The gene can be restored to normal function if a second mutation either (1)converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon, such restorative mutations are called nonsense suppressors.

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Aneuploidy

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Aneuploidy can arise in several ways

Chromosome may be lost during mitosis or meiosis.

The small chromosome generated by a Robertsonian translocation may be lost in mitosis or meiosis.

Aneuploidy may arise through nondisjunction.

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Types of Aneuploidy

Nullisomy is the loss of both members of a homologous pair of chromosomes. It is represented as 2n-2.

Monosomy is the loss of a single chromosome, represented as 2n-1. 

Trisomy is the gain of a single chromosome, represented as 2n+1. 

Tetrasomy is the gain of two homologous chromosomes, represented as 2n+2. 

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Nondisjunction

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SEX-CHROMOSOME ANEUPLOIDIES 

The most common aneuploidies seen in living humans.

Aneuploidy of the human sex chromosomes is better tolerated than aneuploidy of autosomes.

AUTOSOMAL ANEUPLOIDIES

There is no mechanism of dosage compensation for autosomes.

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DOWN SYNDROME Also known as trisomy 21, is the most common autosomal aneuploidy in humans About 75% of the nondisjunction events that cause Down syndrome are maternal in origin, most arising in meiosis I.