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In population geneticslinkage disequilibrium is the non-random association of alleles at two or more loci, that may or may not be on the same chromosome. It is also referred to as gametic phase disequilibrium,[1] or simply gametic disequilibrium. In other words, linkage disequilibrium is the occurrence of some combinations of alleles or genetic markers in a population more often or less often than would be expected from a random formation of haplotypes from alleles based on their frequencies. It is not the same as linkage, which is the presence of two or more loci on a chromosome with limited recombination between them. The amount of linkage disequilibrium depends on the difference between observed and expected (assuming random distributions) allelic frequencies. Populations where combinations of alleles or genotypes can be found in the expected proportions are said to be in linkage equilibrium.

The level of linkage disequilibrium is influenced by a number of factors, including genetic linkage, selection, the rate of recombination, the rate of mutationgenetic driftnon-random mating, andpopulation structure. A limiting example of the effect of rate of recombination may be seen in some organisms (such as bacteria) that reproduce asexually and hence exhibit no recombination to break down the linkage disequilibrium. An example of the effect of population structure is the phenomenon of Finnish disease heritage, which is attributed to a population bottleneck.

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## Definition

Consider the haplotypes for two loci A and B with two alleles each¡ªa two-locus, two-allele model. Then the following table defines the frequencies of each combination:

 Haplotype Frequency

Note that these are relative frequencies. One can use the above frequencies to determine the frequency of each of the alleles:

 Allele Frequency

If the two loci and the alleles are independent from each other, then one can express the observation  as " is found and  is found". The table above lists the frequencies for , and for, hence the frequency of  is , and according to the rules of elementary statistics .

The deviation of the observed frequency of a haplotype from the expected is a quantity[2] called the linkage disequilibrium[3] and is commonly denoted by a capital D:

In the genetic literature the phrase "two alleles are in LD" usually means that D ¡Ù 0. Contrariwise, "linkage equilibrium" means D = 0.

The following table illustrates the relationship between the haplotype frequencies and allele frequencies and D.

 Total Total

is easy to calculate with, but has the disadvantage of depending on the frequencies of the alleles. This is evident since frequencies are between 0 and 1. If any locus has an allele frequency 0 or 1 no disequilibrium  can be observed. When the allelic frequencies are 0.5, the disequilibrium  is maximal. Lewontin[4] suggested normalising D by dividing it by the theoretical maximum for the observed allele frequencies.

Thus:

 =

where

 $D_\max = \begin{cases} \max(-p_1q_1,\,-p_2q_2) & \text{when } D < 0\\ \min(p_1q_2,\,p_2q_1) & \text{when } D > 0 \end{cases}$

Another measure of LD which is an alternative to  is the correlation coefficient between pairs of loci, expressed as

.

This is also adjusted to the loci having different allele frequencies.

In summary, linkage disequilibrium reflects the difference between the expected haplotype frequencies under the assumption of independence, and observed haplotype frequencies. A value of 0 for  indicates that the examined loci are in fact independent of one another, while a value of 1 demonstrates complete dependency.

## Role of recombination

In the absence of evolutionary forces other than random mating and Mendelian segregation, the linkage disequilibrium measure  converges to zero along the time axis at a rate depending on the magnitude of the recombination rate between the two loci.

Using the notation above, , we can demonstrate this convergence to zero as follows. In the next generation, , the frequency of the haplotype , becomes

This follows because a fraction  of the haplotypes in the offspring have not recombined, and are thus copies of a random haplotype in their parents. A fraction  of those are . A fraction  have recombined these two loci. If the parents result from random mating, the probability of the copy at locus  having allele  is  and the probability of the copy at locus  having allele  is , and as these copies are initially on different loci, these are independent events so that the probabilities can be multiplied.

This formula can be rewritten as

so that

where  at the -th generation is designated as . Thus we have

 .

If , then  so that  converges to zero.

If at some time we observe linkage disequilibrium, it will disappear in the future due to recombination. However, the smaller the distance between the two loci, the smaller will be the rate of convergence of  to zero.

## Example: Human Leukocyte Antigen (HLA) alleles

HLA constitutes a group of cell surface antigens as MHC of humans. Because HLA genes are located at adjacent loci on the particular region of a chromosome and presumed to exhibit epistasis with each other or with other genes, a sizable fraction of alleles are in linkage disequilibrium.

