Genetic Analysis of Complex Disease. Группа авторов

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of recombination events occur; when an even number of recombination events occurs between two loci, the resultant gametes appear to be nonrecombinant and hence these recombination events are unobserved. However, two loci that are located closely on the same chromosome have a low likelihood of experiencing a recombination event between them and nearly always segregate together. These loci are considered to be genetically linked (Figure 2.9). Linkage analysis (Chapter 6) is a method of determining whether two loci are genetically linked when passed on from one generation to the next through measuring the recombination fraction (θ) between loci.

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      (Source: Reprinted by permission from Jorde et al. (1995).)

      Following recombination, at least two of the four chromatids become unique, unlike those of the parent. The cellular division process that occurs ensures that one paternal homologue and one maternal homologue are transmitted to each of two diploid daughter cells. This cell division marks the end of meiosis I. The process of genetic recombination helps to preserve genetic variability within a species by allowing for virtually limitless combinations of alleles in the transmission from parent to offspring. Estimates of genetic recombination can also predict distance between two loci: The closer two loci are to one another, the less chance for recombination between them. The frequency of recombination is not uniform through the genome. Some areas of some chromosomes have increased rates of recombination (hot spots), while others have reduced rates of recombination (cold spots). For instance, recombination frequencies may vary between sexes or may vary depending on whether the loci are at the telomere or centromere of the chromosome.

      The second phase of meiosis is identical to a mitotic (somatic cell) division, in which genetic material is transmitted equally, identically, and without recombination to daughter cells. However, in contrast to a mitotic division, which yields two identical diploid daughter cells, the end result of the entire meiotic process in sperm cells is four haploid daughter cells with chromosomal haplotypes different from those originally present in the parent; in egg cells, the final outcome is a single haploid daughter cell, with the remainder of the genetic material lost because of the formation of nonviable polar bodies.

Meiosis Mitosis
Purpose Produce gametes; ensure or produce genetic variability through recombination Replace somatic cells
Location Gonads Body cells
Number of cell divisions per cycle Two: meiosis I and meiosis II (latter identical to mitosis) One
Chromosome number in daughter cells Halved from parental complement to produce gametes; resultant cells are haploid Identical to parental complement; resultant cells are diploid
Recombination Occurs in diplotene of prophase I of meiosis I Occurs rarely, usually result of abnormality

      When Genes and Chromosomes Segregate Abnormally

      Failure of meiosis at either phase (meiosis I or II) is termed nondisjunction and leads to aneuploidy, or abnormal chromosomal complements. The most well‐known aneuploidy is Down syndrome. Most cases of Down syndrome are caused by an extra copy of chromosome 21, but ~5% of cases are caused by a translocation such that the trisomy is not of the whole chromosome. Down syndrome is often called trisomy 21 because most individuals with Down syndrome have a total of 47 chromosomes, with three copies of chromosome 21. A monosomy, or the absence of a second member of a chromosome pair, is rarely viable. A noted exception is Turner syndrome, in which a female has a total of 45 chromosomes, including a single X chromosome. Nondisjunction events that lead to abnormal chromosome complements may occur in the egg or the sperm. However, in contrast to male sperm, which are produced throughout the lifespan, female egg cells are suspended in meiosis II since shortly after that female’s conception. These eggs are subject to aging over the female lifespan, and errors of nondisjunction are more likely to occur.

      Triploidy and tetraploidy are the terms for the presence of one or two entire extra sets of chromosomes, leading to a total of 69 or 92 chromosomes, respectively. These anomalies, which usually are not viable in humans, are due to errors in fertilization such as dispermy (two sperm fertilizing an ovum) or failure of the ovum’s polar body to separate.

      Autosomal Recessive

      A condition described as autosomal recessive maintains that the causative gene is located on an autosome and that both copies of the gene must harbor a causative allele. In most cases of recessive conditions, it is correctly assumed that each parent of an offspring with the trait carries a single causative allele. Carriers for recessive conditions do not typically show phenotypic features. Recessive conditions occur more commonly in offspring of consanguineous unions or in populations with high frequency of carriers for a particular condition.

      Autosomal Dominant

      In a dominant condition, a genetic mutation in a single copy of a gene is sufficient to cause a disease or trait. Dominant conditions may be inherited through an affected parent or alternatively may have occurred as the result of a de novo genetic mutation. Some de novo dominant mutations have been


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