a chromosomal region. Multipoint computational algorithms usually employ a hidden Markov model that assumes that IBD sharing at consecutive loci behaves in a first-order Markov manner.

2.4. Linkage Disequilibrium

The phenomenon of linkage results in the cosegregation of the alleles of two linked loci and thus in within-family association of specific alleles. Among families, it is common to assume in a linkage analysis that no association between the allelic variants of different loci is present, as different marker alleles may cosegregate with the disease allele in different families. If no population association exists between alleles at two loci, the loci are said to be in linkage equilibrium, and the population frequencies of each two-locus haplotype are the products of the single-locus allele frequencies (Table I). If a population association does exist, the loci are said to be in disequilibrium.

As an example of disequilibrium, assume we have two diallelic loci, one a disease locus with alleles A and a (with frequencies pA and 1 — pA, respectively), and the other a marker locus with alleles B and b (with frequencies pB and 1 — pB, respectively). Under random mating, the frequencies of the two-locus genotypes are determined by the products of the frequencies of the four haplotypes AB, Ab, aB and ab, which change over time (usually measured in generations).13"15 Specifically, at a particular point in time, let the haplotype frequencies for AB, Ab, aB and ab be

respectively, where D = hABhab — hAbhaB is the departure from equilibrium; that is, D is the disequilibrium between A and B.

The magnitude of disequilibrium between the disease and marker alleles dissipates as future generations of a population emanate and recombination occurs. How quickly equilibrium is reached depends on the number of new generations that have passed since the disease allele arose (mutation age), and the genetic distance between the disease and marker loci. Fewer recombinations imply that the disease and marker alleles are in stronger disequilibrium. This inverse relation implies that when there is very close linkage, disequilibrium may persist for long periods of evolutionary time.16 Other effects influence allele and haplotype frequencies in the population over evolutionary time, including genetic drift, the random component of allele frequency change over generations. The magnitude of genetic drift is inversely related to the size of the portion of the population relevant to mating and gene transmission, called the effective population size.

Natural Vertigo And Dizziness Relief

Natural Vertigo And Dizziness Relief

Are you sick of feeling like the whole world Is spinning out of control. Do You Feel Weak Helpless Nauseous? Are You Scared to Move More Than a Few Inches From The Safety of Your Bed! Then you really need to read this page. You see, I know exactly what you are going through right now, believe me, I understand because I have been there & experienced vertigo at it's worst!

Get My Free Ebook

Post a comment