6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.
-
Upload
augustine-parks -
Category
Documents
-
view
214 -
download
0
Transcript of 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.
![Page 1: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/1.jpg)
6- GENE LINKAGE AND GENETIC MAPPING
Compiled by
Siti Sarah Jumali
Level 3 Room 14
Ext 2123
![Page 2: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/2.jpg)
LINKAGE AND CROSSING OVER
![Page 3: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/3.jpg)
LINKAGE
• Chromosome is a linkage group• Linkage refers to:• 1) 2 or more genes may be related on the same
chromosome– Physically linked because eukaryotic chromosome
contains a single, continuous linear molecule of DNA
• 2) Genes that are close on the same chromosome tend to be transmitted as a unit– Indicate the linkage has an influence on the pattern
![Page 4: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/4.jpg)
Bateson and Punnet discovered 2 traits that did not assort independently
• They suggested that the transmission of these 2 traits from the parental generation to the F2 generation was somehow coupled and not easily assorted independently
• This is due to linkage
![Page 5: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/5.jpg)
![Page 6: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/6.jpg)
LINKAGE CONT’D
• Traits may not assort independently such as in Mendel’s law, where the traits did not segregate showing linkage
• Therefore it produces bizarre phenotypic ratio than 9:3:3:1 to something like 15:1:1:4
• Chi-square (χ2) can be used to distinguish between linkage and independent assortment
![Page 7: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/7.jpg)
Trihybrid cross- Lets consider this
• Gray body, red eyes 1159
Yellow body, white eyes 1017
Gray body, white eyes 17
Yellow body, red eyes 12
Total 2205
• Red eyes, long wings 770
White eyes, miniature wings 716
Red eyes, miniature 401
White eyes, long wings 318
Total 2205
Nonparental offspring
Nonparental offspring
Nonparental Offsprings are called recombinant
![Page 8: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/8.jpg)
?Question
• Why is the recombinant number so low?
![Page 9: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/9.jpg)
Crossing over • May produce recombinant phenotypes or known as
recombinant or nonparent• Because the genetic information recombine during
Meiosis 1
![Page 10: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/10.jpg)
Chi-square (χ2)
• Is used to test goodness of fit between a genetic hypothesis and observed experimental data
• Must 1st propose a hypothesis; • 2 genes are unlinked, therefore follow Mendel’s law• Null hypothesis is said to be null because it is assumed
that there will be no difference between experimental data and observed data
• If χ2 is low, accept hypothesis, genes assort independently
• If χ2 is high, reject hypothesis, genes are linked
![Page 11: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/11.jpg)
Chi-square (χ2) can be used to distinguish between linkage and independent assortment
• χ2=Σ (o-e)2/ e;– Where o = observed value,
e = expected outcome (theory)
If the data do not fit, we will reject the idea that the genes assort independently and conclude that the genes are linked
![Page 12: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/12.jpg)
Chi-square (χ2) analysis
• The larger the chi-square, the smaller the p value
• P value more than 5% will ensure acception of null hypothesis
• 5% and below will cause rejection of null hypothesis
![Page 13: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/13.jpg)
Experiment: Do the genes for flower color and pollen shape assort independently?
![Page 14: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/14.jpg)
![Page 15: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/15.jpg)
Mendel’s law overruled
• Soon after Mendel's rules were rediscovered, it was found that some loci did not assort independently.
• The simplest explanation is that the loci lie close to each other on the same chromosome. They are linked on the same chromosome.
![Page 16: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/16.jpg)
Thomas Morgan
• He did experiment on genetic mapping on X chromosome
• He concluded that genes are located on the same X chromosome, so they are likely to be inherited together
• Due to crossing over, X chromosome can exchange pieces of chromosomes and create new parental combination of alleles
• The likelihood of crossing over depends on the distance between 2 genes. I f 2 genes are far apart from each other, crossing over is more likely to occur
![Page 17: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/17.jpg)
CROSSOVER VALUE AND GENE MAPPING
• Determining Map DistanceThe percent recombination is calculated as before.
(# of recombinants ÷ # of offspring) x 100 = % of recombinants
1% recombination = 1 map unit, or 1 centimorgan, in honor of T.H. Morgan, one of the first persons to propose this linkage, and first to win a Nobel prize in genetics.
• Two phenotypes are in very high frequency have the same phenotypes as the original parents (P1). These are called non-recombinants or parentals.
• Two phenotypes are in low frequency and combine the phenotypes of the two original parents (P1). These are called recombinants or non-parentals.
![Page 18: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/18.jpg)
Three-point cross
• Determining gene order
• The pair of phenotypes with the highest frequency is always the non-recombinant group.
• The pair of phenotypes with the lowest frequency is always the double cross-over group. The probability of a double cross-over is approximately the product of the probability of the single cross-over.
• compare the wild type class b+pr+c+ to the purple double cross-over class b+prc+ and we can see that the purple locus does not match indicating that the purple locus is in the middle.
• Also note that as the parental phenotypes are composed of a gamete from the female that is either b pr c or b+ pr+ c+,
![Page 19: 6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123.](https://reader031.fdocuments.in/reader031/viewer/2022032702/56649cda5503460f949a42e8/html5/thumbnails/19.jpg)
Determining Map Distance
The next step is to set-up a table that is titled " number of recombinants between". The percent recombination is calculated as before.
# of recombinants
100 x ----------------- = % of recombinants
# of offspring
thus for the distance from b to pr
887
100 x ----------------- = 5.9% or 5.9 m.u.
15,000
The distance from b to c, the two outside loci, (25.4 m.u.) is the sum of the distance from b to pr (5.9 m.u.) and the distance from pr to c (19.5 m.u.).