Isovolumetric Ventricular Relaxation

Closure of aortic valve producing the second heart sound (S2).

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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. 

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Those who have Down syndrome have three full copies of chromosome 21, a condition termed primary Down syndrome.

familial Down syndrome Human condition caused by a Robertsonian translocation in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families.

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