4.3: Chromosomal and Genetic Abnormalities

There are abnormalities that might be traced to problems with genes or chromosomes.

Genetic Disorders

Many genetic disorders are dominant gene-linked; however, the vast majority of dominant gene linked disorders are not serious or debilitating. For example, the majority of those with Tourette's Syndrome suffer only minor tics from time to time and can easily control their symptoms. When dominant-gene linked diseases are serious, they do not tend to become symptomatic until later in life. Huntington's Disease is a dominant gene linked disorder that affects the nervous system and is fatal, but does not appear until midlife.

Recessive gene disorders, such as cystic fibrosis and sickle-cell anemia may actually claim more lives because they are less likely to be detected as people are unaware that they are carriers of the disease. Most of the known truly harmful genetic disorders are carried on recessive genes. If they were not, they would have not survived the process of natural selection.

Some genetic disorders are sex-linked; the defective gene is found on the X-chromosome. Males have only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene such as hemophilia, color-blindness, and baldness. For females to be affected by recessive genetic defects, they need to inherit the recessive gene on both X-chromosomes. But if the gene carrying the disorder is dominant, females are equally at risk.

Recessive Disorders (Homozygous):

1The individual inherits a gene change from both parents. If the gene is inherited from just one parent, the person is a carrier and does not have the condition.

Table \(\PageIndex{1}\): Homozygous recessive disorders[1]

Disorder	Description	Cases per Birth
Sickle Cell Disease (SCD)	A condition in which the red blood cells in the body are shaped like a sickle (like the letter C) and affect the ability of the blood to transport oxygen.	1 in 500 Black births 1 in 36,000 Hispanic births
Cystic Fibrosis (CF)	A condition that affects breathing and digestion due to thick mucus building up in the body, especially the lungs and digestive system. In CF, the mucus is thicker than normal and sticky.	1 in 3500
Phenylketonuria (PKU)	A metabolic disorder in which the individual cannot metabolize phenylalanine, an amino acid. Left untreated, intellectual deficits occur. PKU is easily detected and is treated with a special diet.	1 in 10,000
Tay Sachs Disease	Caused by an enzyme deficiency resulting in the accumulation of lipids in the nerves cells of the brain. This accumulation results in progressive damage to the cells and a decrease in cognitive and physical development. Death typically occurs by age five.	1 in 4000 1 in 30 American Jews is a carrier 1 in 20 French Canadians is a carrier
Albinism	When the individual lacks melanin and processes little to no pigment in the skin, hair, and eyes. Vision problems can also occur.	Fewer than 20,000 US cases per year

Autosomal Dominant Disorders (Heterozygous)

In order to have the disorder, the individual only needs to inherit the gene change from one parent.

Table \(\PageIndex{2}\): Autosomal dominant gene (heterozygous) disorders[2]

Disorder	Description	Cases per Birth
Huntington’s Disease	A condition that affects the individual’s nervous system. Nerve cells become damaged, causing various parts of the brain to deteriorate. The disease affects movement, behavior and cognition. It is fatal, and occurs at midlife.	1 in 10,000
Tourette Syndrome	A tic disorder which results in uncontrollable motor and vocal tics as well as body jerking	1 in 250
Achondroplasia	The most common form of disproportionate short stature. The individual has abnormal bone growth resulting in short stature, disproportionately short arms and legs, short fingers, a large head, and specific facial features.	1 in 15,000-40,000

Sex-Linked Disorders

When the X chromosome carries the mutated gene, the disorder is referred to as an X-linked disorder. Males are more affected than females because they possess only one X chromosome without an additional X chromosome to counter the harmful gene.

Table \(\PageIndex{3}\): Sex-linked Disorders[3]

Disorder	Description	Cases per Birth
Fragile X Syndrome	Occurs when the body cannot make enough of a protein it needs for the brain to grow and problems with learning and behavior can occur. Fragile X syndrome is caused from an abnormality in the X chromosome, which then breaks. If a female has a fragile X, her second X chromosome usually is healthy, but males with fragile X don’t have a second healthy X chromosome. This is why symptoms of Fragile X usually are more serious in males.	1 in 4000 males 1 in 8000 females
Hemophilia	Occurs when there are problems in blood clotting causing both internal and external bleeding.	1 in 10,000 males (only 0.5-1.5% of hemophilia sufferers are female)[4]
Duchenne Muscular Dystrophy	A weakening of the muscles resulting in an inability to move, wasting away, and possible death.	1 in 3500 males 1 in 50million females[5]

Chromosomal Abnormalities

A chromosomal abnormality occurs when a child inherits too many or two few chromosomes. The most common cause of chromosomal abnormalities is the age of the mother. As the mother ages, the ovum is more likely to suffer abnormalities due to longer term exposure to environmental factors. Consequently, some gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the mother’s body.[6]

Autosomal Chromosome Disorders

The individual inherits too many or two few chromosomes.

