6- gene linkage and genetic mapping

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6- GENE LINKAGE AND GENETIC MAPPING Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123

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6- GENE LINKAGE AND GENETIC MAPPING. Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123. Linkage and crossing over. LINKAGE. Chromosome is a linkage group Linkage refers to: 1) 2 or more genes may be related on the same chromosome - PowerPoint PPT Presentation

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Page 1: 6- GENE LINKAGE AND GENETIC MAPPING

6- GENE LINKAGE AND GENETIC MAPPING

Compiled bySiti Sarah JumaliLevel 3 Room 14

Ext 2123

Page 2: 6- GENE LINKAGE AND GENETIC MAPPING

LINKAGE AND CROSSING OVER

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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

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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

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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

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Trihybrid cross- Lets consider this

• Gray body, red eyes 1159Yellow body, white eyes 1017Gray body, white eyes 17Yellow body, red eyes 12Total 2205

• Red eyes, long wings 770White eyes, miniature wings 716Red eyes, miniature 401White eyes, long wings 318Total 2205

Nonparental offspring

Nonparental offspring

Nonparental Offsprings are called recombinant

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?Question

• Why is the recombinant number so low?

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Crossing over • May produce recombinant phenotypes or known as

recombinant or nonparent• Because the genetic information recombine during

Meiosis 1

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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

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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

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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

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Experiment: Do the genes for flower color and pollen shape assort independently?

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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.

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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

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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.

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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+,

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Determining Map DistanceThe 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.).