... The most convincing evidence [about the individuality of the chromosomes] comes from exceptional cases of accidental or irregular distribution of one or more chromosomes, so that an egg, or a cell comes to have one more chromosome than is usually present...

- Thomas Hunt Morgan,
The Physical Basis of Heredity,
1919, page 52

Perhaps a better title for today's lecture would be:

What happens when you don't get the right number of chromosomes?

This poor fly has lost its X chromosome after the 2nd cell division - so it is literally half male (X0) and half female (XX).   Several sex-linked traits that would otherwise have been masked are now visible in half of this fly!  (Most sexual mosaics are called gynandromorphs.)

Yes, the same thing can happen in humans!

There are two different types of variation in chromosome number:

1. Aberrant Euploidy -  this is where you have an entire EXTRA SET (or two, or three...) of the entire set of chromosomes.  Thus a human with 92 chromosomes is said to be an "aberrant euploid" (amongst other things...)

2. Aneuploidy -this is where you have an extra single chromosome (or two) or are missing a chromosome (or two)

I. Aberrant Euploidy

    Monoploids
    Polypoids

•  Triploids
• Autotetraploids
• Allopolyploidy - composed of sets of chromosomes from different species (e.g., wheat)

 

Polyploidy in Animals
 
 
 
 
 

II. Aneuploidy
 

Nullisomics (2n-2)

Monosomics (2n-1)

Trisomics (2n+1)

Disomics (n+1)

Somatic Aneuploids

 

Back to the GENETICS Syllabus

Last modified on: 1 February, 2000 by Dave Ussery