Nondisjunction and Karyotypes

Nondisjunction = having too few or too many chromosomes When chromosomes are not distributed correctly in meiosis

Results of nondisjunction after fertilization: Normal = a diploid cell with 2 of each chromosome

Results of nondisjunction after fertilization: Monosomy = a diploid cell is missing a chromosome in one chromosome pair Example: Turner Syndrome (One X chromosome)

Results of nondisjunction after fertilization: Trisomy = a diploid cell has an extra chromosome in one chromosome pair Examples: Down Syndrome (Trisomy 21) Klinefelter Syndrome (XXY)

Human Karyotyping Two ways to obtain chromosomes from the fetus: Amniocentesis = removing amniotic fluid that contains cells from the fetus from the amniotic sac which surrounds the fetus during development using a syringe Chorionic villi sampling (CVS) = removing cells from the chorionic villi (fingerlike extensions of the placenta that grow in the mother’s uterus)

Karyotype = representation of chromosomes in metaphase of mitosis Arranged in homologous pairs by size 22 pairs are autosomes 1 pair are the sex chromosomes (XY male or XX female) =23 pairs (46 chromosomes) Looking for abnormalities –sets that do not have 2 chromosomes

Chromosome Pair Affected # of Total Chromosomes in a cell Nondisjunction in Humans: Chromosome Pair Affected # of Total Chromosomes in a cell Clinical Syndrome Estimated Frequency at Birth Symptoms Trisomy of chromosome 21 47 Down Syndrome 1/700 Short, broad hands with a single crease across the palm, short stature, hyperflexibility of joints, mental retardation, small head with flattened face, slanting eyes, large tongue X 45 Turner Syndrome 1/2500 female births Female that do not fully develop sexual characteristics, usually sterile, short stature, webbing of skin in the neck, hearing impairment, heart abnormalities XXY Klinefelter Syndrome 1/500 male births Male with small testes, infertile, developed breasts, feminine-pitched voice, long legs XXX Triplo-X Female with normal genitalia, limited fertility, slight mental retardation