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