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