Errors in Meiosis

Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis.

• Nondisjunction may occur during meiosis 1 or meiosis 2

• Abnormal number of chromosomes may result

Non-disjunction is one of the Two major Non-disjunction is one of the Two major occurrences of Meiosisoccurrences of Meiosis

(The other is Crossing Over)(The other is Crossing Over)

Non-disjunction results in the production of zygotes with abnormal chromosome numbers

An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring.

• Should the gamete with the chromosome pair be fertilized then the offspring may or may not develop properly or at all

• If it occurs in one of the smaller genes…

• 13, 15, 18, 21, 22

• There is some chance of viability

• Trisomy 21 and 22 may survive into adulthood

• If it occurs in one of the larger genes…

• There is a failure in early development and termination occurs

Monosomy • If an organism

has Monosomy 23 it has only one chromosome in the 23rd set.

Usually occurs in one of two ways

Trisomy• If an organism

has Trisomy 21, there are three chromosomes in the 21st set.

Genetic TestingGenetic Testing Amniocentesis

Physician removes amniotic fluid from around the fetus. Cells are cultured for karyotype

Chorionic villus sampling Narrow tube

inserted through the cervix to suction out a sample of the placenta (fetal cells)

Immediate karyotype

Down’s Syndrome – Trisomy 21

Turner’s Syndrome – Monosomy 23 (X)

Kleinfelter’s Syndrome – Trisomy 23 (XXY)

Edward’s Syndrome – Trisomy 18

Fertile Normal physically Normal mentally Increase in

testosterone Perhaps more

aggressive Normal lifespan 1 in 1,800 births

Scarce beard Longer fingers and

arms Sterile Delicate skin Low mental ability Normal lifespan 1 in 1,100 births

• Short, broad hands

• Stubby fingers• Rough skin• Impotency in

males• Mentally retarded• Small round face• Protruding tongue• Short lifespan• 1 in 1,250 births

1 in 4400 births• XX=80%• XY=20%

Small head Mentally retarded Internal organ

abnormalities 90% die before

5 months of age

1 in 4400 births 96-98% do not survive to

birth No menstruation No breast development Small hips Broad shoulders and neck Other health issues can

include heart disease, horseshoe shaped kidneys, learning disabilities