Class 3

Ploidy

The number of chromosome sets

Haploid (1n)

One set of chromosomes

Diploid (2n)

Two sets of chromosomes

Polypoid (3x, 4x, 5x, etc)

More than two sets of chromosomes

Euploid

A cell/individual that contains the ‘expected’ number of chromosomes

Aneuploid

Differ from expected by one or a few chromosomes

Nullisomy

2n - 2

Monosomy

2n - 1

Trisomy

2n + 1

What causes aneuploidy?

Nondisjunction

When can nondisjunction occur

Anaphase, Anaphase I, Anaphase II

Autopolyploidy

Having additional sets of chromosome from same species

Allopolyploidy

Having additional sets of chromosome from another species

Fertility of even-numbered autopolyploidal chromosome sets

Fertile, can often give rise to new species

Fertility of odd-numbered autopolyploidal chromosome sets

Sterile, unbalanced gametes

The symbol n

Represents one complete set of nonhomologous chromosomes, the number of distinct chromosome types

Nondisjunction

The failure of chromosomes to separate properly during cell division

Two different homologous alleles from one parent

Indicates Meiosis I Nondisjunction

Two identical copies of an allele from one parent

Indicates Meiosis II Nondisjunction

Meiosis I Nondisjunction

Homologous chromosomes fail to separate

Meiosis II Nondisjunction

Sister chromatids fail to separate

Meiosis I Nondisjunction Offspring

Two n + 1, Two n - 1

Meiosis II Nondisjunction Offspring

Two normal offspring, one n + 1, one n - 1

Mosaic Organism

Result of mitotic nondisjunction after fertilization

Triploid Fertility

Sterile, because three homologs cannot divide evenly during meiosis

Infertile Fruits

Are Triploid

What causes polyploidy?

Double/triple fertilization, Cytokinesis failure in gamete precursor cells, mating between parents of different ploidy