Chromosomal and Sex-Linked Genetic Disorders
Altered Heterochromosome Numbers
Several changes can occur in the number of heterochromosomes, ranging from the duplication of X and Y chromosomes to the presence of four X chromosomes in both women and men. The main heterochromosomal aberrations are:
Turner Syndrome
Only occurs in women. It results from changes in the number of X heterochromosomes, originating from the union of a gamete carrying an X chromosome with a gamete lacking it. This monosomy causes: cardiovascular disease, moderate mental retardation, infantile features, and absence or atrophy of the uterus, fallopian tubes, and ovaries, leading to sterility.
Klinefelter Syndrome
Only occurs in males. It results from a variation in the number of heterochromosomes. Karyotypes vary between 47,XXY and 49,XXXXY. These chromosomal aberrations occur when the gametes have an extra X chromosome. Characteristics include: tall stature, sterility, poorly developed gonads, larger chest, and smaller testicles than average.
Structural Chromosome Aberration
Cri-du-chat Syndrome
Individuals with this syndrome have a deletion on the short arm of one chromosome 5. Features include: severe mental retardation, short stature, difficulty swallowing and sucking, projected front teeth, and various secondary physical problems.
Chromosomal Diseases
In these conditions, the genetic alteration is significant enough to be visible in the chromosomes, either through changes in number or structure.
Altered Autosome Numbers
Patau Syndrome
It is caused by a trisomy of chromosome 13, meaning somatic cells have 44 autosomes, two heterochromosomes, and an extra chromosome 13. Babies born with this syndrome have features such as mental retardation, deafness, polydactyly (more than five fingers or toes), cleft palate, and heart anomalies.
Edwards Syndrome
This aberration, also called trisomy 18, affects infants who rarely survive more than four months, often dying of pneumonia or heart failure. Characteristics include: deformed ears, hypertension, cardiac abnormalities, nervous system alterations, mental retardation, and an elongated skull.
Down Syndrome
One of the best-known conditions in humans, it occurs when somatic cells have three chromosomes 21 instead of two. Also known as trisomy 21, its phenotype includes: mental retardation in varying degrees, short stature, almond-shaped eyes, round face, flattened nose, small ears, short neck, low muscle tone, cardiovascular disorders, infertility, broad hands, and short fingers.
Sex-Linked Disorders in Humans
Color Blindness
Common types of color blindness are inherited as X-linked traits. To understand color blindness, remember that cones are responsible for color perception, containing visual pigments that respond to different light wavelengths. There are three types of cones: red, blue, and green. Color blindness is related to the absence or malfunction of cones and is classified as:
- Monochromatic: Individuals have only one type of cone, seeing only one color.
- Dichromatic: Individuals have two types of cones, leading to variations such as red color blindness (confusing shades of red, green, and yellow) or blue color blindness (confusing shades of green, blue, orange, and pink).
- Anomalous Trichromatic: Individuals have three types of cones with functional defects, confusing one color with another. This is the most common group.
- Achromatic: Cones do not function, resulting in black and white vision only. This is very rare.
Hemophilia
This disease is caused by a recessive gene linked to the X chromosome and almost exclusively affects males. Mothers of affected males are invariably carriers of the recessive hemophilia gene on one of their X chromosomes.
Sex-Linked Disorders in Humans
Color Blindness
Common types of color blindness are inherited as X-linked traits. To understand color blindness, remember that cones are responsible for color perception, containing visual pigments that respond to different light wavelengths. There are three types of cones: red, blue, and green. Color blindness is related to the absence or malfunction of cones and is classified as:
- Monochromatic: Individuals have only one type of cone, seeing only one color.
- Dichromatic: Individuals have two types of cones, leading to variations such as red color blindness (confusing shades of red, green, and yellow) or blue color blindness (confusing shades of green, blue, orange, and pink).
- Anomalous Trichromatic: Individuals have three types of cones with functional defects, confusing one color with another. This is the most common group.
- Achromatic: Cones do not function, resulting in black and white vision only. This is very rare.
Hemophilia
This disease is caused by a recessive gene linked to the X chromosome and almost exclusively affects males. Mothers of affected males are invariably carriers of the recessive hemophilia gene on one of their X chromosomes.