Table \(\PageIndex{4}\): Autosomal chromosomal disorders [7]

Disorder	Description
Down Syndrome/Trisomy 21	Caused by an extra chromosome 21 and includes a combination of birth defects. Affected individuals have some degree of intellectual disability, characteristic facial features, often heart defects, and other health problems. The severity varies greatly among affected individuals.
Trisomy 9 Mosaicism	Caused by having an extra chromosome 9 in some cells. The severity of effects relates to the proportion of cells with extra chromosomes. The effects include fetal growth restriction resulting in low birth weight and multiple anomalies, including facial, cardiac, musculoskeletal, genital, kidney, and respiratory abnormalities.
Trisomy 13	Caused by an extra chromosome 13. Affected individuals have multiple birth defects and generally die in the first weeks or months of life.
Trisomy 18	Caused by an extra chromosome 18 and the affected individual also has multiple birth defects and early death.

Figure \(\PageIndex{1}\): Infant boy with Trisomy 9 Mosaicism.[8]

Figure \(\PageIndex{2}\): Girl with XXX Syndrome.[9]

Sex linked chromosomal abnormality

When the abnormality is on 23rd pair, the result is a sex-linked chromosomal abnormality. This happens when a person has less than or more than two sex chromosomes.[10]

Table \(\PageIndex{5}\): Sex-linked Chromosomal Disorders[11]

Disorder	Description
Turner Syndrome (XO)	Caused when all or part of one of the X chromosomes is lost before or soon after conception due to a random event. The resulting zygote has an XO composition. Turner Syndrome affects cognitive functioning and sexual maturation in girls. Infertility and a short stature may be noted.
Klinefelter Syndrome (XXY)	Caused when an extra X chromosome is present in the cells of a male due to a random event. The Y chromosome stimulates the growth of male genitalia, but the additional X chromosome inhibits this development. The male can have some breast development, infertility, and low levels of testosterone.
XYY Syndrome	Caused when an extra Y chromosome is present in the cells of a male. There are few symptoms. They may include being taller than average, acne, and an increased risk of learning problems. The person is generally otherwise normal, including normal fertility.
Triple X Syndrome (XXX)	Caused when an extra X chromosome is present in the cells of a female. It may result in being taller than average, learning difficulties, decreased muscle tone, seizures, and kidney problems.

Attributions:

Child Growth and Development by Jennifer Paris, Antoinette Ricardo, and Dawn Rymond, 2019, is licensed under CC BY 4.0

[1] Lifespan Development: A Psychological Perspective (pages 36-37) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[2] Lifespan Development: A Psychological Perspective (pages 36-37) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[3] Lifespan Development: A Psychological Perspective (pages 36-37) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[4] Miller, C.H. & Bean, C. J. (2021). Genetic causes of hemophilia in women and girls. Hemophilia, 27(2), e164-e179.. doi: 10.1111/hae.14186

[5] Nozoe, K.T., Akamine, R.T., Mazzotti, D.R., Polesel, D.N., Grossklauss, L.F., Tufik, S., Andersen, M.L., Moreira, G.A.(2016). Phenotypic contrasts of Duchenne Muscular Dystrophy in women: Two case reports. Sleep Sci., 9(3), 129-133. doi: 10.1016/j.slsci.2016.07.004.

[6] Lifespan Development: A Psychological Perspective (page 38) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[7]Trisomy 9 Mosaicism Diagnosed In Utero by Hironori Takahashi, Satoshi Hayashi, Yumiko Miura, Keiko Tsukamoto, Rika Kosaki, Yushi Itoh, and Haruhiko Sago is licensed under CC BY 3.0; Lifespan Development: A Psychological Perspective (page 39) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[8] Image by Ashley Onken used with permission

[9] Image is in the public domain

[10] Lifespan Development: A Psychological Perspective (page 38) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[11] XYY Syndrome by Wikipedia is licensed under CC BY-SA 3.0; Triple X Syndrome by Wikipedia is licensed under CC BY-SA 3.0; Lifespan Development: A Psychological Perspective (page 39) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0