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Aneuploidy
The most common type of chromosome abnormality is having too many or too few chromosomes. This is called aneuploidy. Aneuploidy is always associated with physical and/or mental developmental problems. The condition at birth is directly related to the type of chromosome abnormality present in the embryo at the time of conception.
Having an extra chromosome is called trisomy and missing a chromosome is monosomy. If the extra or missing chromosome is an autosome (chromosomes 1 to 22), the embryo may not implant or may stop normal development soon after attaching and undergo a spontaneous abortion. If the aneuploidy involves chromosomes 13, 18, 21 X or Y, the embryo may implant and carry to term. Down syndrome (trisomy 21) is the presence of three copies of chromosome 21.
Figure: The fertilization of a genetically abnormal egg carrying an extra chromosome 21 (tan) by a normal sperm (green) produces an embryo with Down syndrome (purple).
Figure: Fluorescence in situ hybridization
 
Patau syndrome (trisomy 13) is the presence of three copies of chromosome 13. Edward syndrome (trisomy 18) has three copies of chromosome 18. Other common aneuploidies seen at birth include Klinefelter syndrome and Turner syndrome. Klinefelter syndrome is the presence of an extra sex chromosome (47,XXY), whereas Turner syndrome is missing a sex chromosome (45,X). Embryos affected with Klinefelter syndrome or Turner syndrome may also spontaneously abort.
Structural Chromosome Abnormalities:
Translocations
There are two types of structural chromosome aberrations, Robertsonian and reciprocal translocations. Translocations occur when pieces of a chromosome are attached to the wrong chromosome.
Robertsonian
Robertsonian translocations are the joining together of chromosomes 13, 14, 15, 21 or 22. People with a Robertsonian translocation are normal because they have the correct amount of genetic material (genes). Sperm and eggs from individuals carrying a Robertsonian translocation either contain the correct amount of genetic material (be balanced) or contain an unbalanced amount of genetic material (unbalanced). If sperm or an egg contains an unbalanced amount of genetic material and fertilization occurs, the resulting embryo will have too many copies or parts of one chromosome and too few copies or parts of the other. This results in too many or too few normal genes on a chromosome. An unbalanced state in an embryo may lead to embryo death, miscarriage, or the live birth of an infant with substantial medical problems.
Uniparental Disomy (UPD) and Robertsonian Translocations
Genomic imprinting is defined as the differential expression of genes based on their parent of origin. Imprinting plays an important role in early development. Disrupted imprinting can give rise to birth defects. The fetus can have physical abnormalities and intrauterine growth retardation. Since embryos with UPD of some chromosomes are at risk for medical complications, UPD testing may be considered. Testing can be done for UPD by comparing the DNA from each parent to the DNA of the embryo.
Translocations
Reciprocal translocations are the exchange of chromosomal material between the wrong chromosomes. If this exchange breaks a gene, this person will have a genetic disease. However, if the amount of genetic material present is the same as with normal individuals, the person is balanced and normal. However, the sperm or eggs of these individuals can carry the reciprocal translocation chromosome and are at increased risk of producing an embryo with an abnormal amount of genetic material (be unbalanced). As with Robertsonian translocations, the couple is at increased risk for repeated miscarriages, repeated unsuccessful IVF cycles or the birth of a child with a genetic disorder.
Figure: Reciprocal translocation between chromosomes 4 and 20. This results in part of chromosome 20 attaching to chromosome 4 (derivative 4) and part of chromosome 4 attaching to chromosome 20 (derivative 20).
Chromosome Deletions
Deletions are the loss of a chromosome segment resulting in an imbalance in the number of genes present. If the deletion removes genetic material, the individual will have a genetic disorder. Cri du Chat, Prader-Willi and Angelman’s syndrome are examples of genetic disease caused by a chromosome deletion.
Figure: Genetic material C and D are lost from the chromosome.
Chromosome Inversions
Inversions occur when a single chromosome breaks in two places and the material in-between is reconstituted upside down. If the chromosome breaks and does not disrupt any gene, people with an inversion are normal. However, if a gene sequence is altered, the individual will have a genetic abnormality. One inversion on chromosome 16 may cause a type of leukemia. The presence of an inversion chromosome during egg or sperm development can result in gametes with a duplicated and/or deleted portion of the inversion chromosome. This is considered an unbalanced state. Embryos with too many or too few copies of genes from this inverted chromosome can fail to grow, miscarry or be liveborn with substantial medical problems.
Figure: The chromosome breaks in two places and is reformed upside down.
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