8. Understanding Basic Genetics
Human cells contain 46 chromosomes each (diploid number); each sperm and each egg contain 23 chromosomes (haploid number). Chromosomes
are composed of DNA and proteins that contain genes. Each cell contains
23 pairs of chromosomes—one chromosome of each pair is inherited from
each parent.
Of the 23 pairs, 22 are autosomes
(nonsex chromosomes) and one pair is the sex chromosomes, X and Y. In a
normal woman, the sex chromosomes are represented by two X (XX)
chromosomes, and in the normal man by X and Y (XY) chromosomes.
Chromosome analysis usually involves
blood cells. Chromosomes are viewed through a microscope, then
photographed and enlarged. They are then arranged in pairs, called a karyotype; a karyotype is used to identify abnormalities in the number of chromosomes.
Healthcare providers can examine
individual chromosomes for abnormalities. We have learned that too much
or too little chromosome material can cause an abnormality.
9. Incidence of Genetic Disease
The hoped-for and anticipated outcome
for every pregnancy is a normal, healthy baby. It is expected and taken
for granted. However, that is not always the case. We believe nearly
half of all first-trimester miscarriages are caused by some type of
chromosomal abnormality. About 1% of all babies born have a chromosomal abnormality; 4 to 7% of perinatal deaths (death after birth) are attributed to chromosomal problems.
Among newborns, 3 to 5% are born with a
major birth defect, such as a heart defect; many of these birth defects
have a genetic component. In the United States, this represents as many
as 225,000 children each year.
Mental retardation is another area of
concern. It is estimated that genetic problems cause 70% of mental
retardation. Genetic counseling may offer an indication that this might
occur.
10. Chromosomal Abnormalities
Chromosomal abnormalities can occur in any cell, and they can occur as an abnormal chromosome number or as an abnormality in the structure
of the chromosome itself. With Down syndrome, an individual has 47
chromosomes instead of the normal 46. With Turner’s syndrome, the
individual has 45 chromosomes.
A loss, a gain or a repositioning of
chromosome material identifies a structural abnormality. Terms used to
describe these various conditions include additions, duplications, deletions, inversions and translocations.
A couple of possible causes of
chromosomal abnormalities are advanced parental age and radiation
exposure of mother, father or fetus, which can affect gene regulation. Advanced parental age
usually refers to the mother’s age, but some research has also
implicated the father’s age. Age is a factor for the mother because we
believe females are born with all the eggs they will ever have. Women
are exposed to radiation and other teratogens in the environment
throughout their lives. These exposures have an additive effect—the
more teratogens a woman is exposed to, the greater the chance of
damaging her eggs.
Radiation exposure, such as
X-rays, can damage genetic material and affect gene function and
regulation. Genes control development of the fetus, its organs and
growth. Three of the most common examples of chromosomal abnormalities
are discussed below.
Down syndrome is the most common
chromosomal abnormality; it occurs in about 1 in 800 births. An extra
chromosome causes this syndrome. Those born with Down syndrome can live
fairly long lives and may be moderately to severely mentally retarded.
About half have congenital heart disease; particular physical
abnormalities are common to the syndrome.
Trisomy 18 (Edwards’ syndrome) is a severe chromosome abnormality occurring in approximately 1 in 6000 births. An abnormal chromosome 18 causes the syndrome. Babies with trisomy 18 have multiple abnormalities of major organs.
Trisomy 13 (Patau’s syndrome)
occurs about once in 5000 births. An extra chromosome 13 causes the
syndrome. Babies born with trisomy 13 may have multiple abnormalities.
Sex-Chromosome Abnormalities
Abnormalities of sex chromosomes are
relatively common—1 in every 500 births. It is believed this type of
abnormality causes about 25% of all miscarriages. We discuss three of
these syndromes below.
Turner’s syndrome occurs about
once in every 10,000 female births. It is the most common chromosome
abnormality identified in miscarriages. Instead of having two X
chromosomes, a baby girl born with Turner’s syndrome has only one X
chromosome, called 45X. A mature girl with Turner’s syndrome is
short, sexually underdeveloped, with a webbed neck, very small ovaries
and often heart and kidney problems. The condition may not be
identified until she reaches puberty.
Klinefelter’s syndrome is found in
approximately 1 in every 1000 males. The most common characteristic is
very tall stature when the boy reaches maturity. These boys are born
with an extra sex chromosome, thus the 47XXY label. A boy is underdeveloped sexually and has small testes.
Fragile-X syndrome (also called X-linked)
is one of the most common causes of mental retardation in males. Major
physical characteristics include large ears, large hands and
language-development delays.
Congenital Infections
Rubella was one of the first maternal
infections researchers identified as the cause of fetal malformations.
Other infections that can cause malformations include cytomegalovirus
(CMV), toxoplasmosis, herpes simplex, parvovirus 19 and syphilis.
Congenital infections can cause problems
in a baby. These range from major birth defects, such as heart defects,
to newborn infection at birth.
Velocardiofacial Syndrome (VCFS)
Velocardiofacial syndrome (VCFS) is a genetic condition that may be hereditary. It is known by many names, including Shprintzen syndrome, craniofacial syndrome and conotruncal anomaly face syndrome. VCFS is one of the most common syndromes in humans, second only to Down syndrome in frequency.
Health Risks of Older Pregnant Women by Age
| Risk |
30-34 |
35-39 |
40+ |
| Uterine bleeding |
8% |
9% |
9% |
| Diabetes |
35% |
48% |
65% |
| Heart disease |
7% |
8% |
9% |
| Hypertension |
34% |
38% |
47% |
VCFS is characterized by various medical
problems. The immune system, endocrine system and neurological system
may be involved. Symptoms do not all occur 100% of the time. Most
people with VCFS exhibit a small number of problems; many problems are
relatively minor.
The exact cause of velocardiofacial
syndrome is unknown; however, investigators have identified a
chromosomal defect in people with VCFS. Most children who have been
diagnosed with this syndrome are missing a small part of chromosome 22.
It is estimated that VCFS is
inherited in only 10 to 15% of cases. Most of the time neither parent
has the syndrome nor carries the defective gene. The occurrence of
congenital heart disease is most often the leading factor in diagnosis.
