Nondisjunction

The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction During normal meiosis I, one chromosome from each pair moves to each pole of the cell In nondisjunction, both chromosomes of a pair move to the same pole

Nondisjunction In one form of nondisjunction, two kinds of gametes result 1.Has an extra chromosome 2.Has a missing chromosome The effects are generally not seen until the fusion of gametes has occurred

Trisomy When a gamete with an extra chromosome fuses with a normal gamete, the zygote will have an extra chromosome This condition is called trisomy In humans, if the extra chromosome is located on the 21 st set of chromosomes the zygote will form into a baby that has Down Syndrome

Monosomy Although organisms with extra chromosomes often survive, organism lacking one or more chromosomes usually do not When a gamete is missing chromosomes the condition is called monosomy A non lethal example of this is Turner syndrome, in which human females have only one X chromosome instead of two

Nondisjuction Another form of nondisjunction involves a total lack of separation of homologous chromosomes When this happens, a gamete inherits a complete diploid set of chromosomes When it is fused with a normal haploid gamete, the zygote has three sets of chromosomes and is triploid

Nondisjunction If you have two gametes that are diploid instead of haploid fuse the result is a tetraploid zygote

Polyploidy Organisms with more than the usual # of chromosome sets are called polyploidy Rare in animals and almost always resulting in death of the zygote However, polyploidy frequently occurs in plants Often the fruits or flowers of these plants are larger and healthier

Polyploidy Hexaploid- (6n)- wheat Triploid- (3n) – apples Some plant breeders have learned to produce polyploid plants artificially by using chemicals that cause nondisjunction