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In this article we will discuss about the translocation in humans.
Interchanges are the most common structural chromosome changes reported in human. Partial sterility in interchange heterozygotes is infrequent due to the production of balanced gametes and as a result, interchanges in human are identifiable only through the analysis of karyotype, usually coupled with chromosome banding patterns.
Various kinds of translocations have been reported in human, so that every arm of the chromosome complement has been reported as being involved in translocation. Among the reciprocal translocations “centric fusion” is the most frequent.
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This type of interchange, termed as Robertsonian translocation, occurs between two acrocentric chromosomes resulting in one small chromosome possessing a centromere and minute arms and another large metacentric chromosome.
Such translocations mostly occur in the D (13, 14, 15) and G group (21, 22) of human chromosomes as D/D, D/G or GIG translocations, the most frequently involved chromosomes being 14 and 21. The chromosome region near the centromere is heterochromatic which is liable to breakage more frequently than the euchromatin regions; this may be the reason for a high frequency of the Robertsonian translocations.
In Robertsonian translocations, the small chromosome possessing the centromere and minute arms may be devoid of genes and, as a result, it may be lost. There may be no genetic effects due to such loss, but the chromosome number of the individual is reduced to 45. In such a case, meiosis would show an association of three chromosomes: i.e., two normal and one large metacentric trans-located chromosome (Fig. 14.16).
One arm of the trans-located chromosome is homologous to one normal chromosome, while its other arm is homologous to the other normal chromosome. In alternate disjunction, both the normal chromosomes move to one pole, while the trans-located chromosome moves to the opposite pole resulting in the production of balanced gametes.
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But in case of nondisjunction, two adjacent (one normal other trans-located) chromosomes move to the same pole; fertilization of such a gamete with a normal gamete will produce an offspring having 46 chromosomes of which the trans-located chromosome carries an arm which is the duplication of almost one complete chromosome.
Therefore, such an individual will be, in effect, trisomic for the concerned chromosome although its chromosome number will be normal (2n = 46). If the gamete is of (1421+21) constitution, the resulting individual will be trisomic for chromosome 21, and phenotypically a Down’s syndrome (Fig. 14.16). Other kinds of interchanges in human were reported in different chromosomal groups such as, A/A, A/B etc.
In reciprocal translocation heterozygotes, 3 : 1 disjunction in the quadrivalent may occur, producing n+1 and n-1 types of gamete. After fertilization of such a gamete with a normal one, monosomic (45 chromosomes) or trisomic (47 chromosomes) offspring would be born in a translocation heterozygote.
A total of 20 possible combinations of chromosomes can be derived from 3 : 1 disjunction at Al and nondisjunction at AIL Some of them are given in Fig. 14.6.
A case of reciprocal translocation reported by Lindenbaum and Bobrow in 1975 is presented here as an example. A female was carrier for the translocation involving the long arms of chromosomes 7 and 21; her karyotype can be written as 46, XX, t (7 : 21) (q31 ; q22). She produced three sons of which two were translocation heterozygotes, while the remaining one son showed the characteristics of Down’s syndrome.
This individual was an interchange trisomic with the constitution of 47, XY, -7+der (7), +der (21), t(7 : 21) (q31; q22) mat. In a review paper, Lindenbaum and Bobrow in 1975 concluded that nearly all the aneuploid individuals resulting from 3 : 1 disjunction are born to female translocation heterozygotes.
The mean age of maternal carriers at birth of aneuploids is not different form the age of normal females. Translocations involving G group (21, 22, Y) and D group (13, 14, 15) chromosomes are found more often in families presenting with 3 : 1 disjunctions than those identified through unbalanced 46-chromosome propositi.
One of the reasons may be the differential viability of the resulting aneuploid offspring. Factors for 3 : 1 segregation of chromosomes in a reciprocal translocation include involvement of an acrocentric chromosome, short interstitial segments, extreme disparity in the lengths of the chromosomes involved and l”1 + l1 or 1IV configurations at MI of meiosis.
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Interchange trisomy is generally identified in association with trisomy 18 or trisomy 21 phenotype. Reproductive risks to translocation carriers due to a 3 : 1 disjunction can be divided into the following three categories: (1) Risk of abortion, (2) Risk of chromosomally unbalanced (duplication-deficiency) offspring’s following 2 : 2 disjunction, and (3) Risk of further episode of 3 : 1 disjunction.
The abortion rate is higher (35-38%) in the families presenting with 3 : 1 disjunction than the offspring’s of reciprocal translocation carriers in general. General abortion rates for reciprocal translocation has been estimated to be 27.6% for female carriers and 22.2% for male carriers.
