What is linkage disequilibrium? The opposite of linkage equilibrium! Recall Mendel’s 2nd Law: Allels of different genes are passed on independently. When this law holds, there is no correlation between polymorphisms. In this state of random assortment, they are said to be in linkage equilibrium. However, if there are correlated, then they are in linkage disequilibrium. This is the non-random as...

Haplotypes Linkage equilibrium and disequilibrium Additive, multiplicative, epistatic tness regimes Causes of linkage disequilibrium Extent of disequilibrium in nature Multilocus population genetics is the study of the distribution and dynamics of genetic variation at several loci in biological populations. Haplotypes Even the simplest known biological organisms have thousands of genes and mill...

Many statistical methods have been proposed in recent years to test for genetic linkage and association between genetic markers and traits of interest through unrelated nuclear families. However, most of these methods are not valid tests of association in the presence of linkage when some of the nuclear families are related. As a result, related nuclear families in large pedigrees cannot be inc...

The last decade has seen a dramatic increase in the number of livestock QTL mapping studies. The next challenge awaiting livestock geneticists is to determine the actual genes responsible for variation of economically important traits. With the advent of high density single nucleotide polymorphism (SNP) maps, it may be possible to fine map genes by exploiting linkage disequilibrium between gene...

Flowering time varies among Arabidopsis accessions. Some of the genes that contribute to this polygenic trait have been localized using quantitative trait loci (QTL) mapping, eventually leading to the identification of genetic variants that contribute to observed differences in flowering time (El-Assal et al., 2001). Despite this and other similar successes with the positional cloning of QTL, t...

Linkage disequilibrium (LD) in the human genome, often measured as pairwise correlation between adjacent markers, shows substantial spatial heterogeneity. Congruent with these results, studies have found that certain regions of the genome have far less haplotype diversity than expected if the alleles at multiple markers were independent, while other sets of adjacent markers behave almost indepe...

Crosses between laboratory strains of mice provide a powerful way of detecting quantitative trait loci for complex traits related to human disease. Hundreds of these loci have been detected, but only a small number of the underlying causative genes have been identified. The main difficulty is the extensive linkage disequilibrium (LD) in intercross progeny and the slow process of fine-scale mapp...

Elevated TG [triacylglycerol (triglyceride)] is a significant independent risk factor for cardiovascular disease. LPL (lipoprotein lipase) is one of the key enzymes in the metabolism of the TG-rich lipoproteins which hydrolyses TG from the chylomicrons and very-LDL (low-density lipoprotein). To investigate the relationship between the LPL gene and lipid profiles, especially TG, in 148 hypertens...

We describe the use of multivariate regression for testing allelic association in the presence of linkage, using marker genotype data from sibships. The test is valid, provided that the correct mean structure is modeled but does not require the correlation structure within families to be specified. The test can be implemented using standard statistical software such as the SAS programming langu...