An example of such linkage disequilibrium is between HLA-A1 and B8 alleles in unrelated Danes[5] referred to by Vogel and Motulsky (1997).[6]

Table 1. Association of HLA-A1 and B8 in unrelated Danes[5]
Antigen jTotal
Antigen i
Total
No. of individuals

Because HLA is codominant and HLA expression is only tested locus by locus in surveys, LD measure is to be estimated from such a 2x2 table to the right.[6][7][8][9]

expression () frequency of antigen  :

;

expression () frequency of antigen  :

;

frequency of gene  :

,

and

.

Denoting the '¨D' alleles at antigen i to be 'x,' and at antigen j to be 'y,' the observed frequency of haplotype xy is

and the estimated frequency of haplotype xy is

.

Then LD measure  is expressed as

.

Standard errors  are obtained as follows:

,
.

Then, if

exceeds 2 in its absolute value, the magnitude of  is large statistically significantly. For data in Table 1 it is 20.9, thus existence of statistically significant LD between A1 and B8 in the population is admitted.

Table 2. Linkage disequilibrium among HLA alleles in Pan-europeans[9]
HLA-A alleles iHLA-B alleles j
A1B80.06516.0
A3B70.03910.3
A2Bw400.0134.4
A2Bw150.013.4
A1Bw170.0145.4
A2B180.0062.2
A2Bw35-0.009-2.3
A29B120.0136.0
A10Bw160.0135.9

Table 2 shows some of the combinations of HLA-A and B alleles where significant LD was observed among pan-europeans.[9]

Vogel and Motulsky (1997)[6] argued how long would it take that linkage disequilibrium between loci of HLA-A and B disappeared. Recombination between loci of HLA-A and B was considered to be of the order of magnitude 0.008. We will argue similarly to Vogel and Motulsky below. In case LD measure was observed to be 0.003 in Pan-europeans in the list of Mittal[9]it is mostly non-significant. If  had reduced from 0.07 to 0.003 under recombination effect as shown by , then . Suppose a generation took 25 years, this means 10,000 years. The time span seems rather short in the history of humans. Thus observed linkage disequilibrium between HLA-A and B loci might indicate some sort of interactive selection.[6]

Further information: HLA A1-B8 haplotype

The presence of linkage disequilibrium between an HLA locus and a presumed major gene of disease susceptibility corresponds to any of the following phenomena:

• Relative risk for the person having a specific HLA allele to become suffered from a particular disease is greater than 1.[10]
• The HLA antigen frequency among patients exceeds more than that among a healthy population. This is evaluated by  value[11] to exceed 0.
• 2x2 association table of patients and healthy controls with HLA alleles shows a significant deviation from the equilibrium state deduced from the marginal frequencies.

(1) Relative risk

Relative risk of an HLA allele for a disease is approximated by the odds ratio in the 2x2 association table of the allele with the disease. Table 3 shows association of HLA-B27 with ankylosing spondylitis among a Dutch population.[12] Relative risk of this allele is approximated by

.

Woolf's method[13] is applied to see if there is statistical significance. Let

and

.

Then

follows the chi-square distribution with . In the data of Table 3, the significant association exists at the 0.1% level. Haldane's[14] modification applies to the case when either of is zero, where replace  and with

and

,

respectively.

Table 4. Association of HLA alleles with rheumatic and autoimmune diseases among white populations[10]
DiseaseHLA alleleRelative risk (%)FAD (%)FAP (%)
Ankylosing spondylitisB27909080.89
Reiter's syndromeB27407080.67
Spondylitis in inflammatory bowel diseaseB27105080.46
Rheumatoid arthritisDR4670300.57
Systemic lupus erythematosusDR3345200.31
Multiple sclerosisDR2460200.5
Diabetes mellitus type 1DR4675300.64

In Table 4, some examples of association between HLA alleles and diseases are presented.[10]

(1a) Allele frequency excess among patients over controls

Even high relative risks between HLA alleles and the diseases were observed, only the magnitude of relative risk would not be able to determine the strength of association.[11] value is expressed by

,

where  and  are HLA allele frequencies among patients and healthy populations, respectively.[11] In Table 4, column was added in this quotation. Putting aside 2 diseases with high relative risks both of which are also with high  values, among other diseases, juvenile diabetes mellitus (type 1) has a strong association with DR4 even with a low relative risk.

(2) Discrepancies from expected values from marginal frequencies in 2x2 association table of HLA alleles and disease

This can be confirmed by  test calculating

.

where . For data with small sample size, such as no marginal total is greater than 15 (and consequently ), one should utilize Yates's correction for continuity or Fisher's exact test.[15]

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