Читать книгу Getting Pregnant For Dummies - Sharon Perkins - Страница 68

There are genetic causes of infertility that can be passed down from generation to generation, but these usually involve much less of the genetic code. Some are single gene mutations, and some are structural problems with a part of a chromosome being rearranged but not entire chromosome changes.

For males, myotonic dystrophy is an inherited disease that is called an autosomal dominant. This means that if the person has the mutation in just one of the chromosomes, he will have the disease. Thus 50 percent of his offspring will also have the disease. The problem is caused by areas of the gene, which produces a protein required for normal functioning, have unwanted repeated sequences of the letters. The general term for this type of problem is nucleotide repeat diseases, and women can have a similar problem causing the fragile X syndrome. Some of these men have sperm and thus can have children, but some will have no sperm and be sterile. Males can inherit genetic diseases affecting fertility that are single gene defect problems. For these, a mutation of many mutations within a gene causes the gene to malfunction. An example of this is cystic fibrosis. A man with cystic fibrosis will have no sperm in the ejaculate because the tubes that transport the sperm from the testicle to the penis (the vas deferens) do not develop. This is called congenital bilateral absences of the vas deferens and can be treated using sperm extraction from the testis and IVF/ICIS.

One problem for females that has some genetic basis is polycystic ovary syndrome. No single gene has been identified that causes PCOS. Rather, there are a number of genetic mutations that can cause PCOS. Also, patients with PCOS have DNA that has been modified so that the directions for constructing the human are not read correctly — these modifications are called epigenetic factors. Epigenetic modification of DNA helps explain how the environment can alter the way the genetic code is read, and it plays a major role in a number of diseases.

Another type of problem can occur when there are too many copies of a three-letter word, for example egg (a genetic sequence). Fragile X syndrome is an example of this type of genetic error. Fragile X syndrome results when a region of a gene called the FMR1 gene has too many repeated sequences of the genetic word cgg. The FMR1 gene produces a protein that regulates other proteins to make normal nerve connections. The mutation can result in individuals with severe mental compromise. A family with members with severe mental compromise may benefit from genetic testing to determine whether the family has the abnormal FMR1 gene. A normal number of repeats is 5 to 50. If the sequence has more than 200 repeats, then the result is developmental abnormalities in varying degrees. However, where there are 50–200 repeated sequences, the person may have some developmental problems, or a female may have early ovarian failure. So experts often test a woman with premature ovarian failure to determine whether she has too many repeat sequences.

There are a number of single gene defects that affect the fertility of a person. These are being diagnosed more and more so the list is becoming quite long. It is beyond the scope of this chapter to fully explore these diseases.