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Genetics_CH4
Mapping Eukaryote Chromosomes by Recombination
| Question | Answer |
|---|---|
| gene position | crucial information needed to build complex genotypes required for experimental purposes or for commercial applications |
| knowing sgene position | allows scientists to zero in on structure and function |
| genes present and their arrangement on chromosomes | are slightly different in related species so geneticists can deduce the evolutionary mechanism through which these genes diverged |
| chromosome map | shows gene positions, known as loci, and the distances between the loci based on some kind of scale |
| loci | gene positions |
| 2 types of chromosome maps | Recombination-based maps and physical maps |
| recombination-based maps | map the loci of genes that have been identified by mutant phenotypes showing single-gene inheritance |
| physical maps | show the genes as segments arranged along the DNA molecule that constitue a chromosome |
| recombination maps | usually assembled 2 or 3 genes at a time using linkage analysis |
| linked | the loci of genes are on the same chromosomes, and the alleles of any one homolog are physically joined (linked) by the DNA between them |
| crossing over | when homologous chromosomes pair during meiosis, the chromosomes occasionally break and exchange parts |
| crossover products | the two new combinations formed after crossing over |
| chiasma (pl. chiasmata) | a cross-shaped structure that forms between two non-sister chromatids |
| chromatids | participate in a crossover |
| cis conformation | gene arrangement in which the dominant alleles are present on the same homolog |
| trans conformation | gene arragement in which the dominant alleles are present on different homologs |
| crossover | results from the breakage and reunion of DNA - chromosomes break at the same position and piece with the other chromosome in the same spot (no genetic material is lost or gained) |
| crossover | takes place between nonsister chromatids at the four-chromatid stage |
| two-chromosome stage | before DNA replication |
| four-chromatid stage | when crossover occurs |
| within meiocytes | several crossover can occur along a chromosome pair |
| in any one meiocyte | multiple crossovers can exchange material between more than two chromatids |
| farther apart genes are on a chromosome | the more likely that a crossover will take place and the higher the proportion of recombinant products will be |
| recombination frequencies | range from 0 to 50 depending on their closeness |
| farther apart genes are | the more closely their recombinat frequencies approach 50 percent (causing question if they are linked or on two different chromosomes) |
| genetic map | linkage map |
| genetic map unit (m.u.) | that distance between genes for which 1 product of meiosis in 100 is recombinant |
| recominant frequency (RF) | percent = m.u. |
| centimorgan (cM) | map unit |
| three-point testcross or three-factor cross | cross of a trihybrid (triple heterozygote) with a triply recessive tester |
| three-point (and higher) testcrosses | enable genetisicists to evaluate linkage between three (or more) genes and to determine gene order, all in one cross |
| double recombinant | add into equation twice |
| deducing the gene in the middle | the allele pair that "flipped" position in the double-recombinant classes |
| interference | crossovers inhibit each other somewhat --> use double recombinants to deduce the extent of this |
| coefficient of coincidence (c.o.c.) | the ratio of observed to expected double recombinants |
| interference | 1 - c.o.c. |
| If no double recombinants | c.o.c. = 0 and I = 1 |
| drosophila males | do not crossover |
| women | show a higher recombinant frequency than men do |
| molecular markers | differences in DNA between two chromosomes that do not produce visibly different phenotypes, either because DNA differences are not located in genes or they are located in genes, but do not alter the product protein |
| two types of molecular markers | single-nucelotide polymorphisms and simple-sequence-length polymorphisms |
| single nucleotide polymorphism (SNPS) or "snips" | individual variations in the genetic sequences (very common) --> occur at a frequency of 1 in every 300 to 1000 bases |
| SNPS | do not produce different phenotypes because they do not lie in a gene or they lie in a gene but both versions of the gene produce the same protein product |
| two ways to identify SNPS | sequence a segment of DNA in a homologous chromosomes and compare the homologous segments to spot difference and 2nd if SNPS are located at a restriction enzyme's target site in the case of RFLPs |
| RFLP | 2 RFLP "alleles" or morphs, 1 of which has the restriction enzyme target & the other does not --> the restriction enzyme cuts the DNA at the SNP containing the target and ignore the other SNP... SNPS then detected as different bands on electrophetic gel |
| RFLP sites | can be between or within genes |
| simple sequence length polymorphisms (SSLPs) or variable number tandem repeats (VNTRs) | repetitive DNA strands that tend to have multiple alleles (as many as 15 have been found at one locus) |
| two types of SSLPs | minisatellite and microsatellite markers |
| satellite | refers to the observation that, when genomic DNA is isolated and fractionated with the use of hpysical techniques, the repetitive sequences often form a fraction that is physically separate from the rest |
| minisatellite marker | based on variation in the number of tandem repeats of a repeating unit from 15 to 100 nucleotides long |
| microsatellite marker | based on variable numbers of tandem repeats of an even simpler sequence, generally a small number of nucleotides such as a dinucleotide |
| SSLPs | take advantage of that fact that homologous regions bearing different numbers of tandem repeats will be of different lengths |
| DNA fingerprints | patterns produced on electrophoretic gel (specifically in the case of minisatellites) after the DNA has been amplified using PCR |
| 1 megabase | 1 million base pairs or 1000 kb |
| centromeres | not genes, but regions of DNA imperative to the orderly reproduction of living organisms absolutely depends |
| octad | produced by the meiotic division that takes place along the long axis of the ascus, and so each meiocyte produces a linear array of eight ascospores |
| tetrad | the four products of meiosis |
| first-division segregation patterns (MI patterns) | two different alleles segregate into the two daughter nuclei at the first division of meiosis |
| second-division segregation patterns (MII) | as a result of crossing over in the centromere-to-locus region, the A and a alleles are still together in the nuclei at the end of the first division of meiosis |
| percentage of meioses | not the same as map units |
| MII frequendy divided by 2 | map units |
| null hypothesis | the observed results cause us to reject the hypthesis of no linkage |
| mapping function | an approach meant to correct for double crossovers - a formula that relates an observed recombinant-frequency value to a map distance corrected for multiple crossovers |
| Poisson distribution | describes the occurence of crosovers in a specific chromosomal region |
| Perkins formula | method of compensating for multiple crossovers |
| physical maps | a map of the actual genomic DNA, a very long DNA nucleotide sequence, showing where genes are, how big they are, what is between them, and other landmarks of interest |
| unit of distance on a physical map | number of DNA bases |
| candidate genes | genes in the physical map that are in the general region of the gene of interest on the recombination map |
| heteroduplex DNA | DNA in whih there is a mismatched nucleotide pair in the gene under study |
| double-stranded break | break in the DNA of one of the chromatids participating in the crossover |
| Holliday junctions | a strange structure with two double-stranded junctions that result from the sealing of replicated ends --> potential sites of single stranded breakage and reunion |
Created by:
Nicolekr
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