 Chromosomes are organized by size and banding patterns into 23 pairs.
 Pairs 1-22 are known as AUTOSOMES. These contain genes for characteristics not related to sex
 Pair 23 is known as the SEX CHROMOSOMES. These chromosomes determine the gender of an organism
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If the sex chromosomes are the same size, they are two X chromosomes. This means that the organism is a female
If the sex chromosomes are a different size, there is an X and Y chromosome. Signifying a male.

There is a certain way that a karyotype needs to be written. It is shown as the following.
Total # chromosomes, All sex chromosomes, +/- autosomes
(Lets revisit the previous karyotype and try to complete the notation)

Nondisjunction
The “staying together” of chromosomes during meiosis.
This can lead to genetic disorders and abnormalities in human beings.

 Write the correct karyotypic notation for a male with an extra chromosome at pair 21.
 Write the correct karyotypic notation for a female with an extra X chromosome

 Klinefelter’s Syndrome : Individual contains one or more sex chromosome (XXY). Mostly results in infertile males with incomplete puberty.
 Down’s Syndrome (Trisomy 21): Individual contains an extra chromosome at the 21 st pair. Typical symptoms include small eyes and mouth, decreased muscle tone, wide/short hands, mental retardation, etc…
 Trisomy 13 (Patua Syndrome): Individual contains an extra chromosome at the 13 th pair. Typical symptoms include a cleft palate, small head/eyes/lower jaw, extra fingers and toes, mental retardation, etc…