Diagnosis
Changes in Genes and Chromosomes
Chromosomes
Changes in genes and chromosomes usually happen when a cell divides. This process of cell division works well most of the time, but sometimes a change in the order or number of the genes occurs. These changes are called mutations. Genetic changes can be improvements or adaptations to changes in the environment. They may not cause any problem but be passed on to future children. They can also cause serious medical issues or disabilities. Below are some of the genetic changes that can occur.
Abnormal Number of Chromosomes
Trisomy – During the
copying of the genes, sometimes an extra chromosome is added. This
results in a trisomy, or three chromosomes instead of two. An example of
a common trisomy is Down Syndrome, where the individual has 3 copies of
chromosome 21 (Trisomy 21). Other examples are Trisomy 13 (Patau)
Syndrome, and Trisomy 18 (Edwards) Syndrome.
Monosomy
– In this case, rather than having an extra chromosome, one chromosome
is missing. One example of this is Turner Syndrome, where all or part of
one of the X chromosomes is missing.
Translocations, Deletions, and Inversions
Deletions – In deletions, the chromosomes may be missing parts or be shaped differently. A chromosome may be missing one gene, a few genes, or a section of genes. Examples include Wolf-Hirschhorn Syndrome (affects chromosome 4), Cri-du-chat syndrome (chromosome 5), DiGeorge syndrome (chromosome 22), and Williams syndrome (chromosome 7).
Translocations– In about one in every 500 births, tiny bits of chromosomes have shifted from one chromosome to another. Most of the time there is no gain or loss of genetic material, and the child is unaffected.
Inversions- In about one in 100 births, small parts of the DNA have been taken out of one chromosome and put into another one, but flipped over.
Extra or Missing Sex Chromosomes – Sometimes problems are caused by an extra or missing X chromosome (female) or Y chromosome (male). The usual combination for a girl is XX and for a boy is XY. Boys with Klinefelter Syndrome are born with XXY or XXXY. A missing X chromosome is called Turner Syndrome.
X-Linked Chromosome Changes – Some changes have been identified when the X chromosome has a change in a gene. Fragile X, hemophilia (a bleeding disorder), and Duchenne Muscular Dystrophy are examples of X-linked disorders.
Gene Mutations
Sometimes a change occurs in just one gene. The gene is present but has been altered in some way. Mutations can occur spontaneously, or in response to radiation, a virus, or a chemical substance. Some examples of genetic illnesses caused by inheriting a single gene include: phenylketonuria (PKU), cystic fibrosis, sickle cell anemia, Tay-Sachs disease, and achondroplasia (a type of dwarfism).
Cancer-linked Genes (Oncogenes)
Some genes have been shown to be associated with a higher risk of certain kinds of cancer. Family history of certain kinds of cancer have been linked to these genes.