LM 26: nondisjunction in meiosis onramps bio

Nondisjunction

Chromosome occurs when homologous chromosome pairs fail to separate during meiosis I, or chromatids fail to separate during meiosis II

• Leads to gametes with one too many or one too few chromosomes

Aneuploidy

An abnormal chromosome #

Monosomy

2N-1 = 45, 1 chromosome lost

Trisomy

2N + 1 = 47, an extra chromosome gained

Change in # of autosomes

• In humans, monosomy in autosomes (chromosomes other than sex chromosomes) is a fatal condition, with no monosomic individuals surviving to birth

• Trisomy in autosomes is a near fatal condition

Change in sex chromosomes

Aneuploidy in sex chromosomes has a much higher survival rate than in autosomes.

Turner syndrome

• Genotype: XO

• 1 in 2500 female births

• 20% do not survive to birth

• 75% caused by abnormal sperm

Klinefeter’s syndrome

• Genotype: XXY

• 1 in 1000 male births

Karyotype of an individual with Klinefelter Syndrome

XYY syndrome

• Genotype: XYY

• Phenotypically male

• 1 in 1000 male births

• XYY males are taller than average

XXX Syndrome

• Genotype: XXX

• Phenotypically female

• 1 in 1000 female births

• XXX females taller than average

• Symptoms often extremely mild or not present at all

Chromosomal Rearrangement

In some cases, chromosomes distribute to the gametes correctly, but undergo rearrangement in the genes they carry

2 major classes of chromosome rearrangements

• Inversions

• Translocations

Inversions

• Chromosome inversion

• Pericentric inversion

• Paracentric inversions

Chromosome inversion

Occurs when a portion of a chromosome detaches, flips, and reattaches to the chromosome

Pericentric inversions

Include the chromosome’s centromere

Paracentric inversions

Don’t include the centromere

Translocation

Sometimes, a portion of a chromosome detaches and then reattaches to a different, non-homologous chromosome