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ABC OF CLINICAL GENETICS - PART 4

Đôi khi UPD có thể phát sinh do thụ tinh của một giao tử monosomic tiếp theo là sao chép của nhiễm sắc thể từ giao tử khác (đơn nhiễm cứu hộ). Kết quả cơ chế này isodisomy uniparental. Về mặt lý thuyết, UPD cũng có thể phát sinh bằng cách thụ tinh của một giao tử | Unusual inheritance mechanisms parent normal two identical chromosomes from one parent isodisomy or two different chromosomes from one parent heterodisomy . Occasionally UPD may arise by fertilisation of a monosomic gamete followed by duplication of the chromosome from the other gamete monosomy rescue . This mechanism results in uniparental isodisomy. Theoretically UPD could also arise by fertilisation of a momosomic gamete with a disomic gamete resulting in either isodisomy or heterodisomy. Uniparental disomy may have no clinical consequence by itself. It is occasionally detected by the unmasking of a recessive disorder for which only one parent is a carrier when there is isodisomy for the parental chromosome carrying such a mutation. In this rare situation the child would be affected by a recessive disorder for which the other parent is not a carrier. Recurrence risk for the disorder in siblings is extremely low since UPD is not likely to occur again in another pregnancy. The other situation in which UPD will have an effect is when the chromosome involved contains one or more imprinted genes as described in the next section. Parents loss of one 0 chromosome 0 JDOCD Gametes Trisomic zygote Disomic zygote Figure 7.3 Uniparental disomy isodisomy due to nondisjunction at meiosis II Imprinting It has been observed that some inherited traits do not conform to the pattern expected of classical mendelian inheritance in which genes inherited from either parent have an equal effect. The term imprinting is used to describe the phenomenon by which certain genes function differently depending on whether they are maternally or paternally derived. The mechanism of DNA modification involved in imprinting remains to be explained but it confers a functional change in particular alleles at the time of gametogenesis determined by the sex of the parent. The imprint lasts for one generation and is then removed so that an appropriate imprint can be re-established in the germ cells of .