Chapter 12.9 Heritable Changes in the Chromosome Number AP Biology Fall 2010

Chapter 12.9 Objectives  Explain the difference between aneuploidy vs. polyploidy, including how and when they occur  Describe the various disorders linked to sex chromosomal abnormalities

Changes in Chromosome Number  Abnormal events occur before or during cell division, causing a change in the chromosome number

Changes in Chromosome Number  Aneuploidy: a condition in which the gametes or cells of an affected individual end up with one extra or one less chromosome than is normal  Fatal for humans, cause of most miscarriages  Typically arise through nondisjunction, where one or more pairs of chromosomes do not separate as they should during mitosis or meiosis

Changes in Chromosome Number  Polypoidy: is the presence of three or more of each type of chromosome in gametes or cells  It is common in plants but fatal in humans  A chromosome number can change during mitotic or meiotic cell division or during the fertilization process  Tetraploid germ cells can result if cytoplasmic division does not follow normal DNA replication and mitosis

Changes in Chromosome Number  Nondisjunction: at anaphase I or anaphase II frequently results in a change in chromosome number

Changes in Chromosome Number  Nondisjunction:  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  Trisomy: 3 of one type of chromosome and 2 of every other type  If an abnormal gamete is missing a chromosome, the zygote will be 2n – 1  This is monosomy  Monosomy: 1 of one type of chromosome and 2 of every other type

Autosomal Change and Down Syndrome  Down syndrome: results from trisomy 21  Is the most frequent type of altered chromosome number in humans  Occurring once in every 800 to 1,000 births  About 95% of all cases come from nondisjunction at meiosis  Most children with Down syndrome show mental retardation and have heart defects  Down syndrome occurs more frequently in children born to older women

Down Syndrome

Female Sex Chromosome Abnormalities  Turner syndrome: involves females whose cells have only one X chromosome  Designated XO  Affected individuals (1 in 2,500 – 10,000 newborn girls) are infertile and have other phenotypic problems such as premature aging and shorter life expectancy  About 75% of the cases are due to nondisjunction in the father  About 98% of all XO zygotes spontaneously abort

Female Sex Chromosome Abnormalities  The XXX condition is an inheritance of multiple X chromosomes  About 1 in 1,000 females inherits 3, 4, or 5 X chromosomes  Most of these girls are taller and slimmer than average  Are fertile and fall within the normal range of appearance and social behavior

Male Sex Chromosome Abnormalities  Klinefelter syndrome: is caused by a nondisjunction which results in an extra X chromosome in the cells (XXY) of these affected males  1 in 500 – 2,000 live-born males  About 67% of these result from nondisjunction in the mother  About 3% of these results from nondisjunction in the father  Sterility, slight mental retardation, and body feminization are symptoms

Male Sex Chromosome Abnormalities  XYY condition: the extra Y chromosome in these males (1 in 1,000) does not affect fertility  Are taller than average and slight mentally retarded