Cytosine is deaminated to Uracil.

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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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#Unit 8. Inheritance Biology

HUMAN TRISOMIES

Trisomy 18 - Edward syndrome

Trisomy 13 - Patau syndrome.