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Sexual Reproduction and Meiosis

Genetic Disorders

Several genetic disorders occur that can be passed on to offspring through the inheritance of chromosomes.
A genetic disorder is any one of a number of diseases that are the result of mutations of damaged genes; they are often inherited. There are three ways in which genetic disorders can arise: mutations within genes, changes in the number of chromosomes, and changes in the structure of chromosomes. Many times they happen because the chromosomes fail to separate properly during anaphase I or II. Instead of one member of each homologous pair moving toward the opposite poles (ends of the cell), they stay together, and both move in one direction or the other. This creates a cell that has an abnormal number of chromosomes. When this happens it is called nondisjunction, the failure of chromosomes or chromatids to separate during anaphase I or II of meiosis that results in an uneven distribution of genetic material. The resulting cells either have one too many or one too few chromosomes. It is unclear why nondisjunction occurs. One possibility is that the material that keeps the chromosomes together, the centromere, during prophase I breaks down.

Aneuploidy and Polyploidy

Aneuploidy describes any condition in which an abnormal number of chromosomes is inherited. Polyploidy occurs when additional sets of chromosomes are inherited.

Anytime an organism has an abnormal number of copies of a chromosome, it is known as aneuploidy. Whether the organism has too many or too few chromosomes, problems can arise. Most of the time, additional or missing chromosomes cause the resulting offspring not to be viable; that is, they are not born. Sometimes, however, aneuploidy results in changes to the organism's phenotype, that is, its observable characteristics. When aneuploidy occurs in humans, it often results in genetic disorders.

When an organism has more than the normal number of sets of entire chromosomes, it is known as polyploidy. This is a specific type of aneuploidy because there are extra chromosomes. It is possible for certain organisms to end up with triploid (3n) or even tetraploid (4n) chromosomes. Organisms with these types of chromosome numbers tend to have larger cells. An example of a polyploid organism is a triploid banana. These fruits tend to be larger in size than other bananas and lack seeds. Seedless watermelons are also a triploid fruit. Polyploidy (extra copies of all chromosomes) in humans results in nonviable offspring. Miscarriages or stillbirth occur when this happens.
Wild bananas are diploid. These bananas contain seeds and are smaller than the ones found in supermarkets today. Domestic bananas are aneuploid because they have an abnormal number of chromosomes. Specifically, they are triploid (3n), which makes them a polyploid species-more than two pairs of chromosomes.

Down Syndrome

Down syndrome results from three copies of chromosome 21 in humans, an example of trisomy.

If a zygote or fertilized egg ends up with three copies of a certain chromosome, a condition called trisomy occurs. The most common type of this disorder in humans is called trisomy 21, also called Down syndrome. There is a large continuum of the severity of Down syndrome. People with this disorder tend to have small noses, broad hands with small fingers, eyes that slant upward and outward, and a flat back of the head. They also tend to have impaired cognitive abilities, which causes delayed development.

Trisomy 21 is quite common in humans. About 1 in 800 babies born have the condition. Many more pregnancies result in miscarriage when the fetus has trisomy 21. One of the factors that can contribute to the frequency is the age of the mother. For reasons that are not clear, human females over the age of 35 are 500 times more likely to have a child with Down syndrome than those who are younger.

While nondisjunction is the most common way for Down syndrome to occur, another event called translocation (where a piece of one chromosome breaks off and fuses with another) can also produce Down syndrome in offspring.

Turner's Syndrome

Turner's syndrome results from only a single sex chromosome rather than two, an example of monosomy.

Turner's syndrome is another chromosomal disorder that occurs from nondisjunction during meiosis. This aneuploid disease affects females because it has to do with an incorrect number of X chromosomes being inherited. Normally people have two sex chromosomes, one X chromosome and one Y chromosome for males and two X chromosomes for females. In Turner's syndrome, the female receives only one X chromosome. The other is missing. This missing chromosome impacts development before and after birth.

Females with Turner's syndrome are often short in stature. This becomes obvious when the female is about 5 years old. There are also difficulties with the development of the ovaries. They start to develop normally, but egg cells die prematurely and the ovaries start to disintegrate. This normally results in the female being infertile. Other symptoms may include extra folds of skin on the neck, kidney problems, swelling of the hands, and heart defects.

Turner's Syndrome Karyotype

One genetic disease affecting only females is called Turner's syndrome. This karyotype shows with Turner's syndrome because there is only one X chromosome and no Y chromosomes. This is called aneuploidy, an incorrect number of chromosomes.

Klinefelter Syndrome

Klinefelter syndrome results from three sex chromosomes rather than two.
Klinefelter syndrome is similar to Turner's syndrome in that it has to do with nondisjunction of sex chromosomes during meiosis. However, in this case it impacts males, and they end up with two X chromosomes and one Y chromosome (XXY). This aneuploid condition is trisomy. Having two X chromosomes interferes with normal sexual development in the male. He develops noticeable breasts and other female features. The testes do not develop properly, and levels of testosterone are greatly decreased. Males with this condition also tend to have poor coordination and problems with their speech. There is a lot of variation in the way Klinefelter syndrome affects males. Some show the aforementioned physical traits, while others show no symptoms at all. In fact, these men may not even realize they have the disorder unless they visit a fertility specialist when trying to conceive a child.

Klinefelter Syndrome Karyotype

This karyotype shows this person is a male with Klinefelter syndrome because there is a Y chromosome even though there are two X chromosomes. Aneuploidy exists in this condition because there is one extra chromosome (47, X).

Other Types of Chromosomal Disorders

Some genetic disorders are caused by errors during crossing-over, such that the organism has the correct number of chromosomes but the chromosomes themselves have been damaged.

Some chromosomal disorders are caused not by nondisjunction but by errors during crossing over. When chromatids exchange material, deletions, duplications, inversions, or translocations of chromosomal material can occur. Because of the variable nature of which genetic material is incorrect, many of these disorders affect only a single individual and do not have names. However, a few disorders of this type occur more frequently.

Cri du chat syndrome is one of these disorders and is the result of a deletion of chromosome 5. It is named because of the distinctive cry of children who have this disorder. The name is French for "cry of the cat." It occurs in 1 out of every 35,000 babies born. It involves an abnormal development of the larynx, which is why the cry sounds different. Most of the time this disorder is not inherited, but individuals with the disorder will pass on the defective chromosome to their children if they have any.

Cat eye syndrome, on the other hand, results from duplication of material on chromosome 22. Symptoms of the disorder include mild mental impairment, impaired growth, and malformations of the skull and internal organs.