Term
| What is deletion mapping? (Commonly used in Drosophila |
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Definition
| Deletion mapping is used to find the physical location of a gene on a chromosome. In Drosophila, the binding patterns in the polytene chromosomes are useful as a visible landmark for deletion mapping of genes. The idea behind using this method is that by deleting the dominant allele of a heterozygote results in the appearance of the phenotype of the recessive allele. When the recessive trait is exposed because of the lacking dominant trait it is called pseudodominance. |
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Term
| What are polytene chromosomes? Why are they useful? Why are they so big? How used in mapping? |
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Definition
| Polytene chromosomes are chromosome bundles that result from repeated cycles of chromosome duplication without nuclear or cell division. This process is called endoreduplication. This enables the polytene chromosomes to become up a thousand times the size of the same chromosomes in meiosis or in the nuclei of ordinary somatic cells. The homologues pairs become tightly fused together. Because they are so large, the characteristic binding patterns are very easy to see when the chromosomes are stained. This allows geneticists to identify any segment of a polytene chromosome. In Drosophila melanogster you can count 5000 band and interbands in the four polytene chromosomes. Each of these band has about 30,000 base pairs of DNA, enough to encode several average size proteins. Through DNA cloning and sequencing, geneticists have found many bands can house up to 7 chromosomes and that genes are also found on the interbands. There large size also helps to easily see mutations. |
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Term
| What is somatic cell hybridization, and how is it used to figure out which chromosome a gene is on? |
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Definition
| Somatic cell hybridization or radiation hybrids are a rodent cell line that carries a small pierce of the genome of another organism, like that of a human. Irradiating human cells with X rays to cause the DNA to break randomly producing these pieces. The higher the dosage of X-rays the smaller the pieces of DNA. The irradiation kills the human cells but the fragments can be rescued by fusing them with the rodent cells. Each piece of human DNA is usually a few mega base pairs long. The human DNA in the RH is analyzed for the genetic markers it carries. The basic idea behind RH mapping is: the closer two markers are together, the better chance they have of being on the same DNA piece and therefore ending up in the same RH. Both gene and DNA markers can be used in RH mapping. |
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Term
| What is the FISH technique? |
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Definition
| Fluorescent In-Situ Hybridization, a genome mapping technique. This “cytogenetic technique which can be used to detect and localize the presence or absence of specific DNA sequences on chromosomes” Fluorescently tagged DNA sequences bind to a region on the genome with a similar sequence causing the region to fluoresce. “FISH is often used for finding specific features in DNA. These features can be used in genetic counseling, medicine, and species identification” |
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Term
| Can you rationalize without actually counting offspring, why you get those weird ratios? |
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Definition
Dominance in the heterozygotes can offset the ratio.
One example (given by the prof.) is when the offspring ratio is 9:7 where the heterozygotes and the homozygous recessive offspring look alike. (3:3:1 of the 9:3:3:1 expected are added up to make a count of 7 similar offspring) This means that the dominant phenotype is only expressed when both genes have the dominant allele.
Another scenario is when the ratio is 15:1. (9:3:3 are added up to make a count of 15 similar offspring) This means that the offspring only need one dominant copy of either allele to express the dominant phenotype. |
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Term
| Why is the chicken comb an example of epistasis, and not just a typical dihybrid cross. |
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Definition
| In the example of the shape of the cock combs, four unique combs result in the combination of the two genes with two alleles. (ref. [fig 13.8] pg.. 349) When the walnut and pea combed birds are crossed all four phenotypes are seen. It is an interaction between two dominant alleles where each produces a different phenotype. No modification of the Medelian ratio is involved. |
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Term
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Definition
| in dominant epistasis A/a B/b and A/a b/b individuals have the same phenotype so the phenotypic ratio is 12:3:1. One dominant gene is epistatic to the other (one dominant allele in the first gene masks whatever else is present and is expressed.) |
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Term
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Definition
| in recessive epistasis, A/a b/b and a/a b/b individuals have the same phenotype and thus alters the phenotypic ratio to 9:3:4 |
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Term
| Involving Duplicate Genes |
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Definition
| Duplicate genes are involved when a gene or a genotype at one locus may produce a phenotype identical to that produced by a gene or genotype at a second locus. (locus-singular, loci-plural; a location on a chromosome, usually a fixed position on a chromosome.) Basically there are two places on a chromosome that give the same effect or control the same phenotype. |
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Term
| Duplicate Dominant epistasis |
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Definition
) example shepherd’s purse plant pg. 354.- There are two genes controlling one trait. One dominant allele in either of the two sets is enough to produce a dominant phenotype.
