Which of the following genetic or chromosomal abnormalities is correctly described

Topic Resources

• Hereditary genetic disorders are passed down from generation to generation.

• Spontaneous genetic disorders are not passed down from one generation to another, but they occur when genetic material in the father’s sperm or mother's egg cells or in the cells of the developing embryo is damaged by chance or by drugs, chemicals, or other damaging substances (such as x-rays).

• Before women become pregnant, they and their partner should speak with their health care practitioner about their risk of having a baby with a genetic disorder.

• Risk factors include older age in the woman, a family history of genetic abnormalities, a previous baby with a birth defect or miscarriage, and a chromosomal abnormality in one of the prospective parents.

• Testing for genetic disorders is offered to all women but is particularly important if a couple’s risk is higher than normal.

• Using sperm from another man if the man has an abnormal gene

• Using an egg from another woman if the woman has an abnormal gene

Couples should take time to absorb the information and should ask any questions they have.

All pregnancies involve some risk of genetic abnormalities.

The following percentage of all babies have an abnormality:

• 0.5% have a chromosomal disorder.

• 1% have a single-gene (mendelian) disorder.

• 1% have a disorder caused by defects in several genes.

The percentage of stillborn babies with an abnormality is even higher.

However, certain conditions increase risk.

Some birth defects, such as cleft lip or palate Cleft Lip and Cleft Palate A cleft is an opening that can form in the lip (cleft lip), roof of the mouth (cleft palate), or both if the tissue does not join together completely during pregnancy. Cleft lip and cleft palate... read more

• Family history: The risk of having a baby with a neural tube defect is increased by having a family member, including the couple’s children, with such a defect (family history). For couples who have had a baby with spina bifida or anencephaly, the risk of having another baby with one of these defects is 2 to 3%. For couples who have had two children with one of these defects, the risk is 5 to 10%. However, about 95% of neural tube defects occur in families without a history of neural tube defects.

• Geographic location: Risk also varies based on where a person lives. For example, risk is higher in the United Kingdom than in the United States.

A few neural tube defects result from hereditary abnormalities in a single gene, from chromosomal abnormalities, or from exposure to drugs.

Several factors increase the risk of having a baby with a chromosomal abnormality:

• Woman’s age: The risk of having a baby with Down syndrome increases with a woman’s age—steeply after age 35. The chance of having a baby with Down syndrome is about 1 in 365 at age 35 and 1 in 100 at age 40.

• Family history: Having a family history (including the couple’s children) of a chromosomal abnormality increases the risk. If a couple has had one baby with the most common form of Down syndrome (trisomy 21) and the woman is younger than 30, the risk of having another baby with a chromosomal abnormality is increased to about 1%.

• Chromosomal abnormality in a prospective parent: Rarely, a prospective parent has a structural chromosomal abnormality that increases the risk of having a baby with a structural chromosomal abnormality. A chromosomal abnormality in one or both parents increases the risk, even if the affected parent is healthy and has no physical sign of the abnormality. Doctors suspect such an abnormality when couples have had several miscarriages, problems with infertility, or a baby with a birth defect. For such couples, the risk of having a baby with a serious chromosomal abnormality is increased, as is the risk of miscarrying.

Some chromosomal abnormalities cannot be detected by standard chromosomal testing. Most of these abnormalities are too small to be seen with a microscope, so they are sometimes called submicroscopic abnormalities (or copy-number variants). For example, a very small part of a chromosome may be missing (called a microdeletion), or a chromosome may have a very small extra part (called a microduplication). How often microdeletions and microduplications occur is unknown. However, about 6% of children with structural birth defects have them, even though results of standard chromosomal testing are normal. Tests called chromosomal microarray testing can detect microdeletions and microduplications. Doctors may offer microarray testing before birth in certain circumstances—for example, when birth defects are suspected in a fetus.

However, girls must inherit two abnormal genes to develop an X-linked disorder if the gene is recessive. If the abnormal gene is dominant, only one abnormal gene is required for the disorder to develop.

If the prospective mother and father are related, they are more likely to have the same mutation in one or more of the genes that cause autosomal recessive disorders. Thus, the risk of such disorders is increased.

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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