Lecture Notes-21 Biology 1004

North Arkansas College

Topic: Sex-Linked Disorders

Click below for discussion and diagrams.

http://thirdage.adam.com/ency/article/002051.htm

The Y chromosome, due to its short size, usually does not undergo crossing over - gene makeup is stable (17-24).

1. Hemophilia - "free bleeders" do not make Factor VIII. Disorder occurs in 1:15,000 individuals.
2. Color blindness - 3 genes control color blindness; 2 of them are located on the X chromosome (red = 2%; green = 5%).
3. Duchenne's Muscular Dystrophy - Individuals exhibit waddle gait, toe walking, and fall frequently. Symptoms begin at early age (trouble getting up). Those affected rarely have children (do not live past 20 years of age). This disorder is caused by lack of a protein called dystropin that regulates calcium released from muscles. Calcium leaks into tissue, activating an enzyme that breaks down muscle.

1. There are > 205 known sex-linked abnormalities associated with the X chromosome.
2. They tend to skip a generation.
3. The affected son can have parents that appear normal. They are heterozygous.
4. If a mother carries the sex-linked trait, all of her son's are going to exhibit the characteristic.
5. If the daughter exhibits the trait, her mother must also have it.

1. Male pattern baldness is an example. This allele causes excess testosterone that is responsible for the baldness. Females who are heterozygous for this allele may develop adrenal tumors and hair loss.

Sometimes the chromosomes fail to segregate properly and some gametes wind up with abnormal numbers of chromosomes - seen with X & Y chromosomes. There are several abnormalities of this type.

1. Men with more than 1 Y chromosome (XYY) tend to be abnormally tall, suffer from persistent acne, and may have low intelligence. There were early attempts to associate XYY carriers with criminal behavior, and excessive aggressiveness.
2. Turner's Syndrome - females that receive only 1 functional chromosome (XO). O = no sex chromosome. Turner's Syndrome affected individuals tend to be short, broad-chested, do not go through puberty or menstruation, may have little or no breast development, and may suffer from congenital birth defects. Ovaries do not develop and often appear as white streaks. Those affected often have difficulty interpreting spatial distance.
3. Klinefelter Syndrome - males that receive 2 or more X chromosomes (XXY). Those affected are sterile, have underdeveloped testes, and some may exhibit breast development.
4. Meta females - have more than 2 X chromosomes (XXX). You might expect them to be super females, but this is not the case. They may have physical abnormalities. They do not develop regular menstrual cycles, and often begin menopause early in life.

More than 2 chromosomes (polyploidy) may cause the following:

Triploidy - 3 copies of a specific chromosome.

1. The best example is Down's Syndrome where 3 copies of chromosome 21 cause abnormalities such as founded face, sloping forehead, slanted eyes, thick tongue and mental retardation.
2. The probability of a female carrying a Down's Syndrome child to term increases with age. The immune system begins to decline in later years. It does not allow the individual to discriminate between self and non-self.
3. Fragile X causes problems similar to Down's Syndrome. This is not due to 3 copies of chromosome 21 of polyploidy. It was thought to be caused by genetic information on the end of a tiny piece of chromosome X (attached by a thin thread). This is not the case.

Repeats CCG at the end of the chromosomes cause Fragile X. 6-50 repeats of CCG is normal (up to 50), > 50 is abnormal. Those affected by Fragile X will often have 250 repeats.

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