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Basic Genetics
Birth Defects and Genetic Conditions
Birth defects/genetic conditions are physical or functional problems in an individual. Some begin before birth and may be obvious at the time of birth or they may be found later. Birth defects/genetic conditions can affect:
- the way a child looks, such as a cleft lip
- the internal organs, such as a hole in the heart or a missing kidney
- how a child is able to learn and develop.
One out of 20 babies is born with some type of birth defect.
Birth defects and genetic conditions:
- Some genetic conditions affect the chromosome material in the human cell
- Some genetic conditions affect the DNA or genes in the human cell
- Some genetic conditions follow inheritance patterns in families, such as Autosomal Dominant, Autosomal Recessive or X-linked
- Many isolated birth defects occur because there is already an underlying genetic predisposition that interacts with other environmental conditions (multifactorial disorders)
- Some birth defects are non-genetic, but may be occur because the developing fetus was affected by a viral infection or a medication/drug taken during pregnancy (teratogenic concerns)
Often the exact cause of the problem cannot be found, and many birth defects cannot be identified before birth.
The diagrams are used with permission from Greenwood Genetic Services.
The Human CellCells are the building blocks of our body. There are many different types of cells like heart cells, skin cells, blood cells, etc. They all have similar compartments including a nucleus that holds our genetic material.
ChromosomesChromosomes are microscopic rod-shaped bodies that contain genetic information. Most cells in the human body contain 46 chromosomes. Chromosomes are usually analyzed by growing white blood cells in culture and then treating them with special stains that bring out the characteristic bands that can be seen in the chromosome spread.
Normal human chromosomes come in 23 pairs. One chromosome from each pair comes from the mother and one from the father. There are 22 AUTOSOMES numbered 1-22 that are common to both sexes and two sex chromosomes. Females have two X chromosomes, and males have one X and one Y chromosome.
There are many birth defects and medical conditions caused by extra or missing chromosome material. This may involve one or more autosomes or one or more sex chromosomes. These are two examples:
- The chromosome abnormality in Down Syndrome is an extra chromosome #21. The figure shown has three chromosome #21s instead of the normal pair, thus the term “TRISOMY 21”. This occurs in 95 per cent of people with Down Syndrome.
- Klinefelter Syndrome is an example of a chromosome abnormality involving the sex chromosomes. This figure shows an extra X chromosome in addition to the Y chromosome, which makes him male. The extra X chromosome can lead to problems with fertility.
DNA and GenesA gene is a segment of DNA. Genes give instructions that tell our cells they need to build a protein. They are like the recipes for proteins. There are over 30,000 genes in each cell of our body.
Patterns of InheritanceWhat is an Inheritance Pattern?
Sometimes we see a condition that is passed through that family in a particular way. Depending on whether:
- The condition is caused by a change in a gene
- What chromosome that gene is located on
- What kind of change is made to that gene
- Whether the condition is caused by a change in one copy of the gene or both copies of the gene
There are three types of patterns: Autosomal Dominant, Autosomal Recessive, and X-Linked.
Autosomal Dominant
A person affected with an Autosomal Dominant genetic condition usually has one changed Dominant gene (D) and one normal gene (D). With each pregnancy, the chance is either 50 per cent that the baby (male or female) will inherit the normal gene or 50 per cent that the baby will inherit the Dominant gene (D) for the genetic condition. The person who inherits the normal gene from each parent cannot transmit the gene (or the disorder) to his or her children because they did not receive the changed gene.
What are some examples of Autosomal Dominant Conditions?
- Neurofibromatosis
- Marfan syndrome
- Achondroplasia
- Hereditary Breast/Ovarian Cancer
- Huntington’s disease
Autosomal Recessive
A carrier of an Autosomal Recessive condition has one normal gene (R) and one recessive or changed gene (r). Carriers are healthy individuals. When both parents are carriers of a gene for the same recessive disease, they have a 25 per cent chance that a child (male or female) will receive the recessive changed gene (r) from both of them and be affected with the disease. These parents also have a 25 per cent chance of both passing on their normal gene (R) resulting in a completely normal child who is not a carrier for the disorder. In addition, there is a 50 per cent chance that each child will receive the recessive changed gene (r) from one parent and the normal gene (R) from the other parent, which results in a normal carrier just like both parents. The chance of inheriting a recessive disorder is increased in a child whose parents are “blood” relatives, or consanguineous.
What are some examples of Autosomal Recessive Conditions?
o Albinism
o Cystic Fibrosis
o Tay-Sachs disease
o Thalassemia
o Phenylketonuria (PKU)
X-linked
Normal females have two X chromosomes, while normal males have one X and one Y-chromosome. A carrier of an X-linked genetic condition is usually normal herself. She carries a normal gene on one of her X chromosomes and a changed gene (Xr) on the other X chromosome. With each pregnancy, male offspring will have a 50 per cent chance of inheriting the changed gene (Xr) and having the condition. Female offspring have a 50 per cent change of being a carrier like their mother.
What are some examples of X-Linked Conditions?
- Duchenne Muscular Dystrophy
- Fragile X Syndrome
- Hemophilia
- Colour Blindness
Multifactorial Disorders These types of disorders result from an interaction of several genes with environmental factors. The inheritance pattern in this group of disorders is less well defined and risk depends on observation of large populations. It is known that recurrence risks are increased in close family members or if more than one family member is affected.
Multifactorial Disorders Present at Birth
Some common birth defects can occur as a single isolated condition and are often caused by many factors. With one affected child in the family, the chances of having the condition reoccur are one to five per cent depending on the type of birth defect.
Common Examples of Multifactorial Birth Defects are:
- Cleft lip/and palate
- Club foot
- Congenital Heart Defects
- Neural Tube Defects (Spina Bifida)
Multifactorial Disorders that occur after Birth
Examples of multifactorial disorders that develop after birth and in adulthood include:
- Diabetes Mellitus (Type II)
- Epilepsy (some forms)
- Multiple Sclerosis
- Psychiatric Illnesses (some)
- Heart Disease
Examples of environmental factors that affect adult onset disorders include:
Teratogenic Disorders
The use of prescription medication, over the counter drugs, herbal medication, recreational drugs, alcohol, smoking, chemical exposures, maternal illness, maternal infections and other exposures during pregnancy sometimes brings about concerns about risks to an unborn baby. Not every exposure will pose risks to your unborn baby; however, some do.
We encourage you to speak to your doctor about the relative risks and benefits of any prescription drug therapy. We do not advise you to suddenly discontinue your prescription drug therapy unless it is under the guidance of your physician. Over the counter medication or naturopathic medications should not be taken without first consulting your physician.
The Provincial Medical Genetics Program offers telephone and written information to health care professionals regarding exposures and risks to a pregnancy (604) 875-2157. We will accept referrals from physicians for pregnancies at increased risk for birth defects. [Referral Form]
For more information:
Motherisk at www.motherisk.org
Motherisk's Home Line: (416) 813-6780 - for information about the risk or safety of prescription and over-the-counter drugs, herbal products, chemicals, x-rays, chronic disease and infections during pregnancy. [This is a toll call to the Motherisk Toronto call centre. Callers may have to wait up to 10 minutes to speak to a counsellor.]
Alcohol and Substance Use Helpline: 1-877-327-4636 - for information about the fetal effects of alcohol, nicotine and drugs like marijuana, cocaine and ecstasy.
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