Catherine has taught high school science and has a master's degree in biology.
Imagine you are rolling long, rope-like pieces of playdough. You crisscross the two pieces of rope, creating an X-shaped pattern. For each color, you make a duplicate pair and set them next to each other. When your back is turned, someone cuts a section of rope from one color and replaces it with another. When you return, you see that two of your ropes have had sections swapped out, leaving you with two ropes that are mix-and-matched.
This is similar to what happens when chromosomal translocation occurs. Each X-shaped playdough pair is a set of chromosomes, which contains genetic DNA information. Normally, each pair of chromosomes is similar. Matching pairs are called homologous chromosomes; the prefix homo is Greek for 'same'. The only exception is sex chromosomes, which do not have a pair. The rest are paired autosomal chromosomes.
In the playdough mix-match, it was not two homologous (same colored) chromosomes that were swapped. Instead, two different colored X's, called nonhomologous chromosomes (think of 'non-homo' meaning 'not the same'), were exchanged. Translocation occurs when a section of genes from two nonhomologous chromosomes are interchanged. More specifically, reciprocal translocation took place in the above example, because the mismatched pieces were interchanged on the nonhomologous chromosomes.
Heterozygous Reciprocal Translocation
Remember that each chromosome is shaped like two ropes that cross each other to form an X shape. In addition, each has a matching homologous pair. Imagine each pair of normal chromosomes as the same color playdough pieces.
Sometimes one X, or chromosome, in the set has been left untouched, while the matching X has a section that has been exchanged. This is called a heterozygous reciprocal translocation, because one chromosome of the pair remains normal, while the other has an interchanged part. The prefix hetero means 'not the same,' signifying that the two pairs are no longer alike, since one chromosome has an abnormality.
Most organisms with a heterozygous reciprocal translocation are semi-sterile. They produce about half of the live offspring as an organism that did not have this genetic mutation. This is because, as the chromosomes divide during meiosis, a cellular process used in reproduction, there is a 50% chance of the offspring getting the mutation.
Homozygous Reciprocal Translocation
A homozygous reciprocal translocation occurs when both pairs of homologous chromosomes have the same abnormality. Remember from above that the prefix homo means the same, signifying that both pairs have been similarly changed. If we picture a homologous chromosome set as two matching colored playdough Xs, then each X in the set would have a mismatched color piece. That's is because, in this case, each X (or chromosome) has an identically mismatched gene fragment from a nonhomologous chromosome.
If we imagine a chromosome as being made from two crossed rope pieces in the shape of an X, the point where they crossed would be the centromere. The centromere plays an important role during cell division, and it holds the chromosome together as a whole. In a Robertsonian translocation, the centromere regions of a nonhomologous chromosome are fused. The result is a single, merged centromere holding a mismatched chromosome together. Down syndrome ensues when this type of gene rearrangement occurs at chromosome 21.
Chromosomes are the carriers of genetic information, and when two unlike, or nonhomologous, pairs are mismatched, the result is a translocation. Translocation can occur on just one of the pairs of matching, or homologous, chromosomes; this is termed a heterozygous reciprocal translocation. Heterozygous reciprocal translocation leads to semi-sterility, which results in half the number of viable offspring. When both pairs of chromosomes each have a similar exchanged fragment, it is called a homozygous reciprocal translocation. Robertsonian translocations combine the centromere region of the chromosome, and are implicated in Down syndrome.
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