - no random mixing- genes are arrayed in a linear order along the length of a chromosome - homologs carry the same array of gene loci, but not necessarily the same alleles What represents no recombination?What represents non-recombinant chromatids?What represents a double recombination event?What is the least common class of recombination?- Double recombinant chromatids - They represent 2 crossovers that flank the middle gene How do you determine the gene order?- By comparing non recombinantt and double recombinant classesHow do you know if genes are linked on the X chromsome?- they will not be produced in a 1:1:1:1.. etc ratioWhat is the most common class of recombination?- parental allelic combinations- they represent non-recombinant chromatids What does mapping through recombination allow us to determine?- Genes positions relative to each other What does mapping through recombination NOT allow us to determine? - it does not allow us to precisely position these genes to a particular region of a chromosome What is interference evidence of?- that crossing-over in one area can decrease the likelihood of an exchange in an adjacent areaHow do gene loci assort if they are on separate chromosomes?- they assort independently in a 1:1 ratio If the genes are located far apart on the same chromosome what is the RF?- the recombination frequency will be 50 percentWhat is the maximum map distance between 2 genes?- 50 map units- it can never exceed 50 map units in any given cross What leads to an underestimate of true genetic distance?- the effects of multiple crossovers How are the most accurate map distances obtained?- from summing distances