A/_ B/_, A/a b/b, and a/a B/b all produce the dominant phenotype of heart shaped friut |
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Term
| Duplicate Recessive epistasis |
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Definition
example: sweet pea flower color pg. 353- One gene controls color and the complementary gene controls the ability to express color.
C/c controls color expression, P/p controls the type of color expressed. |
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Term
| complementary gene action |
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Definition
| see duplicate recessive epistasis |
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Term
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Definition
| interaction between two or more genes to control a single phenotype. |
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Term
| How might you cross two individuals and use the new-fangled genetic markers to follow the offspring of the cross, instead of “traditional” phenotypes? |
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Definition
OK. First: “fangled”- adj. New made; hence, gaudy; showy; vainly decorated.
Genetic markers are often mutations that give a distinguishable phenotype. This makes for an easy and obvious count of the offspring from the cross. (I’m not sure what else to say…I called the prof. here is what we need:)
Go beyond physical and visually perceived phenotype and look to sequencing the genome of the offspring. STS’s (sequence tagged sites) and EST’s (expressed sequence tags) will be very helpful in this task. In order to use these you need to have some type of PCR primers to PCR up a sequence of a desired gene. From these you can make a genetic picture of the phenotype. This is especially useful when you are seeking a specific/desired phenotype.
Example application: corn plant height. PCR DNA of all the babies (!?) and see how the gene for height associates on the genome. By knowing half of the genetic picture you have a good idea of what the other half looks like because of what the parents’ look like. |
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Term
| What are all the reasons that in a cross, you don’t get the “expected” ratio of phenotypes, like 9:3:3:1? (I can think of at least 3 reasons) |
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Definition
Any deviation from the expected ratio is tell sign that two or more genes may be involved to produce that phenotype.
Linked* - If the genes are linked certain phenotypes (the parental strand) are more likely to occur and other phenotypes may not be represented.
Mortality* – Certain genotypic combinations will cause death or non-development in offspring. This group will not be represented in the offspring total or in the sub-groups.
Epistasis* – This may add more ( or less ) phenotypes than anticipated.
Mutations – (always causing problems) may give any of the above mentioned results. |
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Term
| What is a dominant negative? Hint: think of the 2 cars in the driveway example. |
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Definition
A loss-of-function mutation normally results from a dominant negative mutation. It is a mutation whose gene product adversely affects the normal wildtype gene product within the same cell. The mutant product usually interacts with the same elements and results in a protein that is structurally similar to the wild type protein but has a loss of function or some aspect of the function.
This is the 2 cars in the driveway example. IF you have a long, narrow driveway and you park one car in front (representing the normal, dominant gene) and then park another broken down car behind it (representing the mutant, gene) it blocks the function of the working car because you can’t get it out without moving the broken-down car that won’t work. |
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Term
| What is a hypomorph? A null? |
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Definition
Hypomorph: a mutant with less than the normal amount of some gene product
Null: a null allele an allele whose affect is either an absence of normal gene product at the molecular level or an absence of normal function at the phenotypic level. |
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Term
| How is penetrance different from expressivity? |
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Definition
Penetrance: the frequency with which a dominant or homozygous recessive gene is phenotypically expressed within a population
Ex. How many of the offspring are “tall” (tall= 6.0 ft and taller)
Expressivity: the degree to which a particular gene is expressed in the phenotype. A gene with variable expressivity can cause a range of phenotypes.
Ex. What height are they exactly IN the tall region. To what degree of tallness? |
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