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Now that we've looked at how genes and chromosomes are normally inherited, let's discuss what happens when meiosis doesn't proceed properly. One common error in meiosis I is called chromosome nondisjunction, which is the failure of homologous chromosomes to separate during metaphase I. We'll first look at what happens if chromosome nondisjunction of the sex chromosomes occurs.
If a female’s X chromosomes don’t separate properly during meiosis I, some of her gametes will receive two X chromosomes and some will get none. When these gametes fuse with normal sperm cells, XXX, X-Oh, XXY, and Y-Oh zygotes can be produced.
In humans, triple X females are sterile. X-Oh females have Turner syndrome, a condition characterized by sterility, short stature, a webbed neck, immature sex organs, and a low IQ. XXY males have Klinefelter syndrome, which is characterized by sterility and mental retardation. Y-Oh zygotes don’t survive, because there are hundreds of essential genes on the X chromosome.
To test your understanding of chromosome nondisjunction, fill in the zygotes that would result from chromosome nondisjunction in a male. Click Submit when you’re done.
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Half the offspring would have Klinefelter syndrome, and half would have Turner syndrome.
Chromosome nondisjunction creates a condition called aneuploidy, in which a cell has lost or gained one or more chromosomes. There are several types of aneuploidy. Most aneuploidies are lethal in humans; although two types, monosomies and trisomies, are found in a small percentage of the population. In a monosomy, one chromosome is missing. In a trisomy, there are three copies of a chromosome.
The most well-known trisomy causes Down syndrome, which is characterized by particular facial features, short stature, heart defects, and mental retardation. Down syndrome is caused by nondisjunction of chromosome 21. Generally the chromosome nondisjunction occurs in the mother.
For unknown reasons, the probability of having a Down syndrome baby is directly related to the mother’s age. While a 20-year-old woman has a 1 in 1900 chance of having a baby with Down syndrome, a 45-year-old woman has a 1 in 32 chance! The association between maternal age and Down syndrome encourages pregnant women over age 35 to check the chromosomes of their unborn children early in pregnancy.
Amniocentesis is a technique for testing the genotype of an embryo or fetus in the uterus. During this procedure, cells from the amniotic fluid surrounding the fetus are extracted with a long needle. Chromosome abnormalities such as monosomies and trisomies can then be detected by doing a karyotype analysis.
A karyotype is the complete set of metaphase chromosomes of an individual or a cell, arranged in order of decreasing size. Karyotypes are prepared by extracting chromosomes out of cells, staining them, and rearranging the images on a computer.
Human autosomes are numbered 1 through 22; 1 is the largest, and 22 is the smallest. The sex chromosomes are just called X and Y. Karyotype analyses can also detect mutations that alter chromosome structure.
There are four classes of structural chromosome abnormalities: deletions, duplications, inversions, and translocations. A deletion results when a chromosomal region is deleted. When a chromosomal region gets copied, it’s called a duplication. An inversion occurs when the direction of a chromosomal region gets reversed. Finally, two nonhomologous chromosomes can exchange parts to produce a translocation.
Chromosomal abnormalities have serious consequences. They can remove or duplicate genes. They can also disrupt genes.
For example, three different types of translocations can cause acute myeloid leukemia, or AML, a disease in which abnormal blood cells invade other tissues. Some patients have a translocation between chromosomes 3 and 21, some between 8 and 21, and some between 12 and 21. Each translocation disrupts a gene on chromosome 21, called AML1, preventing it from performing its normal function in blood cell development.
Biologists can use information from chromosomal abnormalities to learn more about the genes they're interested in, such as disease genes like AML 1.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education