CHANGES TO CHROMOSOMES – NUMBER, SIZE AND STRUCTURE

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 a person may cause a chromosomal condition that
affects growth, development and health
• Chromosomal changes can be inherited from a parent
• Chromosomal changes can also occur when an egg cell or sperm cell is formed or during or shortly after conception
• Chromosomal conditions can be due to having:
– Extra or fewer copies of the autosomes or the sex chromosomes; eg Down syndrome (3 copies of chromosome 21),
Klinefelter syndrome (boys with XXY) and Turner syndrome (girls with only one copy of the X chromosome)
– Extra or less of parts of individual chromosomes (duplications and deletions)
– Chromosomes with structural abnormalities including forming rings or rearrangement of the material (translocations)
– Both copies of a chromosome pair from one parent rather than a copy from each parent
• When the chromosomal change is only in some cells of the body rather than in all the cells, a person is ‘mosaic’ for the
chromosomal change
• The impact of a chromosomal change will depend on
– The type of change
– The chromosomes (and therefore genes) affected by the change
– The number and type of cells that contain the change
• The chance that a child will have a chromosomal change depends on the parents’ family health history, the mother’s age at
the expected date of delivery and the type of change involved
• Testing in pregnancy is available to
– Determine if the pregnancy is at risk for a chromosomal difference
– Diagnose a chromosomal condition where indicated