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GENETIC DISORDERS
Mutation –change in a gene resulting in change in genetic information
• may be spontaneous or caused by a mutagen• Somatic mutations are usually not noticed• Gametic mutations are more severe• Not all mutations are bad!
SINGLE-GENE DEFECTS• Misplacement, insertion or deletion of one base pair • May result in incorrect amino acid• May result in shift of reading frame
• Congenital Defects: –Visible at birth(club foot, spina bifida,
congenital heart defects)–Caused by environmental factors (FAS,
German measles)• Inherited
–Sickle Cell Anemia, Cystic Fibrosis
INVERSIONS• Part of chromosome is
flipped around 180° from its normal orientation.
• Happens when chromosome breaks occur.
• Broken piece may reattach, but not necessarily in the same orientation as before
TRANSLOCATIONS
• Detachment of a segment of a chromosome, and reattachment to another non-homologous chromosome.
• Some genes wind up on a completely different chromosome
ANOMALIES OF CHROMOSOME NUMBER
• Polyploidy is common in plants
• Gametes don’t reduce chromosome #
• Result in cells with multiple copies of the genome
ANOMALIES OF CHROMOSOME NUMBER
• Aneuploidy – more common in humans
• Usually lethal before or shortly after birth
• Monosomic and Trisomic
NONDISJUNCTION
• Occurs when homologous chromosomes fail to separate after synapsis
• Most nondisjunction events are lethal to the fetus
DOWNS SYNDROME
• Results from a third copy of chromosome 21
• Called a trisomy
• Individuals have 47 chromosomes in every body cell
SEX CHROMOSOME NONDISJUNCTION DISORDERS
TURNER SYNDROME• Occurs when nondisjunction causes a gamete
to have a missing sex chromosome (X O)• Sterile females• 1 in 10 000 live births
SEX CHROMOSOME NONDISJUNCTION DISORDERS
KLINEFELTER’S SYNDROME• Genotype of XXY (extra sex chromosome)• Sterile males, breast development• 1 in 800 live births
SEX CHROMOSOME NONDISJUNCTION DISORDERS
½ ½
½
gamete lacking a sex chromosome
XO ¼ Turner’s syndrome
YO ¼ Will not survive(LETHAL CONDITION)
½
gamete formed from nondisjunction
XXX ¼“super female” usually normal
XXY ¼ Klinefelter’s syndrome
XY
X Y
O
XX
XX