The mechanism of pathogenesis here is distinguished from the mechanism of nondisjunction described earlier in this chapter. In contrast to aneuploidy or uniparental disomy, it is not the process of segregation that is abnormal in these cases; rather, it is the random nature of events during segregation that leads to unbalanced karyotypes and thus to offspring with abnormal phenotypes.

In the case of balanced translocations, for example, because the chromosomes involved form a quadrivalent in meiosis, the particular combination of chromosomes transmitted to a given gamete can lead to genomic imbalance, even though the segregation is itself normal.

Another type of familial structural abnormality that illustrates this mechanism involves inversion chromosomes. In this case, segregation of the inverted chromosome and its normal homologue during meiosis is typically uneventful; however, unbalanced gametes can be produced as a result of the process of recombination occurring within the inverted segment, in particular for pericentric inversions. Different inversion chromosomes carry different risks for abnormal offspring, presumably reflecting both the likelihood that a recombination event will occur within the inverted segment and the likelihood that an unbalanced gamete can lead to viable offspring. This overall risk must be determined empirically for use in genetic counseling. Several well-described inversions illustrate this point.

A pericentric inversion of chromosome 3 is one of the few for which sufficient data have been obtained to allow an estimate of the transmission of the inversion chromosome to the offspring of carriers. The inv(3) (p25q21) originated in a couple from Newfoundland in the early 1800s and has since been reported in a number of families whose ancestors can be traced to the Atlantic provinces of Canada. Carriers of the inv(3) chromo some are normal, but some of their offspring have a characteristic abnormal phenotype associated with the presence of a recombinant chromosome 3, in which there is duplication of the segment distal to 3q21 and deficiency of the segment distal to 3p25. The other predicted unbalanced gamete, with duplication of distal 3p and deficiency of distal 3q, does not lead to viable off spring. The empirical risk for an abnormal pregnancy outcome in inv(3) carriers is greater than 40% and indicates the importance of family chromosome studies to identify carriers and to offer genetic counseling and prenatal diagnosis.

Not all pericentric inversions have a risk for abnormal offspring, however. One of the most common inversions seen in human chromosomes is a small pericentric inversion of chromosome 9, which is present in up to 1% of all individuals. The inv(9)(p11q12) has no known deleterious effect on carriers and does not appear to be associated with a significant risk for miscarriage or unbalanced offspring; the empirical risk is not different from that of the population at large, and it is therefore generally considered a normal variant.

اخر الاخبار

اخبار العتبة العباسية المقدسة

شعبة الحرم الشريف تستعد لاستقبال زائري النصف من شهر شعبان

العتبة العباسية المقدسة تنشر لافتات الفرح بمناسبة ذكرى ولادة الإمام المهدي (عجّل الله فرجه)

العتبة العباسية المقدسة تفتتح التسجيل لتأدية الزيارة بالنيابة في ليلة النصف من شعبان

قسم المقام يعلن جاهزيّته لاستقبال زائري النصف من شعبان

المزيد

الآخبار الصحية

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

المزيد

الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

خرافة "ثمانية أكواب يوميا".. كم من الماء يحتاج جسمك فعليا؟

هل توجد حياة خارج كوكبنا؟.. اكتشاف جديد يثير اهتمام العلماء

اكتشاف الخلايا المسؤولة عن فقدان ذاكرة الطفولة

المزيد

مواضيع ذات صلة

Genetic Basis of Cancer

Conditional Mutations Can Be Used to Study Essential Genes in Yeast

Segregation of Mutations in Breeding Experiments Reveals Their Dominance or Recessivity

Recessive and Dominant Mutant Alleles Generally Have Opposite Effects on Gene Function

Genes and Environment in Development

Gene Therapy

The Genetics of mtDNA Disease

The APOE Gene Is an Alzheimer Disease Susceptibility Locus

The Genetics of Alzheimer Disease

Molecular Testing for Spinal Muscular Atrophy

Genetics of Spinal Muscular Atrophy

The Phenotypes of Spinal Muscular Atrophy