Changes in Chromosome Number

Aneuploidy is one extra or one less chromosome (2n + 1 or 2n - 1). It may affect one of every two newly fertilized eggs. It is a major cause of human reproductive failure. Most human miscarriages are aneuploids

Polyploidy is three or more of each chromosome. It is common in flowering plants but is lethal to the zygote if it occurs in humans.

Nondisjunction at anaphase I or anaphase II frequently results in a change in chromosome number.

If a gamete with an extra chromosome (n + 1) joins a normal gamete at fertilization, the diploid cell will be 2n + 1. This condition is called trisomy.

If an abnormal gamete is missing a chromosome, the zygote will be 2n - 1. This condition is called monosomy.

Changes in the Number of Autosomes

Down syndrome results from trisomy of chromosome 21 . 1 in 1,100 live newborns in North America are affected. Most children with Down syndrome show mental retardation, and 40% have heart defects. Down syndrome occurs more frequently in children born to older women.

Changes in the Number of Sex Chromosomes

Turner syndrome (designated XO) involves females whose cells have only one X chromosome, mostly due to nondisinnction in the father. A vast majority of XO embryos and fetuses are spontaneously aborted. Affected individuals are sterile and have other phenotypic problems such as premature aging and shorter life expectancy. It may be treated with hormones and surgery.

Klinefelter syndrome caused by nondisjunction (mostly in the mother) results in an extra X chromosome in the cells (XXY) of affected males. These individuals are taller than average, usually are sterile, and may show some mental slowness, and feminized traits. It can be treated with testosterone injections.

XYY condition results from an extra Y chromosome in males. It does not affect fertility, but affected individuals are taller than average and are slightly mentally retarded. Erroneous correlations have linked these persons with predisposition to crime.

Phenotypic Treatments

Symptoms of many genetic disorders can be minimized or suppressed by dietary controls, adjustments to environmental conditions, surgery, or hormonal treatments.

Genetic Screening

Large-scale screening programs can detect affected persons. For example, newborns in United States are routinely tested for phenylketonuria or PKU, a disease which results from the lack of a particular enzyme. Early detection allows dietary intervention and prevents brain impairment.

Prenatal Diagnosis

Techniques such as amniocentesis can be used to detect potential risks, such as Down syndrome prior to birth.

REVIEW: Nondisjuction can be caused by _____ .

REVIEW: A gamete affected by nondisjuction would have _____ .
a. a change from the normal chromosome number
b. one extra or one missing chromosome
c. the potential for a genetic disorder
d. all of the above