CHANGES TO CHROMOSOME STRUCTURE - TRANSLOCATIONS

Important points
• In each human cell, except the egg and sperm cells, there are 46 paired chromosomes of varying size
• One chromosome of each pair is inherited from each parent
• The autosomes are chromosomes numbered 1-22 (largest to smallest)
• The two sex chromosomes are called X and Y
• Egg cells contain 23 chromosomes, made up of 22 autosomes and an X
• Sperm cells contain 23 chromosomes, made up of 22 autosomes and an X or a Y
• When the egg and sperm join at conception, the baby will have 46 chromosomes in its cells, just like the parents
• Changes in the number, size or structure of chromosomes in the cells of an individual may cause a chromosomal condition that
affects growth, development and health
• A particular type of chromosomal structural change is called a translocation. There are two different types of translocations:
– Reciprocal translocation - material is exchanged between any of the chromosomes and involves pieces of any size
– Robertsonian translocation - material is exchanged between chromosomes 13, 14, 15, 21 and 22
• Where there does not appear to have been any loss or gain of chromosome material, the translocation is described as balanced
• An unbalanced translocation means that an individual has more or less chromosomal material than usual
• A parent who is a carrier of a translocation may not have any problem with their growth, development and health but,
depending on the chromosomes involved, they may experience reproductive problems such as infertility, miscarriage and
having a child with an unbalanced chromosomal complement
– Generally, when a mother is a carrier of a balanced Robertsonian translocation between chromosomes 14 and 21, there is a
substantial risk of the child having Down syndrome of the translocation type
• If a child has the same balanced chromosomal translocation in their cells as their parent, the child is unlikely to be affected by
the chromosomal rearrangement
• Diagnostic testing is available in pregnancy where one parent carries a ‘balanced’ chromosomal rearrangement